Synchronous Motor generator

ssc-je-synchronous-motor-synchronous-generator-questions

Ques.1. The speed of a synchronous motor can be changed by

  1. Changing the supply voltage
  2. Changing the frequency
  3. Changing the load
  4. Changing the supply Terminals

The speed of the synchronous motor is given as

Ns = 120f/P

Therefore by changing the number of poles and frequency, we can change the speed of the synchronous motor.

 

Ques.2. In a synchronous motor running with the fixed excitation, when the load is increased two times, its torque angle becomes approximately

  1. Half
  2. Twice
  3. Four Times
  4. No Change

A synchronous motor runs at an absolutely constant speed called synchronous speed, regardless of the load. Let us examine how the change in load affects its performance.

The power angle or torque angle of a synchronous motor is given as

P \approx \dfrac{{VE}}{{{X_s}}}\sin \delta

Where

δ = Torque angle

Xs = Synchronous Reactance

A synchronous motor operates at the same average speed for all values of the load from no load to peak load. When the load on a synchronous motor is increased, the motor slows down just enough to allow the rotor to change its angular position in relation to the rotating flux of the stator and then goes back to synchronous speed. Similarly, when the load is removed, it accelerates just enough to cause the rotor to decrease its angle of lag in relation to the rotating flux, and then goes back to synchronous speed. When the peak load that the machine can handle is exceeded, the rotor pulls out of synchronism.

Hence if the load increases two times then the torque will also increase the two times so that the synchronous motor does not pull out of synchronism.

 

Ques.76. A three-phase synchronous motor will have

  1. One slip-ring
  2. Two slip-rings
  3. No slip-rings
  4. Three slip-rings

A three-phase synchronous motor basically consists of a stator core with a three-phase winding (similar to an induction motor), a revolving DC field with an auxiliary or amortisseur winding and slip rings, brushes and brush holders, and two end shields housing the bearings that support the rotor shaft. An amortisseur winding consists of copper bars embedded in the cores of the poles. The copper bars of this special type of “squirrel-cage winding” is welded to end rings on each side of the rotor. The function of a slip ring is to transfer electrical signals from rotary to stationary  components or systems

 

Both the stator winding and the core of a synchronous motor are similar to those of the three-phase, squirrel-cage induction motor, and the wound-rotor induction motor.

The rotor of the synchronous motor has salient field poles. The field coils are connected in series for alternate polarity. The number of rotor field poles must equal the number of stator field poles. The field circuit leads are brought out to two slip rings mounted on the rotor shaft for brush-type motors. Carbon brushes mounted in brush holders make contact with the two slip rings. The terminals of the field circuit are brought out from the brush holders to a second terminal box mounted on the frame of the motor. A squirrel-cage, or amortisseur, winding is provided for starting because the synchronous motor is not self-starting without this feature.

 

Ques.77. The maximum speed variation in a synchronous motor is

  1. Zero
  2. 5%
  3. 2%
  4. 10%

A synchronous motor is a constant speed motor, therefore, the variation of speed is zero.

 

Ques.78. In a synchronous motor which loss varies with the load?

  1. Windage loss
  2. Bearing friction Loss
  3. Core Loss
  4. Copper Loss

In the rotating machine whether it is AC or DC machine the type of losses are almost same 

Cooper- loss(I2R) Losses:- All windings have some resistance (though small) and hence there are capper-losses associated with current flow in them. The copper-loss can again be subdivided into the stator copper-loss, rotor copper-loss, and brush-contact loss. The stator and rotor copper-losses are proportional to the current squared and are computed with the dc resistance of windings at 75°C.

The conduction of current between the brushes (made of carbon) and the commutator of a dc machine is via short arcs in the air-gaps which are bound to exist in such a contact. As a consequence, the voltage drop at the brush contact remains practically constant with the load; its value for positive and negative brushes put together is of the order of 1 to 2 V. The brush-contact loss in a dc machine is therefore directly proportional to current. The contact losses between the brushes (made of copper-carbon) and slip-rings of a synchronous machine are negligible for all practical purposes.

Copper-losses are also present in field windings of synchronous and dc machines and in regulating the rheostat. However, only losses in the field winding are charged against the machine, the other being charged against the System.

Stray load Losses:-  Apart from the variable losses mentioned above, there are some additional losses that vary with load but cannot be related to the current in a simple manner. These losses are known as “stray-load loss”.The stray-load loss is difficult to calculate accurately and therefore it is taken as 1 % of the output for a dc machine and 0.5% of the output for both synchronous and induction machines.

 

Ques.79. In a synchronous Motor, the damping winding is generally used to (SSC-2018 Set-1,2)

  1. Provide starting torque
  2. Prevent hunting and provide starting Torque
  3. Reduces the eddy current
  4. Reduces the Noise Level

Use of Damper Winding to Provide the starting torque

To enable the synchronous machine to start independently as a motor, a damper winding is made on pole face slots. Bars of copper, aluminum, bronze or similar alloys are inserted in slots made or pole shoes as shown in Fig. These bars are short-circuited by end-rings or each side of the poles. Thus these short-circuited bars form a squirrel-cage winding. On the application of three-phase supply to the stator, asynchronous motor with damper winding will start at a three-phase induction motor and rotate at a speed near to synchronous speed. Now with the application of dc excitation to the field windings, the rotor will be pulled into synchronous speed since the rotor pole is now rotating at only slip-speed with respect to the staler rotating magnetic field.

Damper-winding

Use of Damper winding to prevent Hunting

During Hunting, the rotor of the synchronous motor starts to oscillate in its mean position, therefore, a relative motion exists between damper winding and hence the rotating magnetic field is created. Due to this relative motion, e.m.f. gets induced in the damper winding. According to Lenz’s law, the direction of induced e.m.f. is always so as to oppose the cause producing it. The cause is hunting. So such induced e.m.f. oppose hunting. The induced e.m.f. tries to damp the oscillations as quickly as possible. Thus hunting is minimized due to damper winding.

 

Ques.96. An over-excited synchronous motor is used for

  1. Variable speed load
  2. Low torque loads
  3. Power factor corrections
  4. High torque loads

An overexcited synchronous machine produces reactive power whether or not it is operating as a motor or as the generator.

When synchronous motors are used as synchronous condensers they are manufactured without a shaft extension, since they are operated with no mechanical load. The ac input power supplied to such a motet can only provide for its losses. These losses are very small and the power factor of the motor is almost zero.

Therefore, the armature current leads the terminal voltage by close to 90°, as shown in Figure a, and the power network perceives the motor as a capacitor bank. As can be seen in Figure b, when this motor is overexcited it behaves like a capacitor (i.e., synchronous condenser), with Ea > Vφ, whereas when it is under-excited, it behaves like an inductor (i.e., a synchronous reactor), with Ea < Vφ.

Synchronous condensers are used to correct power factors at load points or to reduce line voltage drops and thereby improve the voltages at these points, as well as to control reactive power flow. Generally, in large industrial plants, the load power factor will be lagging. The specially designed synchronous motor running at zero loads, taking leading current, approximately equal to 90°. When it is connected in parallel with inductive loads to improve power factor.

Large synchronous condensers are usually more economical than static capacitors. 

 

Ques.97. When any one-phase of a 3-phase synchronous motor is short-circuited, the motor

  1. Will overheat in the spot
  2. Will refuse to start
  3. Will not come upto speed
  4. Will fail to pull into step

Failure of a synchronous motor to start is often due to faulty connections in the auxiliary apparatus. This should be carefully inspected for open circuits or poor connections. An open circuit in one phase of the motor itself or a short circuit will prevent the motor from starting. Most synchronous motors are provided with an ammeter in each phase so that the last two causes can be determined from their indications: no current in one phase in case of an open circuit and excessive current in case of a short circuit. Either condition will usually be accompanied by a decided buzzing noise, and a short-circuited coil will often be quickly burned out. The effect of a short circuit is sometimes caused by two grounds on the machine.

 Difficulties in starting synchronous motors:-  A synchronous motor starts as an induction motor. The starting torque, as in an induction motor, is proportional to the square of the applied voltage. For example, if the voltage is halved, the starting effort is quartered. When a synchronous motor will not start, the cause may be that the voltage on the line has been pulled below the value necessary for starting. In general, at least half voltage is required to start a synchronous motor.

Difficulty in starting may also be caused by an open circuit in one of the lines to the motor. Assume the motor to be three-phase. If one of the lines is open, the motor becomes single-phase, and no single-phase synchronous motor, as such, is self-starting. The motor, therefore, will not start and will soon get hot. The same condition is true of a two-phase motor if one of the phases is open-circuited.

Difficulty in starting may be due to a rather slight increase in static friction. It may be that the bearings are too tight, perhaps from cutting during the previous run. Excessive belt tension, if the synchronous motor is belted to its load or any cause which increases starting friction will probably give trouble. Difficulty in starting may be due to field excitation on the motor. After excitation exceeds one-quarter of normal value, the starting torque is influenced. With full field on, most synchronous motors will not start at all. The field should be short-circuited through a proper resistance during the starting period.

 

Ques.98. Which of the following can be measured by conducting an insulation resistance test on a synchronous motor?

  1. Phase to Phase winding resistance
  2. Rotor winding to earth shaft
  3. Stator winding to an earthed shaft
  4. All option are correct

Insulation Resistance Test

This test is conducted with voltages from 500 to 5000 V and provides information on the condition of machine insulation. A clean, dry insulation system has very low leakage as compared to a wet and contaminated insulation system. This test does not check the high-voltage strength of the insulation system but does provide information on whether the insulation system has high leakage resistance or not. This test is commonly made before the high-voltage test to identify insulation contamination or faults. This test can be made on all or parts of the machine circuit to ground.i.e

  • Field winding test or Rotor winding Test
  • Overall Stator Armature winding test
  • Overall System test for the Motor or generator

The overall system test includes generator neutral, transformer, all stator windings, isolated phase bus, and low side windings of the generator step-up transformer. This test is performed as a screening test after an abnormal occurrence on the machine. If the reading is satisfactory, no further tests are made. If the reading is questionable or lower, the machine terminals are disconnected and further isolation performed to locate the source of the trouble. 

 

Ques.99. The speed of a synchronous motor can be changed by

  1. Changing the supply voltage
  2. Changing the frequency
  3. Changing the load
  4. Changing the supply Terminals

The speed of the synchronous motor is given as

Ns = 120f/P

Therefore by changing the number of poles and frequency, we can change the speed of the synchronous motor.

 

Ques.100. The under-excited synchronous motor takes____ (SSC-2018 Set-2)

  1. Leading current
  2. Lagging current
  3. Both leading and lagging current
  4. None of these

Unlike the induction machine, the synchronous machine can operate at lagging, leading and unity power factors. In an induction machine, the magnetizing current is required to establish flux in the air gap. This magnetizing current lags the voltage and therefore, the induction machine always operates at lagging power factor.

On the other hand, in the synchronous machine, the total air gap flux is produced by dc source and there is no use of lagging current from ac system for production of air-gap flux. If dc excitation is decreased, lagging reactive power will be drawn from ac source to aid magnetization and thus machine will operate at lagging power factor. If dc excitation is more, leading current drawn from ac source to compensate (oppose) the magnetization and the machine will operate a leading power factor.

Thus it can be concluded that an over-excited motor(Eb  > V) draws leading current (acts like a capacitive load) but an under-excited motor(Eb < V) draws lagging current (acts as an inductive load).

 

Ques.80. Power factor correction substations consist of

  1. Rectifiers
  2.  Inverters
  3. Synchronous condenser
  4.  Transformers

Power factor correction substation: It is used for the power factor improvement purpose because due to the impedance of the line, the power factor of the system is decreased. These substations are located near the receiving end of the line. Hence, synchronous condenser or static capacitors are installed to improve the power factor at the receiving end.

Synchronous condenser

A synchronous condenser is a synchronous motor running without the mechanical load. The Synchronous motor takes a leading current when over-excited and therefore behave as a capacitor. When such a machine is connected in parallel with the supply, it takes a leading current which partly neutralizes the lagging reactive component of the load. Thus the power factor is improved.

Now let V is the voltage applied and IL is the current lagging V by angle φL. This power factor φL, is very low, lagging.

The synchronous motor acting as a synchronous condenser is now connected across the same supply. This draws a leading current of Im.

The total current drawn from the supply is now the phasor sum of IL and Im. This total current I now lags V by smaller angle φ due to which effective power factor gets improved. This is how the synchronous motor as a synchronous condenser is used to improve the power factor of the combined load. 

Advantages

  1. A synchronous condenser has an inherently sinusoidal waveform and the voltage does not exist.
  2. It can supply as well as absorb kVAr.
  3. The PF can be varied in smoothly.
  4. It allows the overloading for short periods.
  5. The high inertia of the synchronous condenser reduces the effect of sudden changes in the system load and improves the stability of the system.
  6. It reduces the switching surges due to sudden connection or disconnection of lines in the system.
  7. The motor windings have high thermal stability to short-circuit currents.
  8. By varying the field excitation, the magnitude of current drawn by the motor can be changed by any amount. This helps in achieving stepless control of power factor.
  9. The faults can be removed easily.

 

Ques.95. Synchronizing power of a synchronous machine is

  1. Directly proportional to the synchronous reactance
  2. Equal to the synchronous reactance
  3. Inversely proportional to the synchronous reactance
  4. None of these

Nowadays, almost all alternators are connected it parallel with other alternators. The satisfactory parallel operation of alternators depends upon the synchronizing power. If the synchronizing power is higher, the higher is the capability of the system to synchronism.

The variation of synchronous power with the change of load angle is called the synchronizing power. It exists only during the transient state, i.e. whenever there is a sudden disturbance in load (or steady-state operating conditions). Once the steady state is reached, the synchronizing power reduces to zero.

If an alternator transfers power to an infinite bus at a steady-state power angle δo and a sudden transient disturbance due to increase in load will occur the rotor of the alternator will accelerate and at a load angle δo + dδ, the alternator supplies new power P + dP. Therefore, the operating point shifts to a new line. The steady-state power input remains unchanged. Therefore, the additional load decreases the speed of the machine and hence the alternator comes back to the steady-state position. Similarly, due to sudden transient, if the rotor retards, the load angle will decrease. The operating point will shift to a new line and the load on the machine becomes P – dP. Once again the input to the machine remains unaltered, therefore, the rotor is accelerated due to the reduction in load. Hence, the machine comes back to synchronism. Therefore, we can conclude that the effectiveness of this correction action depends upon the change in power transfer for a given change in load angle

The synchronizing power flows from or to the bus in order to maintain the relative velocity between an interacting stator and rotor field, zero, once the equality is reached, the synchronizing power vanishes.

The synchronizing power coefficient, which is defined by the rate at which synchronous power varies with load angle (S) gives a measure of effectiveness. The synchronizing power coefficient is also called stiffness of coupling, rigidity factor or stability factor. It is denoted by Psyn.

The synchronizing power of the synchronous machine is given as

{P_{syn}} = \frac{{{E_b}V}}{{{X_s}}}\cos \delta

Where

V = supply Voltage

Eb = Back E.M.F

Xs = Synchronous Reactance

Hence from the above expression, it is clear that synchronizing power is inversely proportional to the synchronous reactance.

 

Ques.96. The normal starting methods that are used to start a synchronous motor is

  1. Star-delta starter
  2. Damper winding
  3. Resistance starter in the armature circuit
  4. Damper winding in conjunction with the star-delta starter

To enable the synchronous machine to start independently as a motor, a damper winding is made on rotor pole face slots. Bars of copper, aluminum, bronze or similar alloys are inserted in slots made or pole shoes as shown in Fig.

These bars are short-circuited by end-rings or each side of the poles. Thus these short-circuited bars form a squirrel-cage winding. On the application of three-phase supply to the stator, synchronous motor with damper winding will start at a three-phase induction motor and rotate at a speed near to synchronous speed or sub-synchronous speed. When a de supply is given to the field winding. At a particular instant, the motor gets pulled into synchronism and starts rotating at a synchronous speed.

Since the rotor poles are now rotating at only slip-speed with respect to the stator rotating magnetic field. To limit the starting current drawn by the motor a reduced voltage may be necessary, to apply for high capacity synchronous motors. Reduced voltage can be applied through an auto-transformer or through a star-delta starter. Hence to start a synchronous machine as an induction motor and to limit the starting current drawn by the motor star-delta starter is used.

During starting period before the application of dc excitation, the field windings are kept closed through a resistor. DC is supplied from an independent source or through the armature of the dc exciter, namely the dc generator carried on the shaft extension of the synchronous motor. If this is not done, a high voltage induced in the dc winding during the starting period will strain the insulation of the field winding.

Since the starting of the motor is done as an induction motor, the starting torque developed is rather low and, therefore, large capacity motors may not be able to start on full load.

 

Ques.97. At what condition synchronous motor can be used as a synchronous capacitor

  1. Under-loaded
  2. Under-excited
  3. Over-loaded
  4. Over-excited

Synchronous condenser

A synchronous condenser is a synchronous motor running without the mechanical load. The Synchronous motor takes a leading current when over-excited and therefore behave as a capacitor. When such a machine is connected in parallel with the supply, it takes a leading current which partly neutralizes the lagging reactive component of the load. Thus the power factor is improved.

Now let V is the voltage applied and IL is the current lagging V by angle φL. This power factor φL, is very low, lagging.

The synchronous motor acting as a synchronous condenser is now connected across the same supply. This draws a leading current of Im.

The total current drawn from the supply is now phasor sum of IL and Im. This total current I now lags V by smaller angle φ due to which effective power factor gets improved. This is how the synchronous motor as a synchronous condenser is used to improve the power factor of the combined load. 

Advantages

  1. A synchronous condenser has an inherently sinusoidal waveform and the voltage does not exist.
  2. It can supply as well as absorb kVAr.
  3. The PF can be varied in smoothly.
  4. It allows the overloading for short periods.
  5. The high inertia of the synchronous condenser reduces the effect of sudden changes in the system load and improves the stability of the system.
  6. It reduces the switching surges due to sudden connection or disconnection of lines in the system.
  7. The motor windings have the high thermal stability to short-circuit currents.
  8. By varying the field excitation, the magnitude of current drawn by the motor can be changed by any amount. This helps in achieving stepless control of power factor.
  9. The faults can be removed easily.

 

Ques.98. Synchronous motor do not have self-starting property because

  1. Starting winding is not provided on the machines
  2. The direction of rotation is not fixed
  3. The direction of instantaneous torque reverses after the half cycle
  4. Starters cannot be used on these machines

The synchronous motor works on the principle of magnetic locking. The operating principle can be explained with the help of 2-Pole synchronous machine with the following steps.

Step 1. When a three phase supply is given to the stator winding, a Rotating magnetic field is produced in the stator.

Step 2. Due to Rotating Magnetic field, let the stator poles Ns and Ss rotate with synchronous speed. At a particular time stator pole, Ns coincides with the rotor poles Nr and SS coincides with Sr i.e like poles of the stator and rotor coincide with each other. As we know, like poles experience a repulsive force. So rotor poles experience a repulsive force Fr. Assume that the rote tends to rotate in the anti-clockwise direction as shown in the Fig.(i).

Step-3.After half cycle, the polarity of the stator pole is reversed, whereas the rotor poles cannot change their position due to inertia as shown in Fig. (ii). Now unlike poles coinciding each other and rotor experiences the attractive force fa and tends to rotate in clockwise direction. In brief, we can say, with the rotation of stator poles the rotor tends to drive in the clockwise and anti-clockwise direction in every half cycle. As a result, the average torque on the rotor is zero Hence 3-phase synchronous motor is not a self-starting Motor.

Step 4. Now suppose the rotor is rotated by some external means at a speed almost equal to synchronous speed. At a certain instant, the stator and rotor unlike poles will face each other, then due to the strong force of attraction, magnetic locking is established, the rotor and stator poles continue to occupy the same relative position.

Step 5. Due to this, the rotor continuously experiences a unidirectional torque in the direction of the rotating magnetic field. Hence 3-phase synchronous motor must run at synchronous speed.

 

Ques.99. The over-excited synchronous motor takes

  1. Leading current✓
  2. Lagging current
  3. Both leading and lagging current
  4. None of the above

Unlike induction machine, the synchronous machine can operate at lagging, leading and unity power factors. In the induction machine, magnetizing current is required to establish the flux in the air gap. This magnetizing current lags the voltage and therefore, induction machine always operates at lagging power factor.

When the load on a synchronous motor is constant. the input power V.I.Cosφ drawn from the bus-bar will remain constant. As the bus-bar voltage V is constant, I.Cosφ will remain constant. Under this condition, the effect of change of field excitation of the armature current, I drawn by the motor will be as follows:

When excitation is changed, the magnitude of induced EMF changes. The torque angle α i.e. the angle of lag of E from the axis of ‘remains constant as long as the load on the motor is constant.

On the other hand, in the synchronous machine, the total air gap flux is produced by dc source and there is no use of lagging current from the A.C system for production of air gap flux. If dc excitation is decreased, lagging reactive power will be drawn from A.C source to aid magnetization and thus machine will operate at lagging power factor. If dc excitation is more, leading current drawn from A.C source to compensate (oppose) the magnetization and the machine will operate a leading power factor.

If dc excitation is more, leading current is drawn from the ac source to compensate (oppose) the magnetization and the machine will operate a leading power factor. If a motor is operating at leading power factor at no-load, it is caller synchronous condenser which can work as variable inductor or capacitor.

 

Ques.100. The maximum power developed in a synchronous motor will depend on (SSC-2018 Set-3)

  1. The rotor excitation and supply voltage
  2. The rotor excitation, supply voltage and maximum value of coupling angle
  3. The supply voltage only
  4. The rotor excitation only

The maximum power developed in the synchronous Motor is given by the expression

{P_{\max }} = \dfrac{{{E_b}V}}{{{Z_s}}}\cos (\theta - \delta ) - \dfrac{{E_b^2}}{{{Z_s}}}Cos\theta

Where

δ = load angle

θ = Internal angle

V = Terminal voltage

Eb = Back EMF or excitation because back EMF Eb in Synchronous Motor depends on the DC excitation only because speed is constant

The power developed depends on the excitation, voltage and coupling angle. The maximum value of θ and hence δ is 90°. An increase in the excitation results in an increase of Pmax. Consequently, the load angle decreases for a given power developed. The overload capacity of the motor increases with an increase in excitation and the machine becomes more stable.

For all values of V and Eb, this limiting value of δ is the same but maximum torque will be proportional to the maximum power developed

If the resistance of the armature is negligible, then 

Z ≅ Xs,  θ = 90° ∴ Cosθ = 0

Therefore the power develop is given by

{P_{\max }} = \dfrac{{{E_b}V}}{{{X_s}}}\sin \delta

Hence the power develop will me maximum when δ is 90°.

 

Ques.71. Name the equipment which runs an alternator

  1. Prime Mover
  2. Generator
  3. Motor
  4. Fan

All generators, large and small, ac and dc, require a source of mechanical power to turn their rotors. This source of mechanical energy is called a prime mover. When a prime mover drives the synchronous machine, it functions as an alternator concerting the mechanical energy of the prime mover into electrical energy. 

Prime movers are divided into two classes for generators-high-speed and low-speed. Steam and gas turbines are high-speed prime movers, while internal-combustion engines, water, and electric motors are considered low-speed prime movers. The type of prime mover plays an important part in the design of alternators since the speed at which the rotor is turned determines certain characteristics of alternator construction and operation.

 

Ques.95. Synchronous Motor Shaft is Made of

  1. Alnico
  2. Chrome Steel
  3. Mild Steel
  4. Stainless Steel

A shaft is a rotating machine element, usually circular in cross-section, which is used to transmit power from a machine to a machine which absorbs power. Low carbon steel which is also called the mild steel (MS) is an alloy of iron and carbon. The carbon content varies from 0.05 to 0.15 percentage for dead mild steel and ().15 to 0.3 percentage for mild steel. 

The shaft is generally made of mild steel When high strength is required, an alloy steel such as nickel, nickel-chromium or chromium-vanadium steel is used. Mechanical power is taken or given to the machine through the shaft.

 

Ques.96. When the voltage applied to a synchronous motor is increased, which of the following will reduce?

  1. Stator Flux
  2. Pull in Torque
  3. Pull out Torque
  4. None of these

Let us know the factors, facts, assumptions that are depended on supply voltage for good operation of the synchronous motor.

The synchronous motor is a dual-excited constant speed motor, Basically supply voltage is injected such that it produces RMF at specified phase angles wrt to phases in the stator of our motor.
RMF produces enough constant magnitude flux, Let us assume that initially our synchronous motor was operated at a voltage less than the preset voltage value.

As in the case of Induction motors Variation in the frequency of the source will result in the corresponding change in the flux in the air gap. Hence, in order to operate the motor with fairly constant flux in the air gap, it is necessary to vary the magnitude of the applied voltage in the same ratio as the frequency of the supply (i.e V/f should be kept constant) and to keep the excitation current constant.

If a synchronous motor is driven by an external power source, and the excitation, or voltage applied to the rotor, is adjusted to a certain value called 100 percent excitation, no current will flow from or to the stator winding. In this case, the voltage generated in the stator windings by the rotor, or back EMF, exactly balances the applied voltage. However, if the excitation is reduced below the 100-percent value, the difference between the back emf and the applied voltage produces a reactive component of current which lags the applied voltage. The machine then acts as an inductance.

Similarly, if the excitation is increased above the 100-percent value, the reactive component leads the applied voltage, and the machine acts as a capacitor. This feature of the synchronous motor permits uses of the machine as a power-factor correction device. When so used, it is called as the synchronous condenser.

Note:- In most applications, the voltage applied to the synchronous machine cannot be varied. This is true because in most cases the machine is directly connected to the grid and the terminal voltage is therefore fixed. 

 

Ques.97. A synchronous motor has a better power factor than an induction motor. This is due to

  1. Stator supply is not required to produce the magnetic field
  2. Synchronous Motor has no Slip
  3. Mechanical Load on the rotor remain constant
  4. Synchronous motor has a large airgap

A synchronous motor is a machine that converts electric power into mechanical power at a constant speed called synchronous speed. It is a doubly excited machine because its rotor winding is excited by direct current and its stator winding is connected to an A.C supply.

Stator: Stator is the stationary part of the machine. The three-phase armature winding is placed in the slots of the stator core and is wound for the same number of poles as the rotor.  The stator is excited by a three-phase ac supply as shown in the Figure 

Rotor: The rotor of the synchronous motor can be of the salient pole or cylindrical pole (Non-salient) type construction. Practically most of the synchronous motor use salient i.e., projected pole type construction, except for exceedingly high-speed machines. The field winding is placed on the rotor and it is excited by a separate dc supply.

Although the synchronous motor starts as an induction motor. it does not operate as one. After the armature winding has been used to accelerate the rotor to about 95% of the speed of the rotating magnetic field, direct current is connected to the rotor and the electromagnets lock in step with the rotating field. Notice that the synchronous motor does not depend on the induced voltage from the stator field to produce a magnetic field in the rotor. The magnet field of the rotor is produced by external DC applied to the rotor. This is the reason that the synchronous motor has the ability to operate at the speed of the rotating magnetic field. As the load is added to the motor, the magnetic field of the rotor remains locked with the rotating magnetic field and the rotor contir ues to tuna at the same speed.

It should be noted that the changes in the dc field excitation do not affect the motor speed. However, such changes do alter the power factor of a synchronous motor. If all of the resistance of the rheostat is inserted in the field circuit, the field current drops below its normal value. A poor lagging power factor results. If the dc field is weak, the three-phase ac circuit to the stator supplies a magnetizing current to strengthen the field. This magnetizing component lags the voltage by 90 electrical degrees. The magnetizing current becomes a large part of the total current input. This gives rise to a low lagging power factor.

If a weak dc field is strengthened, the three-phase ac circuit to the stator supplies less magnetizing current. Because this current component becomes a smaller part of the total current input to the stator winding, the power factor increases. The field strength can be increased until the power factor is unity or 100%. When the power factor reaches unity, the three-phase ac circuit supplies energy current only. The dc field circuit supplies all of the current required to magnetize the motor. The amount of dc field excitation required to obtain a unity power factor is called normal field excitation.

The magnetic field of the rotor can be strengthened still more by increasing the dc field current above the normal excitation value. The power factor in this case decreases. The circuit feeding the stator winding delivers a demagnetizing component of current. This current opposes the rotor field and weakens it until it returns to the normal magnetic strength.

Hence Higher PF means the low requirement of MMF for energy transfer, hence low magnetizing current requirement.

The synchronous machine has separate DC excitation which reduces the machine’s excitation dependency on the mains supply, hence better PF. whereas the Induction motor has no such provisions,  hence low Power factor.

 

Ques.98. When the field circuit of an unloaded salient pole synchronous motor gets suddenly open circuited, then

  1. The Motor Stops
  2. It runs at a slower speed
  3. It continues to run at the same speed
  4. It runs at a very high speed

The synchronous motor starts as an induction motor. it does not operate as one. After the armature winding has been used to accelerate the rotor to about 95% of the speed of the rotating magnetic field, direct current is connected to the rotor and the electromagnets lock in step with the rotating field. Notice that the synchronous motor does not depend on the induced voltage from the stator field to produce a magnetic field in the rotor. The magnet field of the rotor is produced by external DC applied to the rotor.

To shut down the motor, the field circuit is de-energized by opening the field discharge switch. The field discharge resistor is connected across the field circuit to reduce the induced voltage in the field as the field flux collapses The energy stored in the magnetic field is spent in the resistor, and a lower voltage is induced in the field circuit thus rotating magnetic field will not develop and the motor will stop.

Note:- The synchronous motor either runs on synchronous speed or it will not run at all.

 

Ques.99. The armature current of a synchronous motor is minimum when operating at

  1. Unity Power factor
  2. 0.707 Power factor lagging
  3. 0.707 power factor leading
  4. Zero power factor leading

The power factor of the synchronous motor can be changed by changing the field current. When the field current is changed, the armature current of the synchronous motor also changes. Now suppose that a synchronous motor is running at no load. If the field current is increased, the armature current la decreased until the armature current becomes the minimum. At this point, the synchronous motor is running at unity power factor. Before this point, the synchronous motor was running at the lagging power factor and the armature current is low lagging. Now if we increase the field current, the armature current increases (high Lagging) and the motor starts operating at a leading power factor.

It must be noted that due to the constant load, the cosine component of the armature current, In cosφ always remains constant.

The V-curves of a synchronous motor show how armature current varies with its field current when the motor input is kept constant and is so-called because of their shape. The minimum armature current corresponds to unity power factor.

 

Ques.100. The resultant armature voltage of a synchronous motor is equal to the_______. (SSC-2018 Set-4)

  1. Vector sum of Eb and V
  2. Vector difference of Eb and V
  3. Arithmetic sum of Eb and V
  4. Arithmetic difference between Eb and V

When a dc motor or an induction motor is loaded, the speed of the motor drops. This is because the load torque demand increases than the torque produced by the motor Hence the motor draws more current to produces more torque to satisfy the load but its speed reduces. In case of synchronous motor speed always remains constant equal to the synchronous speed, irrespective of load condition It is interesting to study how synchronous motor reacts to changes in the load condition.

In a dc motor, armature develops an emf after motoring action starts which opposes the supply voltage called back emf. Eb. The resultant voltage across the armature is (V – Eb) and it causes an armature current la = (V – Eb) ⁄ Ra to flow where Ra is armature circuit resistance.

In case of the synchronous motor also, once the rotor starts rotating at synchronous speed, the stationary stator (armature) conductors cut the flux produced by the rotor. The only difference is the conductor is stationary and flux is rotating. Due to this, there is an induced emf in the stator which according to Lenz’s law opposes the supply voltage. This induced emf is called back emf in case of the synchronous motor. It is denoted as Ebph i.e., back emf per phase. This gets generated as the principle of the alternator and hence alternating in nature.

So back emf in case of the synchronous motor depends on the excitation given to the field winding and not on the speed as speed is always constant. The net voltage in armature (stator) is the vector difference (not arithmetical, as in d.c. motors) of Vph and Ebph. Armature current is obtained by dividing this vector difference of voltages by armature impedance (not resistance as in d.c. machines).

 

Ques.95. In a synchronous motor the rotor copper losses, are met by

  1. Armature input
  2. D.C  source
  3. Motor input
  4. Supply lines

The synchronous motor consist of the two parts:

Stator: Stator is the armature winding. It consists of three phase star or delta connected winding and excited by 3 phase A.C supply.

Rotor: Rotor is a field winding. The field winding is excited by the separate D.C supply through the slip ring.

  • The 3 phase Ac source feeds electrical power to the armature for the following component of the power
    (i)The net mechanical output from the shaft
    (ii) Copper losses in the armature winding
    (iii) Friction and the armature core losses
  • The power received by the DC source is used to utilized only to meet copper losses of the field winding.

 

Ques.96. The change of D.C. excitation of a synchronous motor changes

  1. Motor Speed
  2. Applied voltage of the Motor
  3. Power Factor
  4. All Option are correct

A synchronous motor is a machine which converts a electric power into mechanical power at a constant speed called synchronous speed. It is a doubly excited machine because its rotor winding is excited by direct current and its stator winding is connected to an A.C supply.

Stator: Stator is the stationary part of the machine. The three-phase armature winding is placed in the slots of the stator core and is wound for the same number of poles as the rotor.  The stator is excited by a three phase ac supply as shown in the Figure 

Rotor: The rotor of synchronous motor can be of the salient pole or cylindrical pole (Non-salient) type construction. Practically most of the synchronous motor use salient i.e., projected pole type construction, except for exceedingly high-speed machines. The field winding is placed on the rotor and it is excited by a separate dc supply.

Although the synchronous motor starts as an induction motor. it does not operate as one. After the armature winding has been used to accelerate the rotor to about 95% of the speed of the rotating magnetic field, direct current is connected to the rotor and the electromagnets lock in step with the rotating field. Notice that the synchronous motor does not depend on the induced voltage from the stator field to produce a magnetic field in the rotor. The magnet field of the rotor is produced by external DC applied to the rotor. This is the reason that the synchronous motor has the ability to operate at the speed of the rotating magnetic field. As the load is added to the motor, the magnetic field of the rotor remains locked with the rotating magnetic field and the rotor continues to run at the same speed.

It should be noted that the changes in the dc field excitation do not affect the motor speed. However, such changes do alter the power factor of a synchronous motor. If all of the resistance of the rheostat is inserted in the field circuit, the field current drops below its normal value. A poor lagging power factor results. If the dc field is weak, the three-phase ac circuit to the stator supplies a magnetizing current to strengthen the field or the lagging reactive power will be drawn from A.C source to aid magnetization. This magnetizing component lags the voltage by 90 electrical degrees. The magnetizing current becomes a large part of the total current input. This gives rise to a low lagging power factor.

If a weak dc field is strengthened, the three-phase ac circuit to the stator supplies less magnetizing current and the leading current is drawn from the ac source to compensate (oppose) the magnetization. Because this current component becomes a smaller part of the total current input to the stator winding, the power factor increases. The field strength can be increased until the power factor is unity or 100%. When the power factor reaches unity, the three-phase ac circuit supplies energy current only. The dc field circuit supplies all of the current required to magnetize the motor. The amount of dc field excitation required to obtain a unity power factor is called normal field excitation.

The magnetic field of the rotor can be strengthened still more by increasing the dc field current above the normal excitation value. The power factor in this case decreases. The circuit feeding the stator winding delivers a demagnetizing component of current. This current opposes the rotor field and weakens it until it returns to the normal magnetic strength.

Hence Higher PF means the low requirement of MMF for energy transfer, hence low magnetizing current requirement.

The synchronous machine has separate DC excitation which reduces machine’s excitation dependency on main supply, hence better PF. whereas Induction motor has no such provisions,  hence low Power factor.

 

Ques.97. The advantage of a stationary armature of a synchronous machine is

  1. Reducing the number of slip rings on the rotor
  2. The difficulty of providing high voltage insulation on the rotor
  3. The armature is associated with large power as compared to the field circuits
  4. All option are correct

Explanation:-

 FIELD AND ARMATURE CONFIGURATIONS

There are two arrangements of fields and armatures:

  1. Revolving armature and the stationary field
  2. Revolving field and stationary armature.

 ADVANTAGES OF ROTATING FIELD IN AN ALTERNATOR

In large alternators, rotating field arrangement is usually forward due to the following advantages.

  1. Ease of Construction: Armature winding of large alternators being complex, the connection and bracing of the armature windings can be easily made for the stationary stator.
  2. The number of Slip Rings: If the armature is made rotating, the number of slip rings required for power transfer from the armature to the external circuit is atleast three. Also, heavy current flows through brush and slip ring cause problems and require more maintenance in large alternators. Insulation required for slip rings from rotating shaft is difficult with the rotating armature system.
  3. High voltage generation:- Voltages can be generated as high as 11,000 and 13,800 V. These values can be reached because the stationary armature windings do not undergo vibration and centrifugal stresses.
  4. High current Rating:-  Alternators can have relatively high current ratings. Such ratings are possible because the output of the alternator is taken directly from the stator windings through heavy, well-insulated cables to the external circuit. Neither slip rings nor a commutator is used.
  5. Better Insulation to Armature: Insulation arrangement of armature windings can easily be made from the core on the stator.
  6. Reduced Rotor Weight and Rotor Inertia:  Since the field system is placed on the rotor, hence the insulation requirement is less (for low dc voltage). Also, rotational inertia is less. It takes lesser time to gain full speed.
  7. Improved Ventilation Arrangement: The cooling can be provided by enlarging the stator core with radial ducts. Water cooling is much easier if the armature is housed in the stator.

Hence in almost all of the alternators, the armature is housed in the stator while the dc field system is placed in the rotor.

 

Ques.98. In which of the following motors the stator and rotor fields rotate simultaneously

  1. Reluctance motor
  2. Universal Motor
  3. D.C Motor
  4. Synchronous Motor

The synchronous motor is a truly constant speed motor. This is the specialty of this motor. Yet, it has very limited applications. To develop a steady torque, its rotor must be rotating at synchronous speed, Ns. This is the major defect of synchronous motors. Either it runs at synchronous speed, or it does not run at all.

The stator field rotates at synchronous speed due to the three-phase currents supplied to its windings.  In order to develop a continuous unidirectional torque, it is necessary that the stator and rotor poles do not move with respect to each other. This is possible only if the rotor also rotates at the synchronous speed. Therefore Magnetic Locking between the poles is necessary to do so.

The concept of Magnetic Locking

  • Synchronous motor work on the principle of magnetic locking.
  • When two unlike strong unlike magnets poles are brought together, there exists a tremendous force of extraction between those two poles. In such condition, the two magnets are said to be magnetically locked.
  • If now one of the two magnets is rotated, the other magnets also rotate in the same direction with the same speed due to the strong force of attraction.
  • This phenomenon is called as magnetic locking 

For magnetic locking condition, there must be two unlike poles and magnetic axes of this two poles must be brought very nearer to each other.

  • Consider a synchronous motor whose stator is wound for 2 poles.
  • The stator winding is excited with 3 phase A.C supply and rotor winding with D.C supply respectively. Thus two magnetic fields are produced in the synchronous motor.
  • When the 3 phase winding is supplied by 3 phase A.C supply than the rotating magnetic field or flux is produced.
  • This magnetic field or flux rotates in a space at a speed called synchronous speed.
  • When the rotor speed is about synchronous, stator magnetic field pulls the rotor into synchronism i.e. minimum reluctance position and keeps it magnetically locked. Then the rotor continues to rotate with a speed equal to synchronous speed.
  • The rotating magnetic field or rotating flux has fixed relationship between, the number of poles, the frequency of a.c supply and the speed of rotation.
  • The rotating magnetic field creates an effect which is similar to the physical rotation of magnets in space with a synchronous speed.
  • So for rotating magnetic field
    speed of Synchronous motor
    Where f = supply frequency
    P = Number of poles

  • Suppose the stator poles are N1 and S1 which are rotating at a speed of N and the direction of rotation be clockwise.
  • When the field winding on a rotor is excited by the D.C source, it produces the two stationary poles i.e N2 and S2.
  • To establish the magnetic locking between the stator and rotor poles the, unlike poles N1 and S2 or N2 and S1 should be brought near to each other.
  • As stator poles are rotating and due to magnetic locking the rotor poles will rotate in the same direction of rotating magnetic field as that of stator poles with the same speed Ns.
  • Hence synchronous motor rotates at only one speed that is synchronous speed.
  • The synchronous speed depends on the frequency therefore for constant supply frequency synchronous motor speed will be constant irrespective of the load changed.

At zero speed or at any other speed lower than synchronous speed, the rotor poles rotate slower than the stator field. Therefore, in one cycle of rotation of the stator field, the N-pole of the rotor is for some time nearer to N pole of the stator and for some other time nearer to the S-pole of the stator. As a result, the torque developed is for some time clockwise and for some other time anticlockwise. Consequently, the average torque developed remains zero. Hence the Synchronous Motor Run either on Synchronous Speed Or Not at all.

 

Ques.99. In a 3-phase synchronous motor, If the direction of its field current is reversed

  1. The winding of the motor will burn
  2. The motor will stop
  3. The motor will run in the reverse direction
  4. The motor continues to run in the same direction

The synchronous motor is a doubly excited machine with stator and Rotor. The stator is usually excited by the three-phase supply and the rotor is excited by a DC supply in order to create a magnetic field.

The three-phase supply usually fed to the stator, three-phase current are generated in the stator winding since it is a closed circuit. These current phases are separated by 120 degrees with each other. As a result, three phase flux is generated which is equal in magnitude but are displaced by 120 degrees with each other.

However, the rotor is excited by DC supply which usually creates poles of opposite polarity due to the positive and negative supply of dc. So the rotor is rotated by external means by using some pony Motors or induction motors so that it is interlocked with the stator field this rotates with the same speed as a stator named the synchronous speed.

Interchanging the polarity during the running of the motor

It should be noted that it is the very difficult procedure to change the polarity of the rotor suddenly. However, if we do so, the rotor will lose its magnetic interlocking with a stator for a second. Due to inertia the rotor , it will be rotating with some speed. Now the stator speed is faster than the rotor, as a result, the rotor field is again interlocked with stator at a particular point of time during the run. Think of a magnet. It always looks for opposite polarity. So it will get interlocked at a particular point. Hence it rotates in the same direction as the stator. It neither changes its direction of rotation nor it stops

All this happens in within seconds that it is impossible to trace because the normally operating speed is around 1500 rpm . At that speed, it is impossible to trace this effect.

Hence the direction of rotation of the synchronous motor is determined by its starting direction, as initiated by induction motor action. Thus, to reverse the direction of a 3 phase synchronous motor, it is necessary to first stop the motor and then reverse the phase sequence of the 3 phase connections at the stator.

REVERSING THE CURRENT TO FIELD WINDINGS WILL NOT AFFECT THE DIRECTION OF ROTATION

 

Ques.100. The maximum power developed in a synchronous motor will depend on (SSC-2018 Set-5)

  1. The rotor excitation and supply voltage
  2. The rotor excitation, supply voltage and maximum value of coupling angle
  3. The supply voltage only
  4. The rotor excitation only

The maximum power developed in the synchronous Motor is given by the expression

{P_{\max }} = \dfrac{{{E_b}V}}{{{Z_s}}}\cos (\theta - \delta ) - \dfrac{{E_b^2}}{{{Z_s}}}Cos\theta

Where

δ = load angle

θ = Internal angle

V = Terminal voltage

Eb = Back EMF or excitation because back EMF Eb in Synchronous Motor depends on the DC excitation only because speed is constant

The power developed depends on the excitation, voltage and coupling angle. The maximum value of θ and hence δ is 90°. An increase in the excitation results in an increase of Pmax. Consequently, the load angle decreases for a given power developed. The overload capacity of the motor increases with an increase in excitation and the machine becomes more stable.

For all values of V and Eb, this limiting value of δ is the same but maximum torque will be proportional to the maximum power developed

If the resistance of the armature is negligible, then 

Z ≅ Xs,  θ = 90° ∴ Cosθ = 0

Therefore the power develop is given by

{P_{\max }} = \dfrac{{{E_b}V}}{{{X_s}}}\sin \delta

Hence the power develop will me maximum when δ is 90°.

 

Ques.95. The power factor of a synchronous motor, When the field is under-excited

  1. Leading
  2. Unity
  3. Lagging
  4. Zero

Unlike induction machine, synchronous machine can operate at lagging, leading and unity power factors. In induction machine, magnetizing current is required to establish flux in the air gap. This magnetizing current lags the voltage and therefore, induction machine always operates at lagging power factor.

On the other hand, in synchronous machine, the total air gap flux is produced by dc source and there is no use of lagging current from ac system for production of air gap flux. If dc excitation is decreased, lagging reactive power will be drawn from ac source to aid magnetization and thus machine will operate at lagging power factor. If dc excitation is more, leading current drawn from ac source to compensate (oppose) the magnetization and the machine will operate a leading power factor.

Thus it can be concluded that an over-excited motor(Eb  > V) draws leading current (acts like a capacitive load) but an under-excited motor(Eb < V) draws lagging current (acts like an inductive load).

 

Ques.96. To limit the operating temperature of the synchronous motor, it should have proper

  1. Current Rating✓
  2. Voltage Rating
  3. Power Factor
  4. Speed

When a machine has to be designed and constructed, the choice of suitable materials and manufacturing technology becomes important. The following considerations are required which impose the limitations on the machine design:

Rated current:- Rated current in is the maximum permissible current that can be maintained permanently, which at the same time does not cause any overheating, damages, faults, or accelerated aging. In AC machines, the rated current implies the RMS value of the winding current.

Electrical currents in windings of electrical machine produce losses and develop heat, increasing the temperature of conductors and insulation. Current in the windings creates Joule losses which are proportional to the square of the current. The temperature of the machine is increased in proportion to generated heat. If the current is increased beyond the rated current then the temperature of the synchronous motor will also increase. 

This excessive temperature rise may cause insulation failure. The life of the machine depends upon the life of the insulation. If the machine is continuously operated above the specified temperature limit, the life of the insulation and hence the life of the machine will be reduced. By providing proper ventilation and

Saturation of the Magnetic Circuit: The saturation of the magnetic circuit disturbs the straight line characteristics of magnetization (B-H) curve resulting in increased excitation required and hence higher cost for the field system.

Insulation: The insulating properties and the strength of the insulating materials are considered on account of breakdown due to excessive voltage gradients set up in the machine.

Mechanical Strength: The machine should have the ability to withstand centrifugal forces and other stresses.

Efficiency: The efficiency of the machine should be high for the low running cost. The specific magnetic and electric loading should be low to achieve high efficiency. With the low value of magnetic and electric loadings, the size of the machine will be larger and hence more capital cost (initial investment).

 

Ques.97. A synchronous machine with large air gap has

  1. A higher value of stability limit
  2. A higher synchronizing power
  3. A small value of regulation
  4. All options are correct

In Synchronous Machine the Magnetic Flux is set up separately by Field Winding.The Emf induced in the Stator Armature Winding is not by Mutual induction it is a Dynamically induced Emf due to relative motion between the Field and Conductors.

The length of the air gap greatly influences the performance of a synchronous machine. A large air gap offers a large reluctance to the path of flux produced by the armature MMF and thus reduces the effect of armature reaction. This results in a small value of synchronous reactance and the high value of SCR.

A high value of SCR (short circuit ratio) means that the synchronous reactance has a low value, synchronous machines with the low value of SCR thus have greater changes in voltage under fluctuations of load i.e., the inherent voltage regulation of the machine is poor.

Thus a machine with a large air gap has a high synchronizing power which makes the machine less sensitive to load variations.

 

Ques.98. Synchronous motor speed

  1. Decreases as the load decreases
  2. Increases as the load increases
  3. Always remains constant
  4. None of these

The principle of working of the synchronous motor is magnetic locking. Both stator and rotor and separately excited. The rotor catches the flux speed and gets locked with revolving filed of the stator and then rotates at that speed. In case of synchronous motor speed always remains constant equal to the synchronous speed, irrespective of load condition.

 

Ques.99. The magnitude of field flux in a 3-phase synchronous machine

  1. Varies with speed
  2. Remains constant at all loads
  3. Varies with power factor
  4. Varies with the load

  • In a synchronous motor, the counter Emf is proportional to the speed and field flux.
  • Since the speed is constant in a synchronous motor, therefore, field flux is substantially constant within the normal limit of operation.
  • If field excitation is increased thereby tending to increase the field flux that varies only slightly, there must be an automatic change in the armature MMF  in order to offset the effect of the increased field excitation. The armature current must, therefore, contain a leading component, hence the leading current in a synchronous motor exerts a demagnetizing effect.
  • By the same reasoning, it follows that a weakening of the field excitation tends to draw a lagging current from the source of supply. For any set of operating conditions, there will be some value of field excitation which will cause the power factor to be unity, i.e., the current to be in phase with the terminal voltage.
  • When this condition exists while the motor is carrying its rated load, the motor is said to have normal excitation.

 

Ques.100. In a synchronous motor, the magnitude of back e.m.f depends on (SSC-2018 Set-6)

  1. The speed of the motor
  2. DC excitation Only
  3. Load on the motor
  4. Both the speed and rotor flux

In case of the synchronous motor also, once the rotor starts rotating at synchronous speed, the stationary stator (armature) conductors cut the flux produced by the rotor. The only difference is conductors are stationary and flux is rotating. Due to this, there is an induced e.m.f. in the stator which according to Lenz’s law opposes the supply voltage. This induced e.m.f. is called back e.m f. It is denoted as Ebph i.e. back e.m.f. per phase. The back E.M.F is alternating in nature and its magnitude can be calculated by the equation.

Ebph = 4.44.φ.f.Tph

Where 

ϕ is Flux per pole

Tph is the number of turns connected in series per phase

f be the frequency

As speed is always synchronous, the frequency is constant and hence magnitude of such back e.m.f. can be controlled by changing the flux φ produced by the rotor.

Ebph ∝ Φ

So back e.m.f. in case of the synchronous motor depends on the excitation given to the field winding and not on the speed, as speed is always constant.

 

 

 

 

Ques 11. Speed of rotor magnetic flow in the rotor body is

  1. Synchronous
  2. Asynchronous
  3. Zero
  4. None of these

Answer.2. Asynchronous

Explanation:

An induction motor cannot run at Synchronous speed because if the rotor was to accelerate to the speed of the rotating magnetic field, there would be no cutting action of the squirrel-cage bars and. therefore, no current flow in the rotor. if there was no current flow in the rotor, there could be no rotor magnetic field and. therefore, no torque.

 

Ques 73. Which one of the following can be obtained by the equivalent circuit of an electrical machine? (SSC-2017)

  1. Temperature rise in the cores
  2. Complete performance characteristics of the machine
  3. Type of protection used in the machine
  4. Design Parameters of the windings

Answer.2. Complete performance characteristics of the machine

Explanation:

An equivalent circuit of an electric machine helps us to determine the complete performance of the machine e.g efficiency, losses etc.

 

Ques.26. Which of the following motors is represented by the characteristics curve shown below?  (SSC-2016)

  1. D.C shunt Motor
  2. D.C Compound Motor
  3. D.C series Motor
  4. Asynchronous Motor

The induction motor is also known as an asynchronous motor. To start the induction motor, let us assume that the induction motor has been started without any load on it. The motor will come to its no-load speed, which may be at a slip as low as 0.1 percent.

At full load, the motor runs at a speed of N. When mechanical load increases, motor speed decreases till the motor torque again becomes equal to the load torque. As long as the two torques are in balance, the motor will run at constant (but lower) speed. 

The motor may be loaded continuously till pull out (or break down) torque is developed, at which point the motor will Stop if more load is placed on it. 

 

Ques 6. A 10 pole 25 Hz alternator is directly coupled to and is driven by 60 Hz synchronous motor then the number of poles in a synchronous motor is?

  1. 24 poles 
  2. 48 poles
  3. 12 Poles
  4. None of the above

Number of poles of alternator Pa = 10

F = 25 Hz (alternator)

F = 60 Hz (motor)

Then the number of poles of motor Pm =?

Since synchronous motor is directly coupled hence

Synchronous speed of an alternator = Synchronous speed of the motor

(120 x 25)/10 = (120 x 60)/ Pm

Pm = 24

 

Ques 24. A silent pole synchronous motor is operating at one-fourth full load if the field current is suddenly switched off, it would?

  1. Run at super-synchronous Speed
  2. Stop Running
  3. Run at sub-synchronous Speed
  4. Continue to run at synchronous speed
Silent pole synchronous motor runs either at synchronous speed or not at all. That is while running,  it maintains a constant speed. The speed is independent of load.

 

Ques 46. An alternator is supplying a load of 300 kW at a power factor of 0.6 lagging. If the power factor is raised to unity, How many more kW can alternator supply?

  1. 300 kW
  2. 100 kW
  3. 150 kW
  4. 200 kW

PF = active power/apparent power. So from question

0.6=300/apparent power.

Apparent power=500 Kva.

At unity power factor, active power=apparent power. So active power=500kw. So you will get an additional 200kw at unity power factor

 

Ques 60. The reactive power generated by a synchronous alternator can be controlled by?

  1. Changing the alternator speed
  2. Changing the field Excitation
  3. Changing the terminal Voltage
  4. Changing the prime mover input
If alternator is Overexcited, it will deliver reactive power with lagging current while in Underexcited, it absorb reactive power with leading current

 

Ques 81.  Which of the following motor is not self-starting?

  1. DC series motor
  2. Slip ring Induction motor
  3. Synchronous motor
  4. Squirrel cage induction motor

  • At first synchronous motor has Stator supplied with 3 phase supply and rotor with dc supply.
  • When dc supply is given to rotor, alternate poles form on the rotor.
  • Because of the three-phase supply, the rotating Magnetic field will generate rotating torque at synchronous speed.
  • Consider during the positive half cycle (positive torque),  N pole of stator and S pole of the rotor is in front of each other then they will experience the force of attraction, and the tendency of the rotor will be to rotate in an anti-clockwise direction.
  • Now consider, during the negative half cycle (Negative torque),  Pole of the stator will change to S. Then S pole of stator and rotor will experience the force of repulsion and the tendency of the rotor will be to rotate in the clockwise direction.
  • So combine effect of the whole cycle will result in zero torque. Thus due to a cancellation of torque during positive and negative half cycle synchronous motor can’t be self-starting.
  • For the synchronous motor to be self-starting we prefer damper winding. or mechanical input for starting.

 

Ques 98. Synchronous impedance method of finding voltage regulation of an alternator is called pessimistic method because (SSC-2015)

  1. It is simplest to perform and compute
  2. Armature reaction is wholly magnetizing
  3. It gives regulation value lower than its actual found by direct loading
  4. It gives the regulation value higher than its actual found by direct loading
The regulation calculated from the synchronous impedance method is higher than the actual value found by direct loading hence this method is called as the pessimistic method.

 

Ques 2. Hydrogen is used in large alternator mainly to

  1. Reduce eddy current losses
  2. Reduce distortion of waveform
  3. Cool the machine
  4. Strengthen the magnetic field

Why is hydrogen used for Alternator cooling?

Hydrogen is least expensive, less weight, high thermal conductivity, less density and less viscosity. Less weight, less density & less viscosity attributes to its flow rate. High thermal conductivity helps in better heat exchange. Least expensive helps in balance sheets, more power in less investments.

In order to reduce high temperature of alternator hydrogen gas is used as a coolant. The coolant, Hydrogen gas is allowed to flow in a closed cyclic path around the rotor. Heat exchange takes place and the temperature of hydrogen gas increases, for better cooling of the rotor in next cycle it has to be cooled. Cooling of hydrogen gas is done by passing it through heat exchangers generally constituted with water. Now Hydrogen gas after cooling is allowed to pass through driers ( mainly silica gel which absorbs moisture) and allowed to pass again through the rotor.

 

Ques 11. If a synchronous motor working at leading power factor can be used as

  1. Mechanical Synchronizer
  2. Voltage Booster
  3. Phase Advancer
  4. Noise Generator

  • When the synchronous motor operates at the leading power factor then Rotor is overexcited in such a way that back emf (Eb which is generated in stator due to dc excitation of the rotor ) is greater than the supply voltage (V).
  • At this time resultant flux is greater than that is required for the unity power factor,  then this extra flux will generate reactive power so the motor will generate additional reactive power.  And it will also use active power for mechanical work.
  • Therefore synchronous motor working on leading PF will work as a synchronous condenser or phase advance
  • Ques 30. For V-curve for synchronous motor, the graph is drawn between

    1. Armature current and power factor
    2. Field current and armature current
    3. Terminal voltage and load factor
    4. Power factor and field current

    The Graph plotted between the armature current Ia and field current If at no load the curve is obtained known as V Curve. Since the shape of these curves is similar to the letter “V”, thus they are called V curve of the synchronous motor.

     

    Ques 40. Which of the following condition is NOT mandatory for alternators working in parallel?

    1. The alternators must have the same phase sequence
    2. The terminal voltage of each machine must be the same
    3. The machines must have equal kVA ratings
    4. The alternators must operate at the same frequency

    • There are five conditions that must be met before when two alternators running in parallel.
      1. Equal line voltage
      2. Same frequency
      3. Same phase sequence
      4. Same phase angle
      5. Same waveform
    • We can use 2 alternators of 6 MVA and 4 MVA each instead of using single 10 MVA alternator because it is economical than using a single alternator of the same rating.

     

    Ques 44. Regulation of an alternator supplying resistive or inductive load is

    1. Infinity
    2. Always Negative
    3. Always Positive
    4. Zero

    The voltage regulation of an alternator is defined as the change in its terminal voltage when the full load is removed, keeping field excitation and speed constant, to the rated terminal voltage.

    Where Vph = Rated terminal voltage

    Eph =No load-induced e.m.f

    An increase in the load current in a pure resistive load cause a decrease in the output voltage. For an inductive load an increase in the load current cause a greater voltage drop as compared to the resistive load. Therefore for inductive and resistive load conditions there is always drop in the terminal voltage hence regulation values are always positive.

    In case of leading load that means capacitive load, the effect of armature flux on main field flux is magnetizing i.e, the armature flux is adding up with the main field flux. Since it is adding up, the total induced emf(Vph) will also be more than the induced emf at no load(Eph).Hence the regulation is negative.

     

    Ques 64. The positive, negative and zero sequence impedances of 3-phase synchronous generator are j 0.5 pu, j 0.3 pu and j 0.2 pu respectively. When the symmetrical fault occurs on the machine terminals. Find the fault current. The generator neutral is grounded through reactance of j0.1 pu

    1. -j 3.33 pu
    2. -j 1.67 pu
    3. -j2.0 pu
    4. -j 2.5 pu

    For symmetrical fault

    If = E/(Zi +Zn)

    Where E = Pre fault voltage Which is equal to 1

    Zi = 0.5j & Zn = 0.1 j

    If = 1/(0.5j + 0.1j)

    If = -j 1.67

     

    Ques 67. If the synchronous motor can be used as a synchronous condenser when it is

    1. Overexcited
    2. Under excited
    3. Overloaded
    4. Under Loaded

    • When the synchronous motor operates at the leading power factor then Rotor is overexcited in such a way that back emf (Eb which is generated in stator due to dc excitation of the rotor ) is greater than the supply voltage (V).
    • In this case, a Synchronous motor will be operating at a leading power factor. At this time resultant flux is greater than that is required for the unity power factor,  then this extra flux will generate reactive power so the motor will generate reactive power.  And it will use active power too for mechanical work.
    • Therefore synchronous motor working on leading PF will work as a synchronous condenser.

     

    Ques 68. Which of the following methods would give a higher than the actual value of regulation of an alternator?

    1.  ZPF method
    2.  MMF method
    3.  EMF method
    4. ASA method

    Compared to other methods, the value of voltage regulation obtained by the synchronous impedance method (EMF Method) is always higher than the actual value and therefore this method is called the pessimistic method.

     

    Ques 69. If the excitation an alternator operating in parallel with other alternator is increased above the normal value of excitation, its.

    1. Power factor becomes more lagging
    2. Power factor becomes more leading
    3. Output current decreases
    4. Output kW decreases

    If the excitation an alternator operating in parallel with other alternator is changed then it will change the power factor

    • Suppose the excitation of the alternator is decreased below normal excitation then reactive power will change and active power output (W or KW) of the alternator will remain unchanged.
    • The under-excited alternator delivers leading current to the infinite bus bar.
    • It is because the leading current produces an adding m.m.f to increase the under excitation.
    • Similarly, an overexcited alternator operates at lagging power factor and supplies lagging reactive power to an infinite bus bar.

     

    Ques 70. In an alternator, the effect of armature reaction is minimum at the power factor of

    1. 0.5 Lagging
    2. 0.866 Lagging
    3. 0.866 Leading
    4. Unity

    At unity p.f., the effect of armature reaction is merely to distort the main field; there is no weakening of the main field and the average flux practically remains the same.

    At zero p.f. lagging, armature reaction is directly demagnetizing and the armature reaction weakens the main flux. This causes a reduction in the generated e.m.f.

    At zero p.f. leading, armature flux is now in the same direction as the field flux and, therefore the armature reaction strengthens the main flux. This causes an increase in the generated voltage.

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