# SSC JE synchronous motor & synchronous generator question

Ques.11. The under-excited synchronous motor takes____ (SSC-2018 Set-2)

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

Explanation:-

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 the 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 a leading current (acts like a capacitive load) but an under-excited motor(Eb < V) draws a lagging current (acts as an inductive load).

Ques.12. Power factor correction substations consist of (SSC-2018 Set-2)

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

Explanation:-

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.

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.
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.13. Synchronizing power of a synchronous machine is (SSC-2018 Set-2)

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

Answer.3. Inversely proportional to the synchronous reactance

Explanation:-

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.

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

[latex display=”true”]{P_{syn}} = \frac{{{E_b}V}}{{{X_s}}}\cos \delta[/latex]

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.14. The normal starting methods that are used to start a synchronous motor is (SSC-2018 Set-2)

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

Answer.4. Damper winding in conjunction with the star-delta starter

Explanation:-

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.15. At what condition synchronous motor can be used as a synchronous capacitor (SSC-2018 Set-2)

2. Under-excited
4. Over-excited

Explanation:-

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.

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.
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.16. Synchronous motor do not have self-starting property because (SSC-2018 Set-2)

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

Answer.3. The direction of instantaneous torque reverses after the half cycle

Explanation:-

The synchronous motor works on the principle of magnetic locking. The operating principle can be explained with the help of a 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 the 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 with each other and rotor experiences the attractive force fa and tends to rotate in a 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.17. The over-excited synchronous motor takes (SSC-2018 Set-2)

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

Explanation:-

Unlike the induction machine, the synchronous machine can operate at lagging, leading, and unity power factors. In the induction machine, a magnetizing current is required to establish the flux in the air gap. This magnetizing current lags the voltage and therefore, the 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 the 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 a leading power factor at no-load, it is called synchronous condenser which can work as a variable inductor or capacitor.

Ques.18. 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

Answer.2. The rotor excitation, supply voltage, and maximum value of coupling angle

Explanation:-

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

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.19. Name the equipment which runs an alternator (SSC-2018 Set-3)

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

Explanation:-

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.20. Synchronous Motor Shaft is Made of (SSC-2018 Set-3)

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