# Objective Type question of Synchronous Motor With Explanation

Ques 21. What is the ratio of no-load speed to full load speed of a 200 kVA, 12 poles, 2200 V, 3 phase, 60 Hz synchronous motor?

1. Infinite
2. 1
3. 1.1
4. 1.21

Explanation:

• The synchronous motor has the special property of maintaining a constant running speed under all conditions of load up to full load.
• This constant running speed can be maintained even under variable line-voltage conditions.
• It should be noted that, if a synchronous motor is severely overloaded, its operation (speed) will suddenly lose its synchronous properties and the motor will come to a halt.
• The synchronous motor gets its name from the term synchronous speed, which is the natural speed of the rotating magnetic field of the stator.
• This natural speed of rotation is controlled strictly by the number of pole pairs and the frequency of the applied power.
• Like the induction motor, the synchronous motor makes use of the rotating magnetic field.
• In a synchronous machine, the rotor is magnetized and it runs at the same speed as the rotating magnetic field.
• The principle of operation of the synchronous motor is as follows: a multiphase source of AC is applied to the stator windings and a rotating magnetic field is produced.
• A DC current is applied to the rotor windings and a fixed magnetic field is produced.
• The motor is constructed such that these two magnetic fields react upon each other causing the rotor to rotate at the same speed as the rotating magnetic field.
• If a load is applied to the rotor shaft, the rotor will momentarily fall behind the rotating field but will continue to rotate at the same synchronous speed.
• Once the rotor’s north and south poles line up with the stator’s south and north poles the stator current is reversed, thus changing the south- and north-pole orientation in the stator, and the rotor is pushed again.
• This process repeats until the current in the stator stops alternating or stops flowing. In a three-phase motor, the stator magnetic flux rotates around the motor and the rotator actually follows this rotating magnetic field.
• This type of motor is called a synchronous motor because it always runs at synchronous speed (rotor and magnetic field of the stator are rotating at exactly the same speed).
• Maximum torque is achieved when the stator flux vector and the rotor flux vector are 90° apart.
• Synchronous motors operate at synchronism with the line frequency and maintain a constant speed regardless of load without sophisticated electronic control.
• The synchronous motor typically provides up to a maximum of 140% of rated torque.

Here

Ns = 120 × f/p = 120 × 60/12 = 600 rpm

Speed of the synchronous motor remains constant irrespective of the load of the motor ie speed at no load and full load remains 600 rpm hence ratio of speed from no load to full load will be 1.

Ques 22.  In a synchronous motor if the back emf generated in the armature at no load is approximately equal to the applied voltage, then the

1. The torque generated is maximum
3. Excitation is 100%
4. No Excitation at all

Explanation:

### Effect of Change of Excitation on No-Load in Synchronous Motor

To find how the excitation affects the working of the synchronous motor, let us assume that it is an ideal machine having no armature winding resistance and no rotational losses. Let us also assume that the motor is connected to an infinite bus so that its terminal voltage and the supply frequency can be taken as constant.

The ideal motor on no-load requires no armature current. This is possible only when the applied voltage is equal and opposite to the excitation emf. As the infinite bus frequency remains unchanged, the ideal motor runs at a constant speed and so the excitation emf can be changed only by changing the excitation (or field) current.

When the excitation current is adjusted so as to make the excitation emf, Eb, equal to the applied voltage, V. the excitation is said to be normal or 100% excitation. Since no power is developed by the motor, the power angle, &, is zero.

If the excitation current is increased above the normal value, the excitation emf, Eb, increases and becomes greater than the applied voltage, and the motor is said to be over-excited. In over-excited condition Eb > V

If the excitation current is decreased below the normal value, the excitation emf, Eb, decreased and becomes less than the applied voltage, and the motor is said to be under-excited. In under excited condition Eb < V

Ques 23. In a synchronous motor, the armature current has large values for

1. High excitation
2. Low excitation
3. Both high and low excitation
4. None of the above

Answer 3, Both high and low excitation

Explanation:

The synchronous motor always adjusts its cosφ i.e., power factor nature so that Power component Ia cosφ remain constant when excitation of the motor is changed keeping the load constant. This is the reason why a synchronous motor reacts by changing its power factor to variable excitation conditions.

Under excitation condition: When the excitation is adjusted in such a way that the magnitude of induced emf is less than the applied voltage (Eb < V) the excitation is called under excitation.

Due to this, ER increases in magnitude. This means for constant Synchronous Impedance (Zs), the current drawn by the motor increases. But ER, the phase shift in such a way that, phasor Ia, also shifts (as ER Ia = θ) to keep the Power component of Ia i.e Ia cosφ components constant. So in under excited conditions, the current drawn by the motor increases. The power factor cosφ decreases and becomes more and more lagging in nature.

Over excitation condition: The excitation to the field winding for which the induced emf becomes greater than the applied voltage (Eb > V) is called overexcitation.

Due to the increased magnitude of Eb, ER also increases in magnitude. But the phase of ER also changes. Now (as ER Ia = θ) is constant, hence Ia also changes its phase, So φ changes. The Ia increases to keep Ia cosφ constant. The phase of ER changes so that Ia becomes leading with respect to Vph in over-excited conditions. So power factor of the motor becomes leading in nature. So overexcited synchronous motor works on leading power factor. So power factor decreases as over excitation increases but it becomes more and more leading in nature.

Two important points stand out clearly from the above discussion :

(i) The magnitude of armature current varies with excitation. The current has a large value both for low and high values of excitation (though it is lagging for low excitation and leading for higher excitation). In between, it has a minimum value corresponding to a certain excitation. T

(ii) For the same input, armature current varies over a wide range and so causes the power factor also to vary accordingly. When over-excited, motor runs with leading p.f. and with lagging p.f. when under-excited. In between, the p.f. is unity.

1. When the motor is under excited, the armature current and power factor is lagging. In this case, the motor behaves like an inductive load.
2. When the motor is normally excited, the power factor is unity. In this case, the armature current is minimum and is in phase with the terminal voltage.
3. When the motor is over-excited, the power factor is leading. In this case, the motor behaves like a capacitive load.

Ques 24. The construction of a synchronous motor resembles which of the following machine

1. Differential compound motor
2. Alternator
3. Dc series motor
4. Induction motor

Explanation:

### Synchronous Motor Construction

• The basic construction of a synchronous motor is identical to that of a generator.
• In small sizes, the rotor carries the three-phase windings, and in large sizes, they are provided in the stator and the rotor carries the field winding.
• The stator is made up of laminations with suitable slots to accommodate the winding. The stator winding is also of the same type as in the alternator.
• The rotor is usually of salient pole type except in cases of special application like the synchronous capacitor which is run at high speeds which requires round rotor construction.
• The synchronous motor is likely to hunt and so, damper winding is necessary on the rotor poles.
• Sometimes the damper windings are designed not only for damping out the oscillations in their speed but also for starting.
• When there is an active power reversal at terminals, then the same machine uses it to generate mechanical torque, as now field MMF falls behind armature MMF and in trying to align itself along resultant MMF axis, it generates electromagnetic torque to drive the mechanical load
• Therefore a given synchronous machine can be used as an alternator when driven mechanically.

Ques 25.  The synchronous motors are not self-starting because

1. The direction of torque on the rotor reverses after every half cycle.
2. Slip is not present in synchronous machine
3. Starting winding is not present in the synchronous machine
4. DC excitation is used

Answer 1. The direction of torque on the rotor reverses after every 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.

Let us consider a two-pole synchronous motor as shown in Figure. The three-phase supply is provided to the stator which induces two poles i.e North pole and the South pole on Stator. Since the supply in the stator is alternating in nature, therefore, its polarity changes in every half cycle, thus the poles of the stator also changes after every half cycle.

The synchronous motor rotor is energized by the DC current. The field current (D.C Current) of the motor produces a steady-state magnetic field. Since the polarity of the D.C current is fixed therefore the poles of the rotor don’t vary.

Therefore, there are two magnetic fields present in the machine. Stator poles changes in every half-cycle whereas rotor poles remain the same.

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 i.e North poles (Ns) and South Poles (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. Let us assume that the rotor tends to rotate in the anti-clockwise direction as shown in Fig. (i).

Step-3.

• After half cycle, the polarity of the stator pole is reversed, whereas the rotor poles cannot change their polarity as shown in Fig. (ii).
• Now unlike poles of rotor and Stator coincide with each other and rotor experiences the attractive force fa and the rotor tends to rotate in the 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.
• Hence, to and fro motion is excited on the rotor and as a result, the rotor does not rotate. As a result, the average torque on the rotor is zero. Hence the 3-phase synchronous motor is not a self-starting motor.

Ques 26. Due to which one of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current?

1. Low short circuit ratio
2. High field current
3. High core losses
4. Low field current

Explanation:

• In a synchronous motor, we provide DC excitation to produce require a magnetic field.
• This magnetic field generates the back emf in the synchronous motor.
• The low field current is the main reason when the synchronous motor fails to pull into synchronous after applying the D.C. field current.
• Lowering the field strength lowers the internally generally generated voltage.
• So if the value of excitation is low we can’t generate the required magnetic field and hence back emf will is low.
• When the machine fails to pull into synchronism it operates as an induction motor at a speed slightly less than synchronous speed.
• Therefore the synchronous motor can’t attend the synchronous speed.

### Pull-in torque

• Pull-in (synchronizing) torque is required to accelerate the motor and driven load from the maximum induction motor speed to synchronous speed.
• It is the critical stage in the starting of a synchronous motor rotor. Whether or not the rotor pulls into step depends on the position of the rotor poles relative to the poles of the rotating stator field.
• If like poles are aligned, they tend to repel each other, so that the pull-in torque is at a minimum and fails to pull the rotor into synchronism.
• But if unlike poles are aligned, they attract each other and the pull-in torque is a maximum.
• When the direct current excitation is applied to the field system and because the rotation is running below synchronous speed, the torque is a combination of the pulsating torque due to the d.c. excitation of the field system and the induction motor torque which also varies with speed.
• If the slip is low enough, the attraction between the unlike poles in the stator and rotor will pull the rotor into step with- the rotating stator field.
• For given excitation, system inertia, and load, there is a slip below which the synchronous motor will not pull into step. This is termed the critical slip.

Ques 27.When a synchronous motor is connected to an infinite bus, while. operating on leading power factor.

1. Excitation voltage will be independent of the supply voltage.
2. Excitation voltage will be more than the supply voltage.
3. Excitation voltage will be less than the supply voltage.
4. Excitation voltage will be equal to the supply voltage.

Answer 2. Excitation voltage will be more than the supply voltage.

Explanation:

The bus bars whose frequency and the phase magnitude of potential differences are not affected by changes in the condition of anyone machine connected to it are called infinite bus bars.

or

A network having zero impedance and infinite rotational inertia is also termed as Infinite bus-bars.

An overexcited synchronous motor draws current at the leading power factor. If d.c. field excitation of a synchronous motor is such that the back emf Eb is greater than applied voltage V, then the motor is said to be over excited.

An overexcited synchronous motor acts as a power factor correction device and is also known as a synchronous condenser. The variation of armature current and power factor as a function of field current is plotted to give a better insight.

We can state that an over-excited synchronous motor draws a leading power factor current from the mains. The synchronous motor, therefore, when over-excited, in addition to driving some load, will work as a capacitor or condenser. A capacitor draws a leading power factor current. An over-excited synchronous motor draws the leading power factor current from the mains.

An over-excited synchronous motor is also called a synchronous condenser. Synchronous motors are used as constant-speed drive motors. Over-excited synchronous motors are used to improve the power factor of electrical loads in industries. Generally, the motor is run on load, and by overexcitation, the system power factor is also improved.

Ques 28. Which of the following losses is not dissipated by the stator core surface in a synchronous motor?

1. Eddy current losses in the conductors
2. Iron losses in the stator
3. Windage losses.
4. Copper losses in the slot portion of the conductors

Explanation:

## Losses in Synchronous machine

The losses in synchronous machines are as follows:

(a) Fixed losses:-  Core loss, bearing, friction, and windage loss, brush friction loss. These losses are obtained from the no-load test. Core loss occurs because of the eddy currents and hysteresis caused by the main magnetic field. It is the difference between the power required to drive the synchronous machine with or without field excitation. This is taken at the rated voltage and speed.

### Windage losses in the synchronous motor

• These losses occur during the circulation of moving air around inside the machine.
• The rotor “whips” air around and the air resistance cause losses.
• Sometimes the rotor fan losses are included in windage and sometimes they are calculated separately; however, the physics of both are the same.
• Windage losses vary with the airspeed relative to the motor surfaces squared. In low-speed machines, these are often neglected.
• But in large high-speed machines (like flywheels and some 400Hz generators) windage losses are a dominant loss.
• To reduce windage losses and improve cooling, large power generators are sometimes sealed and cooled with hydrogen rather than air.
• Windage losses are not dissipated by the stator core surface in a synchronous motor.

(b) I2R loss in armature winding, stray loss in iron and conductors:- Armature I2R loss is current2 × dc resistance R corrected at 70°C and not the effective resistance. This can be calculated when I and R are measured. Stray load losses are caused due to changes in the flux distribution due to load. This can be found by a short-circuit test. The short-circuit current is adjusted to the value of load current at which the loss is to be determined, then the stray load loss = mechanical power input friction and windage loss I2R loss. The synchronous machine is run at the rated speed. Ventilation loss is the power required to circulate cooling air in addition to the windage loss.

(c) Excitation circuit losses:- These include field copper loss, rheostat loss. brush contact loss, exciter losses. Field copper loss = I2fRf, where If is the field current and Rf is the resistance of field winding. Rheostat loss is I2fRr, where Rr, is the resistance of the rheostat. Brush contact loss is taken as slip ring current. Exciter loss is considered when it is driven by a Synchronous machine and is part of the whole machine. Otherwise, it is changed to the plant and not to the alternator.

Note:- If the machine is not excited (zero field current) and running on no-load, the core loss will be zero and only windage and friction loss takes place. If the machine is excited (field current is supplied), both windage and friction and core losses take place. Thus, the core loss can be computed by taking the difference of the power consumed by machines with excitation and without excitation. It is common practice to consider core loss under load and no-load conditions the same.

Ques 29. The direction of rotation of the synchronous motor can be reversed by reversing

1. Field winding
2. The polarity of the rotor poles
3. Supply phase sequence
4. None of the above

Explanation:

### Reversing a Synchronous Motor

• The direction of rotation of a synchronous motor is determined by its starting direction, as initiated by induction-motor action.
• Thus, to reverse the direction of a three-phase synchronous motor, it is necessary to first stop the motor and then reverse the phase sequence of the three-phase connections at the stator like an induction motor.
• The direction of rotation of a 3-phase synchronous motor can be changed by altering the phase sequence of the supply. I.e from RBY to RYB. Doing so will change the direction of rotation from clockwise to anticlockwise.
• Reversing the current to the field windings will not affect the direction of rotation. If the current in the field winding is reversed the motor will run in the same direction. The field side will only slip through a pole-pitch due to the reversal of the polarities of the field poles.

Ques 30. In synchronous motor out of the following losses, which one will have the highest proportion?

1. Stator copper losses
2. Iron losses
3. Eddy current losses
4. Friction and windage losses

Explanation:

A synchronous machine is used to convert mechanical energy into electrical energy or vice-versa. While doing so, the whole of input energy does not appear at the output but a part of it is lost in the form of heat in the surroundings. This wasted energy is called losses in the machine. These losses affect the efficiency of the machine.

The various losses occurring in a synchronous machine can be sub-divided as

1. Copper losses.
2. Iron losses.
3. Mechanical losses
4. Stray losses

1. Copper losses: The various windings of the synchronous machine such as armature and field winding are made of copper and have some resistance. When current flows through them, there will be power loss proportional to the square of their respective currents. These power losses are called copper losses.

In general, the various copper losses in a synchronous machine are:

(i) Armature copper loss = I2R

(ii) Field winding copper loss = I2fRf

(iii) Brush contact loss = I2Rb

The brush contact loss is generally included in the field winding copper losses.

2. Iron losses: The losses which occur in the iron parts of the DC machine are called iron losses or core losses or magnetic losses. Iron losses have a high proportion of losses occurring in the synchronous machine. Iron core losses in electrical induction machines operate with sinusoidal power supplies account for 15-25% of the total machine losses which are one of the major losses in electrical machines. These losses consist of the following:

(i) Hysteresis loss: Whenever a magnetic material is subjected to reversal of magnetic flux, this loss occurs. It is due to the retentivity (a property) of the magnetic material. The Hysteresis losses are proportional to the frequency and the maximum flux density Bm in the air gap.
The loss is basically due to the reversal of the magnetization of the armature core. It occurs in the armature (stator core). To minimize this loss, the armature core is made of silicon steel which has low hysteresis constant.

(ii) Eddy current loss: When flux linking with the magnetic material changes (or flux is cut by the magnetic material) an emf is induced in it which circulates eddy currents through it. These eddy currents produce eddy current loss in the form of heat.The eddy current losses are proportional to the square of the electrical frequency. The electrical steel used in the stator and rotor of induction machines

The major part of this loss occurs in the armature core. To minimize this loss, the armature core is laminated into thin sheets (0·3 to 0·5 mm) since this loss is directly proportional to the square of the thickness of the laminations.

3. Mechanical losses: As the field system of a synchronous machine is a rotating part, some power is required to overcome:

(i) Air friction of rotating field system (windage loss).

(ii) Friction at the bearing and friction between brushes and slip rings (friction loss).

These losses are known as mechanical losses. To reduce these losses proper lubrication is done at the bearings.

4. Stray losses:- In addition to the iron losses, the core losses are also caused by the distortion of the magnetic field under load conditions and losses in the insulation of armature and field winding, these losses are called stray lasses. These losses are also included while determining the efficiency of synchronous machines.

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