SSC JE synchronous motor & synchronous generator question

SSC JE synchronous motor & synchronous generator question

Ques.1. The speed of a synchronous motor can be changed by (SSC-2018 Set-1)

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

Answer.2. Changing the frequency


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 (SSC-2018 Set-1)

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

Answer.2. Twice


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

[latex]P \approx \dfrac{{VE}}{{{X_s}}}\sin \delta [/latex]


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.3. A three-phase synchronous motor will have (SSC-2018 Set-1)

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

Answer.2. Two 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

Synchronous Motor1

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.4. The maximum speed variation in a synchronous motor is (SSC-2018 Set-1)

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

Answer.1. Zero


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


Ques.5. In a synchronous motor which loss varies with the load? (SSC-2018 Set-1)

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

Answer.4. Copper Loss


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

DC Machine Losses

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.6. 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

Answer.2. Prevent hunting and provide starting Torque


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.


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.7. An over-excited synchronous motor is used for (SSC-2018 Set-1)

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

Answer.3. Power factor corrections


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 Condenser

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.8. When any one-phase of a 3-phase synchronous motor is short-circuited, the motor (SSC-2018 Set-1)

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

Answer.2. Will refuse to start


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.9. Which of the following can be measured by conducting an insulation resistance test on a synchronous motor? (SSC-2018 Set-1)

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

Answer.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.10. The speed of a synchronous motor can be changed by (SSC-2018 Set-1)

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

Answer.2. Changing the frequency


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.

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