Synchronous Motor Interview Question - Day 1

[Q:-01] What is a synchronous motor and where is it used ?

Ans :

      When an a.c. generator is driven as a motor it is called synchronous motor. It is used to improve the power factor of the line as well as used for load at constant speed.

[Q:-02] Why is it called synchronous motor ?

Ans :

      As the motor runs at synchronous speed in synchronism with supply frequency the motor signifies its name as synchronous motor.

[Q:-03] Does a synchronous motor run at constant speed ?

Ans :

      Yes, it runs at constant synchronous speed which is given by the equation

Ns = 120f/P r.p.m

where f = Supply frequency,

P = Number of poles and

Ns = Synchronous speed of the motor in r.p.m.

[Q:-04] Can we vary the speed of synchronous motor in running condition ?

Ans :

No.

[Q:-05] How can the speed of a synchronous motor be changed ?

Ans :

By varying the frequency with special arrangements.

[Q:-06] What are the characteristic features of synchronous motor ?

Ans :

(a) It runs at synchronous speed.

(b) It is not self-starting.

(c) It is capable of being operated under a wide range of power factors both lagging and leading.

[Q:-07] What is the principle of operation of synchronous motor ?

Ans :

      Synchronous motor is practically an alternator having a field on the rotor and an armature on the stator. The stator armature has three phase winding which is energized from a 3-phase supply and a rotating magnetic field is created on the armature as in an induction motor. The speed of the rotating field is synchronous speed.

      The magnetic field system on rotor consists of one or more pairs of north and south poles as in a d.c. machine. The field is excited by a d.c. supply fed from two slip rings mounted on the shaft. In addition the rotor generally carries a squirrel cage winding (rotor bars short circuited by end rings) keeping the d.c. field winding short circuited. When the supply is with on to the stator winding the rotor starts up as an induction motor the synchronous the help of the squirrel cage winding. When the rotor speed approaches "a synchronous speed the d.c. excitation is switched on. Then the rotor magnetic field suddenly gets pulled up to the synchronous speed and locks up with the rotating magnetic field produced by the stator. The rotor will then be running exactly at synchronous speed and the speed will remain constant as long as the supply frequency is steady.

[Q:-08] In which motor the stator field and the rotor field rotate simultaneously ? rota'

Ans :

Synchronous motor.

[Q:-09] In which motor the air gap is more than that of rho remaining motors ?

Ans :

Salient pole synchronous motor.

[Q:-10] How does a synchronous motor differ from an induction motor with regard to its excitation and the power factor at which it operates ?

Ans :

      The excitation of an induction motor comes solely from the a.c. supply lines to which it is connected whereas the excitation of synchronous motor comes from two different sources of supply. The stator winding receives its excitation from an a.c. source while the rotor field winding is excited from a d.c. source.

      The power factor of an induction motor is always lagging and fixed by the magnitude of the load whereas the power factor of a synchronous motor is variable over wide range with the variation of its d.c. excitation.

[Q:-11] From where is d.c. excitation given in the field of synchronous motor ?

Ans :

      The d.c. excitation is provided by an exciter (d.c. generator) driven either by mounting on the same shaft or by a separate motor.

[Q:-12] What do you mean by exciter ? How much voltage is induced by this exciter ?

Ans :

      It is a d.c. shunt or compound generator by which the voltage is induced upto 250 volts.

[Q:-13] Is the synchronous motor self started ?

Ans :

No.

[Q:-14] Why is it necessary to employ special starting equipments for starting a synchronous motor ?

Ans :

      Because the starting torque of a synchronous motor is practically zero.

[Q:-15] What are the methods of starting a synchronous motor ?

Ans :

Various methods of starting of synchronous motor are as follows :—

(a) By means of a d.c. compound motor directly coupled or belted to the main motor.

(b) By means of a pony motor (a small induction motor directly coupled or belted to the main motor).

(c) The motor is made self starting by providing damper winding on rotor poles.

[Q:-16] What is the advantage of using d.c. motor as auxiliary rotor for starting a synchronous motor ?

Ans :

      After starting synchronous motor the same d.c. motor can be used as d.c. generator for providing field excitation to the synchronous motor.

[Q:-17] What is the advantage and disadvantage of making self starting by damper winding ?

Ans :

      It requires no auxiliary apparatus or separate source of power but requires large lagging motor current resulting in disturbance of voltage regulation and the field winding also suffers from the high induced voltage.

[Q:-18] How much voltage is given to the synchronous motor at the time of starting ?

Ans :

50% to 80% of normal voltage is applied through auto-transformer starter.

[Q:-19] What are the procedures of self starting a synchronous motor ?

Ans :

      Almost all synchronous motors are provided with damper winding or squirrel cage winding consisting of copper bars embedded in the pole shoes and short circuited at both ends. During starting period a synchronous motor starts acting as an induction motor. Before starting the main field winding is short circuited. A reduced voltage is applied to the stator terminal with the help of auto-transformer and the motor starts up. When it attains a steady 'speed a weak d.c. excitation is applied to the main rotor field winding by opening short circuit. If the excitation is sufficient the rotor' poles are magnetically interlocked with the stator poles and the motor will run at synchronous speed. Then full supply voltage is applied to the stator' terminal by cutting out the auto-transformers and the motor may be operated at any desired power factor by changing the d.c. excitation.

[Q:-20] What is the purpose of field sectionalisation during starting ?

Ans :

      At the time of starting when the stator armature is switched on and the field winding is still unexcited the field coils are being cut by rotating armature flux a high voltage is induced in the field winding by transformer action which may reach very high value dangerous both to life and also to the field insulation. In order to limit this high voltage to safe value a field sectionalising switch operated by centrifugal device is provided to sectionalise the field winding in several points along with the provision of extra insulation on the field.

[Q:-21] What Is the advantage of short circuiting the find winding during starting and upto what limit ?

Ans :

      Instead of leaving the field circuit open during starting if the field winding is short circuited current will be induced in it which will produce, extra torque in addition to the torque contributed by the squirrel Cage damping winding. As a result starting torque will be increased but upto the half synchronous speed of the motor beyond which the actual torque will be less than with the field open circuited.

[Q:-22] What will be the effect when a synchronous motor is switched on to an a.c. three phase supply with its field windings shorted on themselves ?

Ans :

      The synchronous motor will start but continue to run as an induction motor.

[Q:-23] When does the synchronous motor run without load

Ans :

For improving power factor of the supply mains.

[Q:-24] What do you understand by normal excitation, under excitation and over excitation of synchronous motor ?

Ans :

      The normal excitation is that excitation which produces unity power factor. Under excitation is that excitation which causes the motor to take a lagging current. Over excitation is that excitation which causes the motor to take a leading current.

[Q:-25] How can you operate a synchronous motor on both lagging and leading power factors ?

Ans :

      A synchronous motor can be operated at both lagging and leading power factor by under-exciting and overexciting the motor respectively.

      For this purpose a regulating resistance is inserted in the field circuit in such a position that the normal excitation is obtained from the (near) mid-point of the regulating resistance.