Saturday , January 22 2022

Viva questions for motors part 3

Viva questions for motors part 3


Q: While the start button is pressed the motor gets accelerated but stops as soon as the button is released?  

Ans: The auxiliary contact of the starter is not working satisfactorily. As this is in parallel with the start button so as long as the start button is on, the supply continues to no-volt release coil but breaks with its releasing because the supply of NVRC is not continued so the plunger is released and stops the motor  

Q: Generally for a 5 HP induction motor, which type of slots, are used? 

Ans: Semi-closed slots with parallel teeth. 

Q: What are the types of slots used for the starter and rotor windings of the induction motor?

Ans: Closed type, open type, and semi-closed types. 

Q: How the starting torque of an induction motor can be increased? 

Ans: Starting torque is directly proportional to the square of voltage and rotor resistance. Generally, rotor resistance is increased (slip ring induction motor application). 

Q: Can you roughly tell, at what load the power factor of the induction motor will be maximum.

Ans: Full load capacity as these are designed for that. 

Read: viva questions for motors part 1

Q: Can two A.C. induction motors of the same h.p. having different efficiencies be started from the same types of starters? 

Ans: yes, they can be started as the efficiencies are different not the capacity of the motor. 

Q: What is the rotor impedance of an induction motor in standstill condition? 


\begin{array}{l}It\;is\;Z_r=\sqrt{R_r^2+(sX_r)^2}\\\\At\;s\tan dstill\;condition\;S=1\\so\;Z_r=\sqrt{R_r^2+X_r^2}\;ohm\end{array}

Where Zr = rotor impedance in ohm,

Rr = rotor resistance in ohm, and

Xr, = rotor reactance in ohm. 3 

Q: What is the value of Zr, in running conditions? 



Read: viva questions for motors part 2

Q: What is the expression for the rotor the power factor in the case of the motor running with S-Slip?




Q: Which change Rrs, Xr, will change/affect the maximum on the rotor power factor? 

Ans: Change in rotor resistance because Rr is directly proportional to the power factor. 

Q: The rotor resistance is also in the denominator. Why don’t you consider the net effect will be neutralized? 

Ans: No, the net effect will be a corresponding change, because the rotor resistance square under-root will decrease the denominator more than the numerator. 

Q: What are the possible faults in an induction motor? 

Ans: Earth fault, short circuit between the circuit in the coil, leakage or failure of the insulation etc.

Q: How earth fault can be tested? 

Ans: By means of test lamp or by means of megger. Connect one terminal on the body on the winding. In the case of series lamp testing, the lamp will glow and in the case of megger testing the megger will give zero reading. In the case of lamp testing method, one thing should be kept in mind that the phase wire may not be directly earthed, phase must be connected through the lamp. 

Read: viva questions for motors part 1

Q: How much leakage current is allowed?

Ans: 1/5000 of the total full load current of the appliance/motor. 

Q: What are the possible tests to be carried out for the proper functioning of the motor? 


  1. Insulation resistance test.
  2. Phase sequence test.
  3. The leakage current should be under permissible limits. 

Q: Why are the windings varnished? 

Ans: To avoid air bubbles in the coil turns, to give sufficient mechanical strength to the conductors and to avoid moisture also. 

Q: Why the motor winding and varnish are heated up before varnishing? 

Ans: So that even a small bubble of air from the coil may come out and the hot varnish may penetrate to the deep and grip the conductors in an effective manner. Thus the winding will be mechanically strong enough. 

Read: viva questions for motors part 2

Q: Why the no load tests conducted? 

Ans: To find out no-load current lo, the no-load power factor cosφ, windage and friction losses, no load input to the motor, no-load resistance Ro and reactance Xo

Q: Why short circuit test is performed for squirrel cage induction motor? 

Ans: Just to find out short circuit current with normally applied voltage, power factor on short circuit, total equivalent resistance, and reactance of the rotor referred to the stator winding. 

Q:How the short circuit test conducted? 

Ans: The rotor is held firmly and stator is connected across variable voltage. It is just equivalent to the short circuit test of the transformer.

Q: What is a single phase induction motor? 

Ans: The motor which works on single-phase supply and converts it into mechanical energy is known as single-phase motor, 

Q: What are the classifications of single-phase motors? 


  1. Synchronous motor.
  2. Induction motor.
  3. Commentator motor. 

Q: What do you know about the commentator motor? 

Ans: The motor in which the rotor is wound and the leads are brought to the commentator is known as the commentator motor. For example, series motor and repulsion motor.

Read: viva questions for motors part 1

Q: How does the repulsion motor works?

Ans: It works on the principle of repulsion between the magnetic field produced by the stator and rotor winding. 

Q: Name the different classifications of the repulsion motor? 


  1. Plane repulsion motor.
  2. Repulsion induction motor. 
  3. Repulsion start induction run motor.

Q: What is the nature of the magnetic field produced by the single-phase alternating current?

Ans: Alternating magnetic field. 

Q: Is this magnetic field a rotating magnetic field? 

Ans: No. 

Q: Are the single-phase motors self-starting? 

Ans: No, these are not self-starting. 

Read: viva questions for motors part 2

Q: How are you getting the splitting of the phases? 

Ans: By providing two windings known as the starting winding and running winding, 

Q: What are the methods of obtaining starting torque in a split-phase motor? 

Ans: The resistance start, the reactance start, the capacitor motors (capacitor start capacitor run, capacitor start induction run, and permanent capacitor motor). 

Q: Can you give an example of a permanent capacitor motor? 

Ans: Ceiling fan or table fan. 

Q: Where the capacitor start capacitor run motors are used? 

Ans: Places where high starting torque is required for example compressors etc. 

Q: Generally which single-phase motor is used in lathe machines? 

Ans: Capacitor starts induction run. 

Read: viva questions for motors part 1

Q: In case of a capacitor start induction run motor the starting winding is cut off in running condition. How is it possible? 

Ans: It is possible by providing a switch generally known as centrifugal switch. In normal condition it is closed and in operation it is opened. 

Q: How will you identify the starting winding and running winding? 

Ans: The starting winding has high resistance and the test lamp will give dim light and the running winding has less resistance the test lamp will give bright light. 

Q: Where the start winding in case of a shaded pole motor is placed? 

Ans: In the groove, where the pole faces are shaded. 

Q: How much pole face is shaded?

Ans: It is 1/4thor 1/3rd of the pole face.

Q: The poles projected or smooth? 

Ans: Projected. 

Read: viva questions for motors part 2

Q: Which action of the magnetic field is responsible for the production of torque in shaded pole motors? 

Ans: Sweeping action of the magnetic field.

Q: The shading ring is made of which material? 

Ans: Generally copper. 

Q: Where single-phase synchronous motors are used? 

Ans: Generally for constant speed for example in clocks etc. 

Q: In which winding circuit the capacitor is used and why? 

Ans: The capacitor is used in series with the starting winding to give a phase displacement to the current drawn by starting and running winding. 

Q: In a capacitor start motor, if the starting winding is not disconnected what will happen? 

Ans: The starting winding circuit is for starting purposes. It has to be disconnected as soon as the motor attains starting. If it is not disconnected the motor will give an abnormal sound and will take more current and finally may burn out.

Read: viva questions for motors part 1

Q: Why the fault of abnormal sound and taking more current is not experienced? 

Ans: It is because the running torque of these fans is calculated on the assumption that the flux will be produced by both the windings in normal working conditions. The design is such that both windings will remain in the circuit during starting and running conditions. 

Q: Sometimes we have seen that a ceiling fan suddenly starts moving in the opposite direction with slow speed. What could be the defect? 

Ans: The starting winding circuit is open-circuited may be due to burnt winding or damaged capacitor or the running winding is damaged. The fan is running only on one winding. The angle of blades and thrust of air in the room accelerated the blades in opposite directions. As we know that the single-phase motors are not self-starting, they will continue to move in the direction in which these are initially accelerated. 

Q: Why a table fan while switched on stick to one side and does not work (revolve)? 

Ans: The brushes are defective and need replacement (the other things oscillating attachment being alright). 

Q: The torque developed in A.C. single-phase motor is proportional to the sine or cosine of the angle between starting winding current and the running winding current? 

Ans: Sine of the angle between starting winding and running winding current. 

Read: viva questions for motors part 2

Q: Generally which type of capacitor is used as a starting capacitor in A.C. single phase motor?

Ans: Electrolytic capacitor. 

Q: Can an induction motor be used for a record player or tape recorder? 

Ans: No, generally single-phase hysteresis motor is used for this purpose. 

Q: Can you specify a particular use of a reluctance motor? 

Ans: It is a suitable motor for signaling device purposes. 

Q: Which motor is suitable for portable tools, the A.C. split-phase induction motor, universal motor, or shaded pole motor? 

Ans: Universal motor, 

Q: The starting torque of a single-phase induction motor is how many times the full load torque?

Ans: The starting torque of A.C..single phase induction motor depends upon the type of starting,

Q: For changing the d.o.r. of a split induction motor what will you do? 

Ans: The DOR of a split-phase induction motor can be changed by changing the direction of current in either the starting or running winding, Generally the starting winding direction is reversed. 

Q: Which capacitor is used in a shaded pole motor? 

Ans: Capacitor is not used in shaded pole motor. 

Read: viva questions for motors part 1

Q: How many sets of wound coils have o four-pole shaded pole induction motors? 

Ans: Four sets of wound coils. 

Q: The main purpose of shaded pole winding is…….? 

Ans: To provide starting winding purpose and to facilitate the starting torque. 

Q: The capacitor helps in splitting a single-phase supply into two phases for producing a rotating magnetic field? 

Ans: Yes, it helps and splits for the production of torque. 

Q: Can a universal motor be started by a 3-phase supply? 

Ans: No, the universal means (here) that this motor can work on A.C. single-phase or D.C.both. 

Q: What do you understand by a synchronous motor? 

Ans: The motor which rotates at the synchronous speed is known as a synchronous motor. 

Q: Can you tell if a synchronous motor is a self-starting one? 

Ans: No, it is not self-starting.  

Read: viva questions for motors part 2

Q: Can you name a single motor in three-phase motors where two supplies work simultaneously? 

Ans: Yes, it is a synchronous motor. 

Q: On which principle does the synchronous motor work? 

Ans: Magnetic interlocking. 

Q: What are the types of synchronous motors? 


  1. Rotating magnetic field type.
  2. Stationary magnetic field type. 

Q: What are the starting methods of the synchronous motor? 


  1. By means of a separate motor.
  2. By means of an induction motor action.
  3. by means of a pony motor. 

Read: viva questions for motors part 1

Q: What are the dampers or damping windings? 

Ans: It is a winding placed in the pole faces and short-circuited at both the ends as that of a squirrel cage winding. It is used for starting purposes and to damp or hunt the oscillations which are caused by the load impacts or removal of loads. 

Q: Do you know the term ‘angle of coupling? 

Ans: It is the maximum angular displacement between the magnetic axis of the stator and rotor poles with which a synchronous motor can be loaded without losing the synchronism. 

Q: What is pulling in torque? 

Ans: The amount of torque that a motor is able to exert when pulling into synchronism is called the pull-in torque. 

Q: What is pulling out torque? 

Ans: The maximum torque that a motor can develop without losing the synchronism is known as pull-out torque. 

Q: If you increase the load on the synchronous motor, what will happen, will the speed drop-down or not? 

Ans: The speed will remain the same synchronous speed but the angle of coupling will increase.

Read: viva questions for motors part 2

Q: What will be the current unity power factor? 

Ans: It will be the lowest possible.

Q: If there is a constant load on the motor and you are changing the field excitation, what will happen to the load current? 

Ans: The current will be more when the motor is over or under excited. But the speed will be the same. 

Q: Will the overexcited synchronous motor improve the power factor? 

Ans: Yes, in that case, it is known as a synchronous capacitor. 

Q: Where is the synchronous motor used? 


  1. To improve the power factor.
  2. To drive the load. 
  3. For constant speed.

Q: What are the advantages of this motor? 


  1. Constant speed.
  2. Power factor correction.

Read: viva questions for motors part 1

Q: What are the disadvantages of this motor? 


  1. Speed cannot be controlled.
  2. More maintenance is required. 
  3. It is not a self-starting motor so starting means is required.
  4. Cannot be started with a load. 

Q: Generally which start is used for starting the synchronous motor? 

Ans: Autotransformer starter. 

Q: Why are oscillations produced in synchronous motor? 

Ans: Because of variation of load.

Q: How is hunting achieved? 

Ans: By using damping winding in the pole faces. 

Q: If you change the field excitation how much variation in speed you can get? 

Ans: No, variation of speed. 

Q: While changing the field excitation, which factor is effective? 

Ans: Poorer factor. 

Read: viva questions for motors part 2

Q: Which motor may have high efficiency, the synchronous motor or induction motor? 

Ans: synchronous motor. 

Q: If the field winding is connected in reverse order will the motor start and if so, will it produce its full load torque? 

Ans: The motor may start, but will not produce its full torque.

Read: Transformers viva questions part 1

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