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AC motor viva questions part 1

AC motor viva questions part-1

 

Q: What is an A.C motor? 

Ans: the motor which works on A.C supply is known as A.C motor.

Q: What are the main classifications of A.C motors? 

Ans: these are divided into:

  1. Synchronous motor
  2. Asynchronous motor. 

Q: What do you know about the synchronous and asynchronous or non-synchronous motor? 

Ans: The motor which works on synchronous speed is known as the synchronous motor and that which does not run at synchronous speed is known an asynchronous or non-synchronous motor. 

Q: Can you give some names of non-synchronous motors? 

Ans: These are squirrel cage induction motor, slip-ring induction motor or wound rotor motor, double squirrel cage induction motor. 

Q: What is meant by an induction motor? 

Ans: The motor in which supply is not given to the rotor winding, but it is induced in it because of the induction, is known as an induction motor. 

Read:AC motor viva questions part 2

Q: What is the working principle of an induction motor? 

Ans: Whenever a current-carrying conductor is placed in the magnetic field, a torque is developed over the conductor, which tends to rotate. 

Q: When the rotor winding of the motor is not fed, how does the current circulates in the rotor conductors? 

Ans: Because of induction some e.m.t. is induced in the rotor bars. The bars are short-circuited so the current circulates and torque is developed.

Q: How the direction of torque developed is found? 

Ans: By means of Fleming’s Left Hand Rule. 

Q: What is the Fleming’ Left Hand Rule? 

Ans: According to the rule, stretch your left hand in such a way so that the thumb, first finger and middle fingers are at a right angle to each other, then if the first finger indicates the direction of magnetic flux, the middle finger the direction of current in the conductor then thumb will indicate the direction of torque developed on the conductor. 

Read: Viva questions of ac motor part 3

Q: What are the constructional parts of the induction motor? 

Ans: These can be divided into two-the stationary parts and rotating parts. These are the body, side covers, bed, eyebolt, terminal box, shaft, rotor, bearings, and fan. 

Q: What do you mean by stator and rotor? 

Ans: The stator is the stationary part of the motor and that which rotates is known as a rotor. 

Q: Can you tell the construction of the rotor? 

Ans: It is a rotating part. It has so many slots. There are copper bars or aluminum bars placed in the slots. These bars are short-circuited at both ends. These short-circuiting rings are known as end rings. 

Q: What is the function of short-circuited bars in the rotor? 

Ans: It provides the current-carrying conductor in the magnetic field.

Q: How does the current circulates in the rotor bars?

Ans: The e.m.f induced in the bars when short-circuited by the end-rings causes the current to circulate through the bars.

Read:AC motor viva questions part 2

Q: What do you understand about the wound rotor?

Ans: If in the slots of a rotor, the winding is placed and brought to slip rings, that rotor is called wound rotor.

Q: What types of slots are used in the rotor of slip ring induction motors? 

Ans: Open type slots. 

Q: Why the rotor is made of laminations? 

Ans: To reduce the eddy current losses. 

Q: What is done to reduce the hysteresis losses? 

Ans: Some percentage of silicone is added. 

Q: Which material is used for short-circuited bars? 

Ans: Copper or aluminum base. 

Q: Sometimes it is observed that the bars are skewed, why? 

Ans: To avoid magnetic interlocking between the stator and rotor and to increase the length of the rotor conductor to enable the production of more torque. 

Read: Viva questions of ac motor part 3

Q: What types of slots are used for a stator or medium size squirrel cage motor?

 Ans: Semi-open type. 

Q: Name the slot insulation generally used for medium size squirrel cage motors? 

Ans: Leatheroid or mill ax paper. 

Q: How many Terminals are brought out from the slip ring induction motor? 

Ans: Three from stator and three from the rotor and sometimes six from the stator winding and three from the rotor winding. 

Q: How many terminals are brought out from the three-phase squirrel cage motor? 

Ans: Three or six. 

Q: What is the nature of the magnetic field produced because of the three-phase stator winding?

Ans: Rotating magnetic field.

Read:AC motor viva questions part 2

Q: What are the characteristic of the rotating magnetic field? 

Ans: It has constant magnitude rotating at a constant speed, i.e. synchronous speed. 

Q: Can you change the d.o.r. of the rotating magnetic field? 

Ans: Yes, by interchanging the two phases (line). 

Q: What will be the direction of rotation of the rotor with respect to the rotating magnetic field?

Ans: Same as that of a rotating magnetic field. 

Q: If you change the d.o.r. of the rotating magnetic field, what will happen to the d.o.r. of the rotor? 

Ans: It will also change. 

Q: How many electrical degrees are there in a four-pole machine? 

Ans: 720º elect. 

Q: How are the electrical degrees calculated? 

Ans: 360 x No. of pair of poles.

Q: Define synchronous speed? 

Ans: the speed of the rotating magnetic field is known as the synchronous speed. It is represented by Ns, and measured in r.p.m. it is given by the formula Ns =120f/P rpm. 

Read: Viva questions of ac motor part 3

Q: On what factors does the synchronous speed Ns depend? 

Ans: Frequency and number of poles. 

Q: If you increase the number of poles of a motor will the speed decrease or increase? 

Ans: Speed is inversely proportional to the number of the poles so the speed will decrease with the increasing of the poles. 

Q: What is the angular displacement of the magnetic field produced by each phase? 

Ans: 120° elect. 

Q: What is torque? 

Ans: It is defined as the turning or twisting moment of a force about its axis.

Q: In an induction motor, on what factors does the torque depend? 

Ans: Flux produced by the stator winding, the rotor current, and rotor power factor. 

Q: Can the rotor speed be equal to synchronous speed in the case of induction motors? 

Ans: No.

Read:AC motor viva questions part 2

Q: Why the Nr (rotor speed) and Ns (synchronous speed) cannot be equal? 

Ans: Because in an induction motor, the induction can only be possible if there is any relative motion between the rotor bars and the magnetic field so if there is no induction, there will not be any e.m.f. induced in rotor bars, there will not be current and hence no torque. So Nr, cannot be equal Ns.

Q: What do you know about the slip speed? 

Ans: It is the difference between the rotor speed and synchronous speed.

Q: Can you tell the formula for calculating the percentage slip of the motor?

Ans: %S=[(Ns-Nr)/Ns] x 100 . 

Q: Has the e.m.f. induced in the rotor winding any frequency? 

Ans: Yes. 

Q: What is the rotor frequency and on what factors does it depend? 

Ans: Rotor frequency = Slip x supply frequency. It depends upon the slip and supply frequency both. 

Read: Viva questions of ac motor part 3

Q: What is the rotor frequency at the starting of a 50 c/s squirrel cage induction motor? 

Ans: It is 50c/s but decreases with the increase of the speed of the motor.

 Q: Why are running conditions the rotor frequency is less? 

Ans: Because in running conditions the slip is less and thus the flux linking the rotor conductors has less speed producing less frequency. 

Q: What do you mean by the fractional slip? 

Ans: If the slip speed is shown in terms of synchronous speed as =(Ns-Nr)/Ns It is called as fractional slip. 

Q: Will there be any induced emf in rotor winding or an induction motor if its rotor is rotated at synchronous speed? 

Ans: No, there will not be any e.m.f. induced. 

Q: Why will there be no induced e.m.f? 

Ans: Because the speed of the magnetic field produced is the synchronous speed and the rotor speed is also the same so there will not be any linkage and no e.m.f, will be induced in the rotor.

Read:AC motor viva questions part 2

Q: How the power factor of the rotor winding is calculated?

Ans:Rotor\;power\;factor=\frac{Rotor\;resis\tan ce}{Rotor\;impedance}

Q: Do you know something about sX2? 

Ans: It is the slip multiplied by the rotor reactance. It is the reactance of the rotor in running condition. 

Q: Give any formula for calculating the torque in an induction motor?

Ans:

\begin{array}{l}T=\frac{Rotor\;input}{2{\mathrm{πN}}_{\mathrm s}}\\or\\\\T=\frac{Rotor\;putput}{2{\mathrm{πN}}_{\mathrm r}}\\\end{array}

Q: How has the starting torque of the induction motor been improved? 

Ans: By increasing the resistance of the rotor winding at the time of starting (slip ring induction motor) 

Q: How slip is concerned with the torque? 

Ans: Torque ∝ Slip. 

Read: Viva questions of ac motor part 3

Q: In the case of the squirrel cage motor if the resistance of rotor bars is increased how will it affect the performance of the motor? 

Ans: Though by increasing the rotor resistance, the torque developed in starting is increased, but the efficiency of the motor will decrease because of losses in running conditions. 

Q: What is the effect of change in voltage on the production of torque of an induction motor?

Ans: Torque is directly proportional to the square of the voltage. 

Q: What are the variable losses and why these are called so? 

Ans: Generally, the copper losses are called the variable losses as these losses are proportional to the square of the current. 

Q: Which losses are constant losses and why? 

Ans: The stray losses, frictional losses, and windage losses are the constant losses. These are proportional to the speed of the motor which is almost constant hence are called constant losses. 

Q: What is efficiency? 

Ans: It is the ratio of output to input generally shown in percentage. 

Read:AC motor viva questions part 2

Q: Can you calculate the output of a machine if the losses and input are given? 

Ans: Yes. Output = Input – Losses. 

Q: What are the characteristics of an induction motor? 

Ans: The behavior and performance of the induction motor at different load currents and speeds are termed as characteristic. These are:

  1. Speed and load current characteristic. 
  2. Torque and load current characteristic.
  3. Speed and torque characteristics. 

Q: What is the speed load characteristic? 

Ans: It shows the relationship between the speed and the load put on the motor. 

Q: What is the torque and speed characteristic? 

Ans: It shows the relationship, how much load is put on the machine, and what will be the speed of the motor on that load.

 

 

Read: Alternator viva questions part-1

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