# 300+ TOP INDUCTION MOTOR Interview Questions and Answers

INDUCTION MOTOR Interview Questions :-

1. How does the Induction motor work? (OR) Why does the Rotor rotate?

• When the 3 phase stator windings are fed by 3 phase supply, a magnetic flux of constant magnitude which is rotating at synchronous speed is set up.
• The flux passes through the air-gap and sweeps past the rotor surface thus it cuts the rotor conductors.
Due to the relative speed between the rotating flux and the stationary rotor conductors, an emf is induced in the stationary rotor conductors as per the Faraday’s laws of electromagnetic induction.
• The frequency of the induced emf is the same as the supply frequency.
• Its magnitude is proportional to the relative velocity between the flux and the conductors.
• Its direction will be as per Fleming’s right hand rule.
• Since the rotor conductors form a closed circuit the rotor current is produced.
• This current’s direction will oppose the very cause producing it ( as per Len’s law)
• Here the cause is the relative velocity between the rotating flux of the stator and the stationary rotor conductors.
• Hence, in order to reduce the relative speed, the rotor begins to rotate in the same direction as that of the rotating magnetic flux and tries to catch up with the rotating flux. Thus the rotor of induction motor starts to rotate.

2. What is the general working principle of Induction motor?

• The conversion of electrical power into mechanical power takes place in the rotating part of an electric motor.
• In DC motors the electrical power is conducted directly to the armature through brushes and commutator.
Thus the DC motor can be called as conduction motor. But in case of AC motors, the rotor receives electric power, not by conduction but by induction.
• This is exactly in the same way as the secondary of two winding transformer receives its power from the primary.
• That is why such motors are known as induction motors.
• Thus an induction motor is also known as rotating transformer ( ie, one in which primary winding is stationary and the secondary is free to rotate)

3. What is the advantage of skewed stator slots in the rotor of Induction motors?

1. In the induction motor design, the rotor slots are purposely made with a slight skew arrangement. It will not be parallel to the shaft.
2. This is for the purpose of reducing magnetic locking or reducing magnetic attraction between stator and rotor teeth.
3. In addition to that this arrangement will help to reduce the magnetic hum and noise.

4. What is meant by cogging in the Induction motor? How to prevent the cogging?
When the number of teeth in stator and rotor are equal, the stator and rotor teeth have a tendency to align themselves exactly opposite to each other, since this corresponds to minimum reluctance position. In such case the rotor may refuse to accelerate. This phenomenon is called as magnetic locking or cogging.
This problem can be prevented by proper choice of stator and rotor slots and also by skewing the rotor slots by one slot pitch.

5. What are the various methods of measuring slip?

1. By actual measurement of rotor speed
2. By measurement of rotor frequency
3. Stroboscopic method

6. What are the various methods of speed control in three phase induction motors?
(i) Control from stator side
1. By changing the supply frequency
2. By changing the number of stator poles
3. By changing the supply voltage
(ii) Control from rotor side
1. By inserting resistance in rotor circuit
2. By various ways of cascade connection
3. By injecting EMFs in the rotor circuit.

7. What is meant by crawing in the induction motor?
In induction motors, particularly squirrel cage type induction motors, sometimes exhibit a tendency to run stably at speeds as low as one-seventh of their synchronous speed Ns. This phenomenon is known as crawling of an induction motor and the speed is called as crawling speed.

8. What is an Induction Motor?
An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction.

9. What is an Electric Motor?
An Electric Motor converts electrical power to mechanical power in its rotor.

10. How to supply power to rotor?
In a DC motor this power is supplied to the armature directly from a DC source, while in an AC motor this power is induced in the rotating device.

11. Why an Induction Motor sometimes called Rotating transformer?
An induction motor is sometimes called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side.

12. Who invented Induction Motor?
Nikola Tesla.

13. What is the basic difference between Synchronous motor and an Induction Motor?
The basic difference between an induction motor and a synchronous AC motor is that in the latter a current is supplied onto the rotor. This then creates a magnetic field which, through magnetic interaction, links to the rotating magnetic field in the stator which in turn causes the rotor to turn. It is called synchronous because at steady state the speed of the rotor is the same as the speed of the rotating magnetic field in the stator.

14. Why stator windings are arranged around the rotor?
the induction motor does not have any direct supply onto the rotor; instead, a secondary current is induced in the rotor. To achieve this, stator windings are arranged around the rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern can induce currents in the rotor conductors. These currents interact with the rotating magnetic field created by the stator and the rotor will turn.

15. Why the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different?
the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different, or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced.

16. What is the SLIP?
This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is unitless and is the ratio between the relative speed of the magnetic field as seen by the rotor to the speed of the rotating field. Due to this an induction motor is sometimes referred to as an asynchronous machine.

17. How many types of Induction Motor ?
Based on type of phase supply
Three phase induction motor (self starting in nature)
Single phase induction motor (not self starting)
Other
Squirrel-cage induction motor
Slip ring induction motor

18. What is the relationship between the supply frequency number of poles and sycnchronous speed?
f = Pns/2

19. What is a Synchronous Speed?

20. What is a Rotor Speed?
Rotor speed, nr = ns(1-s)

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21. How to calculate the Slip?
Slip is calculated using:

s = [ns-nr]/ns

22. What is a Stator?
The stator consists of wound ‘poles’ that carry the supply current that induces a magnetic field in the conductor. The number of ‘poles’ can vary between motor types but the poles are always in pairs (i.e. 2, 4, 6, etc.).

23. How many types of Rotor are there?
There are two types of rotors

1. Squirrel Cage rotor.
2. Slip Ring rotor.

24. What techniques is used to produce a desired Speed?
The most commonly used technique is Pulse Width Modulation in which a DC signal is switched on and off very rapidly, producing a sequence of electrical pulses to the inductor windings.

25. What is the difference between DC motors and the Induction motors?
The induction motor has no brushes and is easy to control, many older DC motors are being replaced with induction motors and accompanying inverters in industrial applications.

26. How an Induction Motor is started? Why the Starter is Used?
When the motor is started, the slip is equal to 1 as the rotor speed is zero, so the induced emf in the rotor is large. As a result, a very high current flows through the rotor. This is similar to a transformer with the secondary coil short circuited, which causes the primary coil to draw a high current from the mains. Similarly, when an induction motor starts, a very high current is drawn by the stator, on the order of 5 to 9 times the full load current. This high current can damage the motor windings and because it causes heavy line voltage drop, other appliances connected to the same line may be affected by the voltage fluctuation. To avoid such effects, the starting current should be limited. A soft start starter is a device which limits the starting current by providing reduced voltage to the motor. Once the rotor speed increases, the full rated voltage is given to it.

27. What is a Rotor?
The rotor is the non-stationary part of a rotary electric motor or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor’s axis. In some designs, the rotor can act to serve as the motor’s armature, across which the input voltage is supplied. The stationary part of an electric motor is the stator. A common problem is called cogging torque.

28. What is a Stator?
The stator is the stationary part of an electric generator or electric motor. The non-stationary part on an electric motor is the rotor.
Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the armature, receiving its influence from moving field coils on the rotor.

29. What is Commutator?
A commutator is an electrical switch that periodically reverses the current direction in an electric motor or electrical generator. A commutator is a common feature of direct current rotating machines. By reversing the current direction in the moving coil of a motor’s armature, a steady rotating force (torque) is produced. Similarly, in a generator, reversing of the coil’s connection to the external circuit produces unidirectional current in the circuit. The first commutator-type direct current machine was built by Hippolyte Pixii in 1832,

30. What is an Armature?
An armature is one of the two principal electrical components of an electromechanical machine–a motor or generator. The other is the field winding, field magnet. The role of the “field” component is simply to create a magnetic field (magnetic flux) for the armature to interact with, so this component can comprise either permanent magnets, or electromagnets formed by a conducting coil. The armature, in contrast, must carry current so it is always a conductor or a conductive coil, oriented normal to both the field and to the direction of motion, torque (rotating machine), or force (linear machine). The armature’s role is two-fold: (a) to carry current crossing the field, thus creating shaft torque (in a rotating machine) or force (in a linear machine), and (b) to generate an electromotive force (“EMF”).

31. What is a Cogging Torque?
Cogging torque of electrical motors is the torque due to the interaction between the permanent magnets of the rotor and the stator slots of a Permanent Magnet (PM) machine. Also termed as detent or ‘no-current’ torque, it is an undesirable component for the operation of such a motor. It is especially prominent at lower speeds, with the symptom of jerkiness.

• It is simple and rugged in construction
• It is relatively cheap
• Induction motors require less maintenance
• Induction motor has high efficiency and reasonably good power factor
• 3-phase induction machines are self starting

• Induction motors are essentially a constant speed motor and its speed cannot be changed easily
• Induction motors always operate under lagging power factor and during light load conditions they operate at very worst power factor (0.2 to 0.4 lagging). Some of the disadvantages of poor power are increase in I2R losses in the system, reduction in the efficiency of the system. Hence some power factor correction equipments such as static capacitor banks should be placed near to these motors to deliver the reactive power to them.
• One of the main disadvantages of induction motors is that speed control of induction motors are difficult. Hence for fine speed control applications dc motors are used in place of induction motors. Due to advance in power electronics, variable frequency drives using induction motors are used in industries for speed control now a days.

34. Slip ring induction motor advantages and disadvantages compared to squirrel cage motors?

• High starting torque with low starting current
• Smooth acceleration under heavy loads
• No abnormal heating during starting
• Good running characteristics after external rotor resistances are cut out

• The initial and maintenance costs are greater than those of squirrel cage motors
• The speed regulation is poor when run with resistance in the rotor circuit

35. Methods to control speed of Wound Rotor Motors?
Answer: The speed of wound rotor motors are changed by changing the slip of the motor. This can be achieved by:

1. Varying the stator line voltage
2. Varying the resistance in the rotor circuit
3. Inserting and varying a foreign voltage source in the rotor circuit

36. Explain how Torque-Slip Characteristics vary when adding resistance to rotor circuit?
Answer: The addition of resistance to the rotor circuit does not change the value of maximum torque but it only changes the value of the slip at which the maximum torque occurs

37. Disadvantages of Star-Delta Starting of Induction motor?
Answer: In Star-Delta starting induction motor stator is connected in star connection for starting after picking up speed it is connected to delta connection. When induction motor is connected in star connection stator phase voltage reduced by 1/(31/2 ) times the line voltage. This also results in reduced starting torque (1/3 times compared to delta connection).