Induction Motor Interview Question Answer - 1

Important Questions of Three Phase Induction Motor

In this post, some of important questions of three phase induction motor are given. It includes induction motor as rotating transformer, conduction motor and induction motor, advantages of induction motor, synchronous speed, actual speed, types of induction motor, skewing of rotor slots, rotor winding in the induction motor, speed of rotor magnetic field, rotor frequency, starting torque and running torque in the induction motor.

Rotating Transformer

Why the induction motor is called as rotating transformer?

Rotating Transformer

• The power transfers from stator to rotor in the induction motor is same as that of in the transformer ( from primary side to the secondary side ) but the rotor of the induction motor rotates. 
• Therefore, the induction motor is called as rotating transformer.

Conduction Motor & Induction Motor

What do you mean by the conduction motor and induction motor?

Conduction motor

• The power transfer in the armature of the DC motor through brushes and commutator therefore it is called as conduction motor.

Induction motor

• The power transfers in the induction motor stator to rotor via air gap or inductively so it is called as induction motor.

Describe the advantages of induction motor over DC motor.

Advantages of induction motor over DC motor

• Simple and rugged construction
• Low cost
• No brushes are required so friction losses are reduced
• Low maintenance
• Sparkless operation

Synchronous Speed & Actual Speed

Define: Synchronous speed, Actual speed

Synchronous speed

• The speed of the stator magnetic field in the three-phase squirrel cage induction motor is called as synchronous speed.

             Synchronous speed N_S = 120f / P

             f = Supply frequency

             P = Number of poles

Actual speed

• The speed of the rotor in the induction motor is called as actual speed. It is given by N = N_S ( 1 – s )

             N = Rotor speed

             N_S = Synchronous speed and

             s = Slip

Describe the different types of induction motor.

Types of induction motor

• Squirrel cage induction motor
• Slip ring induction motor

Skewing Rotor Slots

Why rotor slots are kept slight skew to the rotor shaft in the squirrel cage rotor?

The rotor slots are not quite parallel to the rotor shaft due to following reasons

• Motor run quietly by reducing magnetic hum.
• To reduce locking of rotor teeth to the stator teeth.

Which type of rotor construction is used in small rating squirrel cage induction motor?

Rotor construction

• The small rating squirrel cage induction motor rotor consists of entire rotor core in the mould and casting all the bars and end – rings in the one piece.

Whether rotor winding is wound for two phases or three phases when the stator winding is wound for two phase supply?

• The rotor is always wound with three phase even when stator is wound with two phase or three phase

Describe the position of slip – rings and brushes in the wound rotor induction motor under running condition?

            OR                                                        

Explain: The working of the phase wound induction motor is same as that of Squirrel cage induction motor under running condition?

• The slip rings of the wound rotor induction motor in the running condition are automatically short circuited by means of a metal collar which is pushed along the shaft and connects all the slip –rings are together.
• The brushes are automatically lifted from the slip – rings to reduce the wear tear and friction loss. 
• Therefore, the wound rotor induction motor works as a squirrel cage induction motor under running conditions.

Which material is used for slip – ring in the wound rotor induction motor?

• Phosphorous – bronze

When the three-phase supply is given to stator winding, resultant flux rotates around the stator at synchronous speed…. True or false. Explain the above statement.

Revolving flux

• There is no actual revolving of the flux. The flux due to each phase change periodically according to change in the phase current but the magnetic flux itself does not move around the stator.

How the rotor does rotate in the three-phase induction motor?

Rotation of rotor

• When a three phase supply is given to induction motor, a magnetic field of constant magnitude is produced. 
• The magnetic field rotates at synchronous speed. The rotor conductor cuts this magnetic field via air gap. 
• The emf induced in the rotor according to faraday’s law of electromagnetic induction. 
• The magnitude of induced emf is directly proportional to relative velocity between two flux and the conductors. 
• The direction of the induced emf is given by Fleming’s right hand rule.
• As the rotor itself closed circuit, current passing through it. 
• According to Fleming’s left hand force act on rotor conductors which is same as that of direction of stator magnetic field. 
• Therefore, the rotor rotates is in the same direction as that of stator magnetic field.

Give reason: The induction motor rotor speed is always less than the stator magnetic field.

Speed of rotor

• If the rotor magnetic field and stator magnetic field rotates same speed then there is no relative speed between them resulting no rotor emf and hence no rotor current and no torque. 
• Therefore, speed of the rotor is always less than the stator magnetic field.

What is speed of stator and rotor magnetic speed?

• The stator and rotor field rotates synchronously to each other it means that they are stationary with respect to each other.

Rotor Frequency

What is frequency of rotor current in the induction motor?

Frequency of rotor current

• The frequency of the rotor current is slip times the supply frequency.
•  Therefore f ’ = sf

         f ’ = Rotor frequency

           s = Slip

          f = Stator supply frequency

Starting torque and Running torque

What is meaning of the starting and running torque in the induction motor?

Starting torque

• The torque developed is the induction motor at starting is called as starting torque of the induction motor. It is roughly 1.5 to 2 times that of full load torque.

Running torque

• The torque developed in the induction motor during running condition is called as running torque.

Describe the effect of rotor power factor on the induction motor torque.

Effect of rotor power factor on rotor torque

• The torque in the induction motor is given by T = kE₂I₂ Cos Φ₂

             Cos Φ₂ = Rotor power factor

• As the rotor power factor increases the torque increases and vice versa

How the starting torque of the slip – ring induction motor can be increased?

• The starting torque of the slip – ring induction motor can be increased by adding external resistance in the rotor circuit by slip – rings.

Describe the effect of rotor torque in the induction motor when rotor is ( a ) Purely resistive ( b ) Purely inductive

• Rotor – Purely resistive: The torque is always positive.
• Rotor – Purely inductive: The backward and forward torque have equal and opposite value resulting total torque becomes zero.

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