Induction Motor Interview Question Answer - 3

Important Questions of Three Phase Induction Motor

In this post, some of the important questions of three phase induction motor are given. It includes relation between gross rotor output and rotor copper loss, shaft torque and gross torque, definition of synchronous watt, back emf in the induction motor, sector induction motor, no load current in the induction motor, mechanical load on induction motor and circle diagram of induction motor.

Describe the relation between rotor copper loss and rotor input in the three-phase induction motor.

• The rotor copper loss is equal to slip times rotor input in the three-phase induction motor.

Gross Rotor Output & Rotor Copper Loss

Describe the relation between gross rotor output and rotor copper loss in the induction motor.

Gross rotor output / Rotor copper loss

                     =  ( 1 – s ) / s  

      Where s =  Slip

Shaft Torque & Gross Torque

Why the shaft torque is always less than the gross torque in the induction motor?

Shaft torque

• The shaft torque is always less than the gross torque in the induction motor because there is some loss due to friction and windage in the rotor.

Synchronous Watt

Explain : Synchronous watt in the induction motor

Synchronous watt

• The power developed across the air – gap of the induction motor is called as synchronous watt.

Back Emf in Induction Motor

What is difference between powers developing in the three-phase induction motor and that of DC motor?

Back emf in the Induction motor

• The power developed in the armature of the dc shunt motor is equal to E_b* I_a

              Where  E_b  = Back emf and

                           I_a  = Armature current

• The rotor induced emf in the induced emf decreases from  ( E₂ to sE₂ ) during rotation of the induction motor. 
• The difference between E₂ and sE₂ is equal to ( 1 – s ) E₂ is called as back emf in the induction motor.

What is direction of the flux in the sector induction motor?

• The direction of the flux in the sector induction motor is continuously from one end to the other end.

Linear Synchronous Speed

Write the equation of the synchronous speed and linear synchronous speed.

The synchronous speed is given by N_s  = 120 × f / p

           f  = Supply frequency

           P = Number of poles

The linear synchronous speed is given by V_S = 2 × ω × f

           ω = Width of pole – pitch and

           f  =  Supply frequency

Give the practical example of rotor is stationery and stator are fixed in three phase induction motor.

• Magnetic levitation train

Why the no load current in the three-phase induction is much higher than that of transformer?

No load current

• The core of the transformer consists of low reluctance path therefore magnetizing current is very small resulting no load current is small. 
• The flux passes through stator to rotor in the three-phase induction motor through air gap which consists of very high reluctance path therefore the magnetic current and hence no-load current is 40 to 50% of the full load current.

What is speed of rotating field of rotor with respect to the stator magnetic field?

• The rotating field of the rotor rotates at synchronous speed with respect to the stator magnetic field.

What is mechanical load on the induction motor at unity value of slip?

• Mechanical load at unity slip : The mechanical load in the induction motor can be represented by R₂ { ( 1 / s ) – 1 }. When the slip is equal to unity, mechanical load is zero.

       Where s = Slip and R₂ = Rotor resistance of the induction motor

What is condition for maximum power output in the induction motor?

• Condition for maximum output : The induction motor transfers maximum power when the load resistance is equal to leakage impedance of the motor.

Which losses are known as no load losses in the induction motor?

No load losses

• When the induction motor run on no load following losses occurs : stator copper loss, stator core loss and loss due to friction and windage.

Which losses are known as fixed losses? Why it is called fixed losses?

Fixed losses

• The stator core losses and friction and windage losses are collectively known as fixed losses. 
• As its name suggests that its value does not change with change in load current.

Circle Diagram of Induction Motor

Why the circle diagram of the induction motor is semi circle?

The equivalent circuit of the induction motor consists of fixed reactance and variable resistance.

• Fixed reactance as referred to stator side X₀₁
• Fixed resistance as referred to stator side R₀₁
• Variable resistance as referred to stator side R_L^’

When supply voltage V is given to induction motor, it consists of series circuit with constant reactance and variable resistance.

• The stator current vector is circle with diameter V / X₀₁

           I = V / √ ( R² + X₀₁² )

             =  ( V / X₀₁ ){ X₀₁ / √ ( R² + X₀₁² ) }

             =  ( V / X₀₁) sin Φ

As we concerned only positive stator current in the induction motor, the locus of the stator current is semi circle.

Give reason : The input power of the three-phase induction motor during no load is W1 - W2 ( Difference between two wattmeters reading ).

Induction motor on no load

• When three phase power is measured by two-watt meter method, the wattmeter reading is directly proportional to

           W₁  α cos ( 30 + Φ )

                 W₂ α cos ( 30 – Φ )

           Where Φ is power factor angle

• The no load power factor of the induction motor is very low i.e. less than 0.5 ( Power factor angle > 60⁰ ) resulting one watt meter shows negative reading.

Which losses do not depend upon load current?

• Stator core loss, friction and windage loss

Which test to be performed on the induction motor in order to draw circle diagram of the induction motor?

Test to be performed for Circle diagram

• No load test
• Block rotor test
• Stator resistance test

Function of Starter

Why starter is necessary to start induction motor?

Function of starter

• The working of the induction motor is similar to transformer with short circuit secondary. 
• When the rated voltage is applied to the induction motor, it will draw heavy current during starting period. 
• The function of the starter is to limit starting current. The overload relay in the starter circuit protects motor against overload.

Describe the methods of starting induction motor.

• Types of Starter: Primary resistor, Star – delta starter and Auto transformer starter

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