## 08/03/2020

### DC Motor Interview Question Answer - 1

At what principle the DC motor operates?

Principle of DC motor

When a current carrying conductor is placed in the magnetic field, it experiences a mechanical force whose direction is given by Fleming’s left hand rule and magnitude is given by

F = BILsinθ

F = Force in Newton

B = Maximum flux density in Wb / m2

I = Current in Ampere

L= Length of conductor in meter

θ = Angle between position of conductor and direction of magnetic field

State the Fleming’s left hand rule.

Fleming’s left hand rule

If first finger shows a direction of magnetic field, middle finger shows direction of current then thumb indicates direction of force.

Describe the function of commutator in the DC motor.

Function of commutator

The function of the commutator is to convert alternating current into continuous and unidirectional current and also torque.

Explain: Magnetic drag or backward force in the DC generator

Magnetic drag

When the armature rotates in the influence of magnetic field of poles, it experiences a force whose direction is opposite to that rotation of the armature. This opposition force is known as backward force or magnetic drag in the DC generator.

What is opposition force produces in the motor and generator?

The force of opposition in the

DC motor: Back emf

DC generator: Magnetic drag

Define: Back emf

Back emf

When supply is given to the DC motor, dynamically induced emf is produced in the armature conductors.  The direction of the dynamically induced emf is opposite to the main supply voltage therefore it is called as back emf.

The armature torque in the DC motor is due to force on the armature conductors - True or False. Comment on the above statement

False, The torque in the DC motor is not due to mechanical force on the armature conductor but actually it is due to tangential pull on the armature teeth.

Describe the effect on armature current due to change of speed in the DC motor.

The armature current is given by

Ia = V – Eb / Ra

The back emf is given by

Eb = ФZNP / 60 × A

From the above equation

When speed of the DC motor increases, Eb increases and armature current decreases. When speed of the DC motor decreases, Eb increases and armature current increases.

Explain the term: Motor efficiency

Motor efficiency

The motor efficiency is given by ratio of power developed in the armature to the input power to the DC motor.

Describe the condition for maximum power transfer in the DC motor.

Condition for maximum power

When the back emf is equal to half of the supply voltage, armature develops maximum power in the DC motor.

Explain the term: Lost torque

Lost torque

The difference between armature torque and shaft torque is known as lost torque.

Explain: Speed regulation

Speed regulation

It is defined as the change in the speed of motor from no load to the full load at a percentage of full load speed.

Speed regulation = ( N0 – Nf ) / N0 × 100 %

Where N0 = No load speed

Describe the relation between armature torque and armature current for DC shunt motor and DC series motor.

Torque in the DC motor is Ta α ФIa

DC shunt motor

The flux is practically constant if supply voltage is constant therefore armature torque is directly proportional to armature current.

Ta α Ia

DC series motor

The series field winding also carries armature current therefore flux is directly proportional to armature current up to point of saturation Ф α Ia

Therefore    Ta α Ia2

After saturation Ta α Ia

The speed in the DC motor depends upon torque and vice versa - True or False

The speed in the DC motor depends upon torque, but torque does not depend upon the speed.

Give reason: The DC series motor never start at no load.

DC series motor at no load

The speed of DC motor is inversely proportional to field flux at constant voltage and at any given armature current.

N α Eb / Ф

As the armature winding and field winding both are in series, the field flux is only due to residual flux in the field winding which is very small at no load. Therefore the DC series motor runs at dangerously high speed particularly at no load. Hence the DC series motor never runs at a load.

What is relation between torque and armature current at light load and heavy load in the DC series motor?

Torque – armature current characteristic

The field winding of the DC series motor also carries the armature current therefore the torque is directly proportional to square of the armature current before saturation at light load. The flux in the DC series motor independent armature current after saturation therefore the torque is directly proportional to armature current. The torque – armature current characteristic is parabola at light load and straight line at heavy load.

The flux in the DC shunt motor is constant. - comment on above statement

Flux in the DC shunt motor

The field winding of the DC shunt motor is connected to the parallel of the supply voltage.  If the supply voltage is kept constant, the field flux is also constant. However the value of field flux slightly decreases at higher load condition due to de – magnetizing effect of armature reaction.

The DC shunt motor is constant speed motor. – comment on above statement

DC shunt motor – Constant speed motor

The speed of DC shunt motor depends upon back emf and field flux.

N α Eb / Ф

However the back emf and flux both decrease with increase in the load. But the back emf decrease slightly more than that of field flux. Therefore there is some decrease in the speed at higher load condition.

The drop in the speed from 5% to 10% of full load condition and it depends upon armature reaction, saturation and brush position.

Explain: Cumulative compound generator, Differential compound generator

Cumulative compound generator

If the direction of the series flux is in the same direction that of shunt flux, the DC motor is called as cumulative compound motor. As the both fluxes are in the same direction, the total flux increases with increase in the load and vice versa.

Differential compound generator

If the direction of the series flux is opposite to that of shunt flux, the DC motor is called as differential compound motor.  As the both fluxes are in the opposite direction, the total flux decreases with increase in the load and vice versa.

Which type of DC motor is suitable for pulsating load? Why?