Which of the following statements about the speed control of DC motors is correct?

Question

Question

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SSC JE Electrical 15 Nov 2022 Shift 2 Official Paper

The flux control method is otherwise called constant torque drive method.
In the speed control by varying field flux, the speed can be regulated at the base speed.
The field flux control is otherwise called constant power drive method.
The armature resistance method of speed control is employed to obtain speed above the base speed.

Answer 1

The correct answer is option 3):(The field flux control is otherwise called constant power drive method.)

Concept:

The speed of a DC motor can control in three ways

Armature resistance control:

We know that,

N ∝ \(Eb\over ϕ\)

N ∝ \(V−Ia(Ra+R)\over ϕ\)

We can control the speed by increasing armature resistance.

This method gives only below base speeds. In this method, the motor acts as constant torque and a variable power drive

Field control method:

We know that,

N ∝ \(Eb\over ϕ\)

N ∝ \(V−Ia(Ra+R)\over ϕ\)

By varying flux or field current, we can increase the speed more than its base speed.
In this method, the motor is a constant power and variable torque drive.The field flux control is otherwise called constant power drive method. option 3 is correct
Also known as a tapped field control method, especially for DC series motor.

Armature voltage control:

We know that,

N ∝ \(Eb\over ϕ\)

N ∝ \(V−Ia(Ra+R)\over ϕ\)

By controlling the armature voltage below the rated value, the speed can be controlled only below the rated value.
Because the voltage can't be above the rated value due to the insulation failure of winding.
This method is used for the separately excited motor but not suitable for the shunt motor because flux will change due to a change in voltage.