This question was previously asked in

UGC NET Paper 2 (Electronic Science) December 2019 Official Paper

Option 4 : For a speed more than the base speed of a DC motor, the armature voltage is varied to control the speed while armature and field currents are maintained constant

General Awareness Subject Test 1

7367

20 Questions
20 Marks
20 Mins

The relationship given below gives the speed of a D.C. motor:

\(N = \frac{{V - {I_a}{R_a}}}{{K\varphi }}\)

From this equation we can say that the speed of a D.C. motor depends on:

- Supply voltage, V
- The armature circuit resistance, R
_{a} - The field flux (φ) which is produced by the field current

__Observations__**:**

- We can control the speed of the D.C motors by varying the applied voltage to the armature. Hence statement (1) is correct.
- Since the field current produces the flux, and if we control the field current the speed can be controlled. Therefore, Statement (2) is correct.
- The speed of a DC motor can be varied by controlling the torque demand, which corresponds to an armature current (I
_{a}) for a fixed field current (I_{f}). Hence Statement (3) is correct. - For a speed, less than the base speed, the armature current, and field currents are maintained constant to meet the torque demand and the armature voltage (V
_{a}) is varied to control the speed. **For a speed more than base speed, the field current is varied to control the speed while armature voltage is maintained constant.**

∴ Statement (4) is incorrect.