Question

Choose the incorrect statement.
1. The speed of a DC motor can be varied by controlling armature voltage
2. The speed of a DC motor can be varied by controlling the field current
3. The speed of a DC motor can be varied by controlling the armature current that is a measure of the torque demand
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

Answer 1

Correct Answer - 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

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.