Question

A 220 V DC motor has an armature resistance of 0.8 Ω. Find the induced emf when the motor has a full load armature current of 40 A.
1. 220 V
2. 180 V
3. 188 V
4. 32 V

Answer 1

Correct Answer - Option 3 : 188 V

Concept:

In a DC generator, generated emf is given by

Eg = Vt + IaRa

In a DC motor, back emf is given by

Eb = V – IaRa

Where, Ia is armature current

Ra is armature resistance

Calculation:

Armature resistance (Ra) = 0.8 ohm

Voltage (V) = 220 V

Armature current (Ia) = 40 A

Back emf (Eb) = 200 V

Eb = V – IaRa

= 220 – 40(0.8)

= 188 V