Question

A dc shunt generator has a speed of 800 rpm when delivering 20 A at 220 V. If this machine is run as a motor and takes a line current of 20 A at 220 V supply, the speed will be:-
1. 800 rpm
2. more than 800 rpm
3. less than 800 rpm
4. zero

Answer 1

Correct Answer - Option 1 : 800 rpm

Concept:

In a DC shunt generator, generated emf is given by

Eg = Vt + IaRa

In a DC shunt motor, back emf is given by

Eb = V_t – IaRa

Where Ia is armature current

Ra is armature resistance

N is speed 

E ∝  N

Given:

Speed of generator (N_g) =  800 rpm

Terminal voltage (V_t) = 220 V

Armature resistance (Ra) = 0 Ω 

Armature current (I_a) = 20 A

Eg = V + IaRa

= 220 + 20 (0) = 220 V

Eb = V - IaRa

= 220 - 20 (0) = 220 V

\(\frac{E_g}{E_b}=\frac{N_g}{N_m}\)

\(\frac{E_g}{E_m}=\frac{220}{220} = 1\)

E_m = E_g = 800 rpm