Question

A separately excited dc generator rotating at 3000 rpm produces an emf of 157 V and delivers a current of 20 A. The braking torque exerted by the armature is
1. 17 N-m
2. 10 N-m
3. 12 N-m
4. 12.5 N-m

Answer 1

Correct Answer - Option 2 : 10 N-m

Concept:

For separately excited dc generator armature current and load current is the same.

The output power of generator = Terminal voltage × Armature current.

Torque exerted by the armature T = Power output / ω 

where ω = (2 π N) / 60

N = speed of the armature in rpm.

Calculation:

Given sped of the generator N = 3000 rpm

Terminal voltage V_t = 157 V

Delivered load current I_L = 20 A

Power developed by generator P = V_t × I_L

P = 157 × 20 = 3140 W

Torque exerted by armature T = P / ω 

where ω = (2 π N) / 60

ω = (2 π × 3000) / 60 = 314 rad / sec

∴ T = 3140 / 314 = 10 N-m