Question

What is the armature current for a DC motor if terminal voltage is 255V and open circuit voltage is equal to 250V, where armature resistance is 0.05Ω?

(a) 10A

(b) 100A

(c) 1KA

(d) 1A

This question was posed to me in my homework.

Query is from Electromagnetic Power and Circuit Models in section Circuit Model, Emf and Torque of DC Machines

Answer 1

The correct choice is (b) 100A

The explanation: Open circuit voltage means the voltage at armature current equal to 0, i.e. Vt=Ea= 250V. Actual Vt at loaded condition is 255V. For motoring mode, Vt – Ea = Ia*Ra. By substituting all the given values, we get Ia= 100A.