Current drawn by a motor operating at half speed.


i’m unsure of how to complete the final part of this question;

1. The problem statement, all variables and given/known data

A motor is designed to operate at 118V and draws a current of 12A when it first starts up. At it’s normal operating speed, the motor draws a current of 2A. Compute:

a) the resistance of the armature of the coil.

b) the back emf developed at normal speed.

c) the current drawn by the motor at half the normal speed.

2. Relevant equations
3. The attempt at a solution

I have completed parts a and b.

a) [itex]V = IR[/itex]

[itex]R = \frac{V}{I} = \frac{118}{12} = 9.83 \Omega[/itex]

b) [itex]2A[/itex] flow through the armature with resistance [itex]9.83 \Omega[/itex].

[itex]V = IR = 19.66V[/itex]

Supply voltage = back emf + voltage across armature

back emf = 118 – 19.66 = 98.34V

But how do I figure out c?


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