A circular armature coil is used in a motor with a uniform magnetic field of 4.1

Question

A circular armature coil is used in a motor with a uniform magnetic field of 4.15 T. The coil has a diameter 95 cm, a resistance of 25 , and generates a back emf of 94 VRMSwhen running a full speed under no load at the angular frequency A.
(A) At what angular frequency, B relative to A does the coil be need to be rotated to generate a back emf of 74 VRMS?

B =  A

(B) If this motor is operating on 120 VRMS now with a load (e.g running a fan) and at the angular frequency A, how much current is flowing throught the motor?

A

Explanation / Answer

On part A you'll just want to take the rotated emf value (smaller Vrms) and divide it by the given back emf of the coil at full speed under no load at A (larger Vrms).

On part B just take the 120 Vrms and subtract the A emf value at full speed under no load and then divide by the resistance.

So,

for (A): 74/94 = 0.787

And for

(B): (120-94)/25 = 1.04

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