The armature of an electric drill motor has a resistance of 15. When connected to a 120.0 V outlet, the motor rotates at its normal speed and develops a back emf of 108 V. What is the current when the motor runs at only half speed?

Question

The armature of an electric drill motor has a resistance of $15 Ω$ . When connected to a 120.0 V outlet, the motor rotates at its normal speed and develops a back emf of 108 V. What is the current when the motor runs at only half speed?

Smart Achievers · Accepted Answer

(2)

$At half speed, back emf reduced to half e = \frac{B ω r^{2}}{2}$

$So i = \frac{120 - 54}{12} = \frac{66}{15} = 4.4$

Solution

Q.
The armature of an electric drill motor has a resistance of $15 Ω$ . When connected to a 120.0 V outlet, the motor rotates at its normal speed and develops a back emf of 108 V. What is the current when the motor runs at only half speed

1 0.3 A

2 4.4 A

3 1.2 A

4 2.0 A

Ans.
(2)

$At half speed, back emf reduced to half e = \frac{B ω r^{2}}{2}$

$So i = \frac{120 - 54}{12} = \frac{66}{15} = 4.4$

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