Worksheet: Generators, Motors, and Back emf

In this worksheet, we will practice calculating the emf induced by the motion of an electric motor and the effect back emf has on the power output of a motor.

Q1:

A series-wound dc motor draws a current of 0.50 A from its 120 V power source when it operates at full speed. The resistance of the armature coils of the motor is 10 Ω.

What is the resistance of the motor’s field coils?

What is the back emf of the motor when it is operating at full speed?

The motor operates at a different speed, at which it draws a current of 1.0 A from the its power source. What is the back emf of the motor when it operates at this speed?

Q2:

A series-wound motor of 120 V has a field resistance of 80 Ω and an armature resistance of 10 Ω. When it is operating at full speed, a back emf of 75 V is generated.

What is the initial current drawn by the motor?

When the motor is operating at full speed, what is the current drawn by the motor?

When the motor is operating at full speed, what is the power output of the source?

When the motor is operating at full speed, what is the power output of the motor?

When the motor is operating at full speed, what is the total power dissipated in the two resistances?