Chapter 25, Problem 68A

To determine

To Explain: Why the initial start-up current is high in an electric motor and describe how the Lenz’s law is applied at the instant the electric motor starts.

Answer to Problem 68A

The initial start-up current is high in an electric motor as no magnetic field is formed around the stationary conductor because of absence of current in the armature.

Explanation of Solution

Introduction:

Lenz law states that the induced current will oppose the change in current. The law follows the conservation of energy.

The EMF that will be produced if the conductor doesn’t rotate will be nil. Also, there is no induced magnetic field because of the zero current in the armature.

The instant at which the armature will begin to rotate, it will be cutting the lines of force and will have an induced potential difference. The induced EMF will have a polarity in such a way that it produces a magnetic field that would opposes the field that created it. This results in reduction of the current in the motor. Thus the motion of the motor increases its apparent resistance.

Conclusion:

Therefore, the initial current is high in an electric motor due to absence of current around the armature in the motor.