Electric Motor Control

10th Edition
Publisher: CENGAGE L
ISBN: 9781133702818



Electric Motor Control

10th Edition
Publisher: CENGAGE L
ISBN: 9781133702818
Chapter 44, Problem 1SQ
Textbook Problem

Why is a starter necessary on a DC motor?

To determine

Explain the necessity of starter on a DC motor.

Explanation of Solution

A starter is an electrical device used to start and accelerate a motor. It is used to limit the inrush current.

Consider the equation of armature current in a DC motor as follows:

Ia=EEcRa (1)


E is the terminal voltage,

Ec is the counter emf,

Ra is the armature resistance.

When a motor is started, the armature is stationary at the moment and no counter EMF will be generated. Therefore, Ec=0.

As the counter emf is zero, equation (1) becomes as follows:

Ia=ERa (2)

Typically, a motor armature resistance is very low. From equation (2), the armature current is inversely proportional to the armature resistance where the armature current is high at the starting of a motor. This high current can create damage to the windings, commutator, and brushes.

Therefore, a starter is used to limit the excessive flow of current and prevent damage to the motor due to the lack of a counter emf and inductive reactance.


Thus, the necessity of starter on a DC motor is explained.

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