Chapter 44, Problem 1SQ

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:

$I_{\text{a}}=\frac{E-E_c}{R_{\text{a}}}$ (1)

Here,

$E$ is the terminal voltage,

$E_{\text{c}}$ is the counter emf,

$R_{\text{a}}$ is the armature resistance.

When a motor is started, the armature is stationary at the moment and no counter EMF will be generated. Therefore, $E_{\text{c}}=0$.

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

$I_{\text{a}}=\frac{E}{R_{\text{a}}}$ (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.

Conclusion:

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