The reason for using starter while starting of DC motor is

Question

Examoogle · Accepted Answer

To restrict armature current as there is no back E.M.F at starting

Explanation: At the moment of starting, a DC motor is stationary, meaning the armature speed is zero. Consequently, the back E.M.F ($E_b = \frac{P\Phi NZ}{60A}$), which is directly proportional to speed, is zero, causing the armature to act as a short circuit with very low resistance ($R_a$).

Working Principle: According to the voltage equation of a DC motor, $V = E_b + I_aR_a$. At the start, $E_b = 0$, so the starting armature current is $I_{st} = \frac{V}{R_a}$. Since $R_a$ is very small, this current is dangerously high and can damage the commutator, brushes, and windings. A starter inserts external resistance in series with the armature to limit $I_{st}$ until the motor gains speed and generates sufficient $E_b$.

Key Formulas: $E_b = \frac{P\Phi NZ}{60A}$ — Back EMF equation; $I_a = \frac{V - E_b}{R_a}$ — Armature current equation

Key Points: A DC motor is self-starting due to the interaction of flux and armature current.; The primary function of a starter is to limit the high initial inrush current.; As the motor accelerates, $E_b$ increases, allowing the external resistance to be gradually cut out.; The starting current can be 10 to 20 times the rated current without a starter.

Advantages: Protects armature windings from thermal damage; Prevents severe sparking at the commutator; Prevents excessive voltage dip in the supply lines

Disadvantages: Adds cost and complexity to the motor starting system; Requires manual or automatic control maintenance

Applications: DC Shunt Motors; DC Series Motors; Compound Wound DC Motors

Additional Info: Common starter types include 3-point starters for shunt motors and 4-point starters for compound motors.. Option B is incorrect because DC motors are inherently self-starting due to the Lorentz force.. Option C is incorrect because limiting the current incidentally limits the torque, but the primary protective purpose is current restriction.

Answer: A is correct — The starter provides an external resistance to limit the high inrush current $I_a = \frac{V}{R_a}$ that occurs when $E_b = 0$.

Exam Tip: Always remember that for a DC motor, $E_b$ acts as a self-regulating mechanism; as speed increases, $E_b$ rises, which automatically reduces $I_a$.

Related Questions

Discussion (0)