Which of the following is not a function of a motor starter?

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Examoogle · Accepted Answer

Option C: It provides high starting torque to the motor

Explanation & Concepts:
A motor starter is a protective and switching device used to safely start and stop electric motors. Its main functions are to limit the high initial starting current and provide protection against overloads and under-voltage conditions. A starter does not inherently increase the motor's starting torque; in fact, reduced-voltage starters actually decrease the starting torque.

Working Principle: At standstill (speed $N = 0$), the back EMF ($E_b = \frac{\Phi Z N P}{60 A}$) in a motor is zero. Since armature resistance ($R_a$) is very small, applying full line voltage directly results in a huge starting current ($I_s = \frac{V - E_b}{R_a} = \frac{V}{R_a}$). A starter inserts variable resistance or reduces voltage at startup to limit this current to a safe value until the motor speeds up and builds $E_b$.

Key Formulas: $E_b = \frac{\Phi Z N P}{60 A}$ — Back EMF equation of a motor; $I_s = \frac{V - E_b}{R_a + R_{ext}}$ — Motor starting current with starter resistance; $T_s \propto I_s^2$ or $T_s \propto V^2$ — Starting torque relationship (torque decreases if starting voltage/current is reduced)

Key Points: At startup, speed $N = 0$, so back EMF $E_b = 0$, leading to high inrush starting current (typically 5 to 8 times rated current).; A motor starter limits this heavy starting current to protect armature windings and prevent line voltage drops.; Reduced voltage starting methods (like Star-Delta or Autotransformer starters) actually reduce the starting torque ($T \propto V^2$).; Starters provide protective functions such as No-Volt Release (NVR) and Overload Release (OLR).

Advantages: Protects motor windings from thermal damage due to excessive starting current.; Prevents severe voltage dips in the supply network during motor startup.; Provides safety protection against power failure (under-voltage) and overload conditions.

Disadvantages: Adds extra cost, complexity, and maintenance requirements to the motor system.; Reduced-voltage starters lead to a reduction in starting torque, which may not suit heavy load startup.

Applications: Used with 3-phase induction motors (Star-Delta, Direct-On-Line, Autotransformer starters).; Used with DC motors (3-Point, 4-Point starters) to limit armature starting current.

Additional Info: Option A is a valid function: At standstill ($N = 0$), $E_b = 0$, so limiting starting current is a primary function.. Option B is a valid function: Overload Relay (OLR) and No-Volt Coil (NVC) provide overload and under-voltage protection.. Option D is a valid function: Pushbuttons/switches on the starter facilitate safe manual start and stop operations.. Option C is incorrect because starting torque $T_s$ is proportional to $V^2$ or $I_s$; reducing voltage/current decreases starting torque rather than increasing it.

Answer: C is correct — Providing high starting torque is not a function of a motor starter; in fact, reduced-voltage starting techniques decrease starting torque.

Exam Tip: Remember that starting torque $T_s \propto V^2$ in induction motors. When using a Star-Delta starter, line voltage per phase drops by a factor of $\frac{1}{\sqrt{3}}$, reducing starting torque to $\frac{1}{3}$ of its direct-on-line value.

Examoogle · Answer

Option A: To prevent high starting current due to $N = 0$

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Option B: It provides overload and under-voltage protection (if overload coil used)

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Option D: It provides a manual start and stops function

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