What is the function of the motor starter?

Question

Examoogle · Accepted Answer

Option D: All of these

Explanation & Concepts:
A motor starter is a specialized electrical device used to safely start, stop, and protect electric motors. At standstill (speed $N = 0$), a motor generates zero back EMF ($E_b = 0$), causing an abnormally high starting current that can damage the windings. Therefore, the starter limits this surge current and incorporates protective relays for overloads and under-voltage conditions.

Working Principle: When an electric motor is energized, the armature current is governed by $I_a = \frac{V - E_b}{R_a}$. At start, since $N = 0$, $E_b = 0$, leading to $I_a = \frac{V}{R_a}$, which can be 5 to 8 times the rated current. A starter temporarily inserts external resistance or reduces applied voltage to lower $I_a$ until the motor accelerates and builds up sufficient $E_b$. Additionally, the overload trip coil (OLC) releases the starter mechanism if current exceeds safe limits, while the no-volt coil (NVC) automatically drops out during power failure to prevent sudden restarting upon voltage restoration.

Key Formulas: $E_b = \frac{\Phi Z N P}{60 A}$ — Back EMF equation of a DC Motor; $I_a = \frac{V - E_b}{R_a}$ — Armature current equation during running condition; $I_{start} = \frac{V}{R_a}$ — Starting armature current when $N = 0$ ($E_b = 0$)

Key Points: At the instant of starting ($N = 0$), back EMF $E_b$ is zero, leading to a heavy inrush current.; A starter limits starting current to a safe value by reducing applied voltage or adding external resistance.; The No-Volt Coil (NVC) provides under-voltage protection and prevents sudden automatic restart after supply failure.; The Overload Coil (OLC) protects the motor windings against continuous thermal overloads.

Advantages: Protects motor windings from thermal damage due to heavy inrush currents.; Prevents supply line voltage dips that affect other connected equipment.; Ensures automatic disconnection during supply voltage failure via under-voltage protection.

Disadvantages: Adds initial capital cost and requires regular maintenance.; Involves slight power loss during starting in resistance-type starters.

Applications: Starting 3-phase squirrel cage and slip ring induction motors.; Starting and controlling DC shunt, series, and compound motors.; Industrial automation systems requiring safe manual or automatic motor control.

Additional Info: Common types of starters include 3-point starter, 4-point starter, Direct On Line (DOL) starter, Star-Delta starter, and Auto-Transformer starter.. Option A is correct because at $N=0$, $E_b=0$ causes high starting current which the starter restricts.. Option B is correct because starters incorporate Overload Release Coils (OLC) and No-Volt Coils (NVC).. Option C is correct because starters provide manual pushbuttons or levers to start and stop the motor.

Answer: D is correct — The motor starter performs all three functions: limiting high starting current, offering overload and under-voltage protection, and providing manual start/stop control.

Exam Tip: For competitive exams, remember that 3-point starters suffer from speed control limitations via field weakening because NVC is in series with field winding; 4-point starters resolve this by putting NVC in an independent circuit.

Examoogle · Answer

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

Examoogle · Answer

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

Examoogle · Answer

Option C: Its provide manual start and stop function

Correct Answer

Discussion (0)

Correct Answer

Related Questions

Discussion (0)