What is the role of the starter in 3-phase Induction Motor?

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Examoogle · Accepted Answer

Option D: All of these

Explanation & Concepts:
A starter in a 3-phase induction motor is primarily used to safely start the motor by limiting the high initial inrush current, which can be 5 to 7 times the full-load current. Additionally, it incorporates protective devices to safeguard the motor against overload and under-voltage conditions. Consequently, limiting the starting current prevents excessive line voltage drops that could disrupt other electrical equipment connected to the same supply.

Working Principle: At the instant of starting, the rotor of an induction motor is stationary, making slip $s = 1$. This acts like a short-circuited transformer, causing a very high starting current ($I_{st} = \frac{V_{ph}}{Z_{e1}}$) to flow through the stator windings. Starters introduce reduced voltage (e.g., Star-Delta or Autotransformer) or additional resistance (e.g., Slip-Ring Rotor Starter) during startup to bring the current down to safe limits. Built-in Overload Relays (OLR) and No-Volt Coils (NVC) automatically disconnect the motor during overcurrent or power failure.

Key Formulas: $I_{st} = \frac{V_{ph}}{Z_{e1}}$ — Direct-on-Line (DOL) starting current; $I_{st(line)} = \frac{1}{3} I_{DOL}$ — Line starting current in Star-Delta starter; $T_{st} \propto V^2$ — Starting torque proportional to the square of applied stator voltage

Key Points: At standstill ($s=1$), back EMF is zero, causing the motor to draw a heavy initial inrush current.; A Direct-On-Line (DOL) starter is generally restricted to small motors (below 5 HP) as it does not reduce the starting current.; Star-Delta and Autotransformer starters reduce the applied stator voltage to limit the starting current.; Overload relays (OLR) protect windings from continuous overcurrent and overheating.; No-Volt Coils (NVC) disconnect the motor during supply failure, preventing unexpected self-restarting when power restores.

Advantages: Protects motor windings from thermal stress caused by excessive current.; Prevents severe voltage dips in the feeder supply lines, protecting adjacent equipment.; Provides automated thermal overload and under-voltage protection.; Ensures operational safety for equipment operators.

Disadvantages: Reduces starting torque since torque varies as the square of the applied voltage ($T \propto V^2$).; Adds initial equipment cost, complexity, and maintenance requirements.

Applications: DOL Starter: Small squirrel-cage induction motors (; Star-Delta Starter: Medium-capacity induction motors (5 HP to 20 HP).; Autotransformer Starter: High-capacity squirrel-cage motors (> 20 HP).; Rotor Resistance Starter: Slip-ring (wound rotor) induction motors requiring high starting torque.

Additional Info: Standard industrial practice dictates using reduced-voltage starters for motors exceeding 5 HP rating.. Option A is correct because high current produces thermal stress and line disturbance.. Option B is correct because starters integrate OLR and NVC coils for protection.. Option C is correct because heavy inrush currents cause line drop $\Delta V = I_{st} Z_{line}$, triggering malfunctioning in parallel loads.. Therefore, Option D (All of these) is the correct answer.

Answer: D is correct — The starter limits the heavy starting current, provides overload and under-voltage protection, and prevents line voltage drop across the power supply.

Exam Tip: Always remember that for reduced voltage starting methods (like Star-Delta), both starting current and starting torque drop by a factor of 3 ($I_{st} = \frac{1}{3} I_{DOL}$ and $T_{st} = \frac{1}{3} T_{DOL}$).

Examoogle · Answer

Option A: To limit Heavy starting current

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Option B: To provide Overload and Under-voltage Protection

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Option C: To avoid malfunctioning produces due to a large voltage drop in the line

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