Why is direct line-starting discouraged for induction motors in favor of starters__________?
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Answer: A. motor takes five to seven times its full load current
Explanation
At standstill, zero rotor speed creates maximum slip and high transformer action currents. Connecting the motor directly to the line draws massive starting current surges. These high currents typically range between five and seven times normal full load values. Direct-on-line starting applies full line voltage instantly to stationary motor stator windings. High starting currents can cause severe voltage sags across local electrical distribution systems. Windings must be thermally robust to withstand high starting current surges without failing.
Starters and variable frequency drives are mandatory for large motors to limit starting current. Electrical utilities enforce strict current limits on industrial motor startups to protect grids. Small fractional horsepower motors are often the only units permitted for direct-on-line starting. Option b is incorrect because motors accelerate smoothly rather than going out of step instantly. Option c is incorrect because phase sequence determines rotation direction, not starting method. Option d is incorrect because starting torque is often relatively low despite high current draw.
- Subject
- Engineering
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- Electrical Engineering
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- —
- Difficulty
- medium
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