Exam Question
Q3
What is soft starting of an induction motor? Describe with a circuit using thyristors used for soft starting. Discuss its advantages and dis-advantages.
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ETO
ETO Exam papers
📅 Jan 2026
Reference Answer
### 1. Definition of Soft Starting
Soft starting is a method of reduced voltage starting for AC induction motors. Its primary purpose is to mitigate the issues associated with Direct-On-Line (DOL) starting, which connects the motor directly to the full supply voltage. DOL starting results in:
* **High Inrush Current:** A starting current that can be 5 to 8 times the Full Load Current (FLC). This causes significant voltage dips in the ship's electrical system, potentially affecting other sensitive equipment and overloading the generator.
* **High Mechanical Stress:** An abrupt application of high starting torque (1.5 to 2.5 times full load torque) creates a mechanical shock on the motor shaft, couplings, gearboxes, and the driven load. This can lead to premature wear and failure.
Soft starting is a method of reduced voltage starting for AC induction motors. Its primary purpose is to mitigate the issues associated with Direct-On-Line (DOL) starting, which connects the motor directly to the full supply voltage. DOL starting results in:
* **High Inrush Current:** A starting current that can be 5 to 8 times the Full Load Current (FLC). This causes significant voltage dips in the ship's electrical system, potentially affecting other sensitive equipment and overloading the generator.
* **High Mechanical Stress:** An abrupt application of high starting torque (1.5 to 2.5 times full load torque) creates a mechanical shock on the motor shaft, couplings, gearboxes, and the driven load. This can lead to premature wear and failure.
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