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Which of the following statements are INCORRECT about auxiliary motor starting in synchronous motors? A) The function of the auxiliary motor is to run the synchronous motor at a speed less than its synchronous speed. B) The rating of the auxiliary motor is much lower than that of the synchronous motor. C) This method is used only for loaded synchronous motors. D) Auxiliary motor starting is not a commonly used starting method in modern days.
B and D
A and D
A, C and D
A and C
A, C and D
Auxiliary motor starting is a method where a separate small motor (pony motor) brings the synchronous motor up to synchronous speed before it is synchronized to the supply grid. The incorrect statements are A (must reach synchronous speed, not less), C (usually used for unloaded machines), and D (though less common, it remains a valid technical method, and the statement is too absolute).
Auxiliary motor starting is a method where a separate small motor (pony motor) brings the synchronous motor up to synchronous speed before it is synchronized to the supply grid. The incorrect statements are A (must reach synchronous speed, not less), C (usually used for unloaded machines), and D (though less common, it remains a valid technical method, and the statement is too absolute).
NsтАЛ=P120fтАЛ тАФ where NsтАЛ is synchronous speed in RPM, f is frequency, and P is number of poles.
An induction motor or DC motor is mechanically coupled to the shaft of the synchronous motor. The auxiliary motor accelerates the synchronous machine until its speed is equal to the synchronous speed, defined by NsтАЛ=P120fтАЛ. Once synchronized, the auxiliary motor is often decoupled or remains idle, and the synchronous motor is excited to maintain magnetic locking.
The auxiliary motor must accelerate the main machine to exact synchronous speed.
The process involves bringing the rotor to synchronous speed, then applying DC excitation to establish magnetic poles.
This method avoids the high starting current associated with induction-type starting via damper windings.
It is primarily used for large synchronous motors starting under no-load conditions.
Eliminates the need for damper windings for starting purposes.
Provides smooth acceleration and precise control during synchronization.
Requires an additional machine and mechanical coupling mechanism.
Increases the overall system footprint and cost.
Complexity in switching the auxiliary motor off once synchronization is achieved.
Large industrial synchronous condensers.
Synchronous motors where damper winding design is not feasible.
Statement A is incorrect because the motor must reach synchronous speed (N=NsтАЛ) for synchronization.
Statement C is incorrect because this method is typically used for no-load starting to minimize the rating of the auxiliary motor.
Statement D is considered incorrect because 'not commonly used' is a relative and non-technical assertion that discounts its specific niche applications in modern power systems.
C is correct тАФ Statements A, C, and D are technically incorrect regarding the mechanism and application context of auxiliary motor starting.
Always remember that for successful synchronization, the frequency and phase of the motor voltage must match the supply grid, which is why reaching exactly NsтАЛ is mandatory.
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