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Which of the following statements is FALSE in the context of the characteristics of synchronous motors?
They have constant speed operation.
Synchronous motors have high power factor correction.
They have low operating efficiency.
They are made in large sizes.
They have low operating efficiency.
Synchronous motors are characterized by very high efficiency, often exceeding 95% in large capacity machines, due to the absence of slip-related losses. Therefore, stating they have low operating efficiency is technically incorrect and contradicts their performance profile in industrial applications.
IS 2253:1974
Synchronous motors are characterized by very high efficiency, often exceeding 95% in large capacity machines, due to the absence of slip-related losses. Therefore, stating they have low operating efficiency is technically incorrect and contradicts their performance profile in industrial applications.
NsтАЛ=P120fтАЛ тАФ Synchronous speed formula
PinтАЛ=3тАЛVLтАЛILтАЛcos╧Х тАФ Input power formula
A synchronous motor operates based on the magnetic locking between the stator revolving magnetic field and the rotor poles. Since the rotor runs at the same speed as the stator field (NsтАЛ=P120fтАЛ), there is no rotor copper loss associated with slip, contributing to superior efficiency compared to induction motors.
Synchronous motors maintain a constant speed regardless of load variations until the pull-out torque is reached.
By adjusting excitation, the motor can operate at leading, lagging, or unity power factor, serving as a synchronous condenser.
They are typically constructed in large sizes for industrial drives requiring high power and precise speed control.
High efficiency is a result of low rotor losses since the excitation is provided by a DC source.
Ability to improve overall plant power factor.
Constant speed operation irrespective of load.
High efficiency in large capacity units.
Requires a separate DC excitation source.
Not inherently self-starting; requires auxiliary starting methods.
Complex control circuitry compared to induction motors.
Power factor correction (Synchronous Condenser).
Large industrial constant speed drives (Centrifugal pumps, compressors, and blowers).
Option A is true as speed is locked to supply frequency.
Option B is true; over-excited synchronous motors provide reactive power to the grid.
Option D is true; synchronous motors are typically used in the MW range.
C is correct тАФ Synchronous motors are known for high operating efficiency, not low efficiency.
Always remember that synchronous machines act as 'Synchronous Condensers' when over-excited, providing reactive power to the system to improve the power factor.
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