What operating modes are available for a synchronous motor____________?
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Answer: D. lagging, leading and unity power factors
Explanation
Adjusting the direct current field excitation changes the internal back electromotive force of the motor. Varying this excitation allows the machine to draw leading, lagging, or unity power factor currents. This unique capability makes the synchronous motor highly versatile in industrial electrical power systems. Power factor describes the phase relationship between alternating voltage and alternating current waveforms. A unity power factor means voltage and current are perfectly aligned with zero phase difference. Leading and lagging power factors indicate capacitive and inductive reactive power behaviors respectively.
Industrial facilities utilize synchronous motors to improve overall plant power factor and efficiency. Utilities often penalize factories operating with poor lagging power factors due to increased losses. Flexible power factor control helps stabilize voltage levels across large electrical distribution networks. Restricting operation to a lagging power factor ignores the benefits of over-excitation and field control. Restricting operation to a leading power factor ignores normal and under-excited operating conditions. Restricting operation to unity power factor ignores the full range of adjustable excitation capabilities.
- Subject
- Engineering
- Category
- Electrical Engineering
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- —
- Difficulty
- medium
Related questions
Q1. A synchronous machine with a small short-circuit ratio features___________?
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Answer: A. lower stability limit
A smaller short-circuit ratio means a weaker magnetic field inside the synchronous electrical machine. This weaker magnetic field reduces the overall capability to maintain synchronization under heavy loads. Consequently...
Q2. Under what conditions will a synchronous motor stall and stop:______________?
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Answer: C. excitation winding gets disconnected
Disconnecting the excitation winding of a synchronous motor removes its rotor magnetic field, causing it to lose synchronism, stall, and stop. Without rotor excitation, the synchronous motor cannot produce the necessary...
Q3. When is synchronous torque produced in a synchronous motor:
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Answer: C. only at synchronous speed
Synchronous torque relies entirely on the permanent magnetic locking between the stator and rotor fields. This magnetic locking only occurs when both magnetic fields rotate at the exact same synchronous speed. Any speed...
Q4. A synchronous motor running unloaded at a leading power factor is referred to as_________?
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Answer: C. synchronous condenser
An unloaded synchronous motor running with over-excitation draws a leading current that resembles a capacitor. Because it acts like a variable capacitor without providing mechanical shaft work, it is termed a condenser....
Q5. At what load angle does a synchronous motor deliver its maximum power____________?
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Answer: C. 90°
A synchronous motor delivers its maximum theoretical torque and power output when the load angle reaches 90 degrees. The power-angle equation dictates that power varies sinusoidally with the load angle, peaking at the si...
Q6. When a normally excited synchronous motor experiences a load increase, its armature current rises because
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Answer: D. Er (net resultant voltage) in armature is increased
When a mechanical load is applied, the rotor momentarily lags behind the rotating stator field. This mechanical lag increases the load angle between the rotor and stator magnetic fields. The larger load angle directly in...