A. No load with over-excited fields
B. No load with under-excited fields
C. Normal load with minimum excitation
D. Normal load with zero excitation
Answer: A. No load with over-excited fields
Explanation:
When the Synchronous motor is operated at no load with overexcitation, it takes current that leads the voltage by nearly 90° which means the motor works like a capacitor.
When operated under this condition, the synchronous motor is called a synchronous capacitor or synchronous compensator.
A synchronous compensator is an overexcited synchronous motor running on no load. It can generate or absorb reactive power by changing the field current. In overexcitation mode, it delivers inductive power or absorbs capacitive VAr.
While in under excitation mode, it delivers capacitive power or absorbs inductive VAr. Specially designed synchronous machines, called synchronous condensers or synchronous capacitors, do not consist of any shaft coming out through the frame so that no load could be connected with it.
Applications of Synchronous Motor
Synchronous motors find extensive application in the following classes of services.
(1) Power Factor Correction
Overexcited synchronous motors operate at leading power factor and hence they are widely used for improving the power factor of those power systems which are connected with various induction motors and other lagging loads like welders and fluorescent lights, etc.
However, nowadays, static capacitors are widely used for power factor correction because they are more economical than synchronous capacitors.
(2) Constant Speed Application
A synchronous motor runs at synchronous speed.
Therefore, this motor is a perfect choice where constant speed is necessary like in centrifugal pumps, belt-driven compressors, blowers, line shafts, rubber and paper mills, etc.
(3) Voltage Regulation
When large inductive loads are connected, the voltage at the end of a long transmission line changes a great deal.
As such, if a particular load is disconnected suddenly, the voltage may increase greatly going above the normal operating voltage which can damage the connected equipment. Using a synchronous motor with the field regulator, this voltage can be limited to a prescribed value.
Moreover, when the line voltage decreases due to inductive load, by increasing motor excitation the power factor is increased compensating for the line drop.
Similarly, if the line voltage increases due to the line capacitive effect, by decreasing the motor excitation its pf is made lagging, thus maintaining the line voltage at its normal value.