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Synchronous Motor: Features, Applications, Advantages and Disadvantages
Let us first take a look at the block diagram of a synchronous motor −
Main Features of Synchronous Motor
Following are the characteristic features of a synchronous motor −
A synchronous motor either runs at synchronous speed or not at all, i.e., while running, it maintains a constant speed from no-load to full load.
The speed of a synchronous motor is independent of load.
Synchronous motors are not inherently self-starting. Some auxiliary means have to be provided for starting.
A synchronous motor will stall if, while running, the load on the shaft is increased beyond the maximum limit that the machine can drive.
A synchronous motor can be made to operate under wide range of power factor both lagging and leading.
Applications of Synchronous Motor
Synchronous motors are used in the following applications −
Synchronous motors were primarily used in constant speed applications. But, with the development of solid-state variable frequency drives like inverters and cyclo converters has allowed their use in variable speed applications also.
The synchronous motors are particularly used for low speed (below 300 RPM) applications because at low speed, the power factor can be adjusted to unity and hence the efficiency is high.
An over-excited synchronous motor (called synchronous condenser) can be used to improve the overall power factor of the plant while carrying their rated load.
Since a synchronous motor behaves like a variable inductor or a variable capacitor, it can be used to improve the voltage regulation of transmission lines.
Power electronic converters generating very low frequency enable us to use the synchronous motors for ultra-low speed applications such as to drive crushers, rotary kilns and variable-speed ball mills, etc.
Some industrial applications of synchronous motors are such as high power and high speed compressors, blowers, mainline traction, induced and forced draft fans, servo drives, etc.
Advantages of Synchronous Motor
The synchronous motors have the following advantages −
It operates at a constant speed (i.e., synchronous speed) from no-load to full-load.
Power factor of a synchronous motor can be easily controlled by changing the excitation of the motor.
For low speed (< 300 RPM) applications, synchronous motors are more economical than induction motors.
The synchronous motors have high starting torque as compared to induction motors.
A single synchronous motor can be used for driving the load as well as correcting the power factor of the plant.
Disadvantages of Synchronous Motor
Synchronous motors have some disadvantages as well, which are given below −
Synchronous motors are not self-starting
Synchronous motors require DC excitation at the rotor.
The construction of a synchronous motor is more complicated than induction motors.
Synchronous motors are costlier than induction motors.
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