Synchronous motor and induction motor are the most widely used types of AC motor. Construction of a synchronous motor is similar to an alternator ( Ac Generator ) . A same synchronous machine
can be used as a synchronous motor or as an alternator. Synchronous
motors are available in a wide range, generally rated between 150kW to
15MW with speeds ranging from 150 to 1800 rpm.synchronous motor construction:
The construction of a synchronous motor (with salient pole rotor) like any other motor, it
consists of a stator and a rotor. The stator core is constructed with
thin silicon lamination and insulated by a surface coating, to minimize
the eddy current and hysteresis losses . The stator has axial slots inside, in which three phase stator winding
is placed. The stator is wound with a three phase winding for a specific
number of poles equal to the rotor poles.
The rotor in synchronous motors is mostly of salient pole type.
DC supply is given to the rotor winding via slip-rings. The direct
current excites the rotor winding and creates electromagnetic poles. In
some cases permanent magnets can also be used.
- Now, the stator poles are revolving with synchronous speed (lets say clockwise). If the rotor position is such that, N pole of the
rotor is near the N pole of the stator , then the poles of the stator and rotor will repel each
other, and the torque produced will be anticlockwise.
- The stator poles are rotating with synchronous speed, and they
rotate around very fast and interchange their position. But at this very
soon, rotor can not rotate with the same angle (due to inertia), and
the next position poles of the stator will attract the poles of rotor, and the torque produced will be clockwise.
- Hence, the rotor will undergo to a rapidly reversing torque, and the motor will not start.
But, if the rotor is rotated upto the synchronous speed of the stator by means of an external force (in the direction of revolving fiels of the stator), and the rotor field is excited near the synchronous
speed, the poles of stator will keep attracting the opposite poles of
the rotor (as the rotor is also, now, rotating with it and the position
of the poles will be similar throughout the cycle). Now, the rotor will
undergo unidirectional torque. The opposite poles of the stator and
rotor will get locked with each other, and the rotor will rotate at the
synchronous speed.
a synchronous motor Characteristic:
- Synchronous motor will run either at synchronous speed or will not run at all.
- The only way to change its speed is to change its supply frequency. (As Ns = 120f / P)
- Synchronous motors are not self starting. They need some external force to bring them near to the synchronous speed.
- They can operate under any power factor, lagging as well as leading. Hence, synchronous motors can be used for power factor improvement.
synchronous motor Application:
- As synchronous motor is capable of operating under either leading and lagging power factor, it can be used for power factor improvement. A synchronous motor under no-load with leading power factor is connected in power system where static capacitors can not be used.
- It is used where high power at low speed is required. Such as rolling mills, chippers, mixers, pumps, pumps, compressor etc.
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