Basics Of Ac Synchronous Motor

Generally speaking, electrical motor is an electro-mechanical device which main purpose is to convert energy from electrical domain to mechanical domain. According to the type of input, there are two types of electro-mechanical motors: single phase and three (3) phase motors. Among the 3 phase electrical motors, induction and synchronous motors are the most commonly used.
The construction of an Ac synchronous motor is similar to the alternator - AC generator one. There are different types of synchronous motors available on the market and they are rated between 150kW to 15MW.

Construction Of An Ac Synchronous Motor

An Ac Synchronous Motor consists of stator and rotor. To minimize hysteresis losses and eddy current, the stator core of synchronous motors must be constructed with thin silicon structure, insulated by a surface coating. The stator has axial slots inside, where in is placed three phase stator winding.
The rotor in Ac synchronous motor is generally of salient pole type. Direct current supply is given to the rotor through slip-rings. The DC fires the rotor winding and creates electromagnetic poles.

The Work Of A Synchronous Motor

Ac synchronous motor has two electrical inputs. Actually, it is stator winding that consists of a 3 phase winding, which is provided with 3 phase supply and rotor, which is provided with DC supply. The 3 phase stator winding that carries 3 phase currents, produces 3 phase rotating magnetic flux andthe rotor carrying DC supply produces a constant flux.
For instance, if the frequency is taken to be 50 Hz, the 3 phase rotating flux will rotate about 3000 revolution in one minute or 50 revolutions in one second.
When talking about particular instant rotor and stator, its poles can be of same polarity (N-N or S-S) causing repulsive force on rotor and in the next second it'll be N-S causing attractive force. However, due to inertia of the rotor, the rotor cannot rotate in any direction because of the repulsive or attractive force and remain in standstill condition.
To overcome all the inertia situation, the rotor is provided with some mechanical input that rotates in the same direction as the magnetic field to a speed which is similar to the synchronous speed. After a while the Ac synchronous motor starts rotating in synchronization with the frequency.

Application Of Ac Synchronous Motor

  • Because synchronous motors are capable of operating under leading and lagging power factor, these types of motors can be used for power factor improvement. capacitors can not be used.
  • Synchronous motors are applied where high power at low speed is required. For instance, pumps, chippers, rolling mills, mixers, compressor and so on.


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