Demonstrative Video
Synchronous Motor - Introduction & Characteristics
A SM is electrically identical with an alternator
Syn Machines operated as alternator (when driven mechanically) and motor (when driven electrically)
Rating of SM : 150 kW to 15 MW and 150-800 rpm
Some characteristics features of SM are:
not a self-starting motor.
For a given frequency, it operates only at one speed called synchronous speed, \(N_s = 120 f / P\).
It can be operated under a wide range of PF both lagging and leading.
In addition to the motor being used for mechanical load, it is also used as a PF improvement equipment as synchronous condenser.
At NL it draws a very small current from the mains to meet the internal losses of the motor.
load \(\uparrow\), torque angle \(\delta\) \(\uparrow\) due to which motor draws \(I~\uparrow\)
After the input current reaches maximum ( \(\delta \approx 90^{\circ}\)) no further increase in load is possible.
If the motor is further loaded it goes out of synchronism and stops.
Principle of Operation
When a 3-phase supply is given to the stator of a 3-phase wound SM, a revolving field is set up (say in anticlockwise) which rotates at a synchronous speed (\(N_s = 120f/P\) ).
This field is represented by the imaginary stator poles.
The opposite poles of stator and rotor are facing each other (for simplicity two-pole machine is considered).
As there is a force of attraction between them, an anticlockwise torque is produced in the rotor as the rotor poles are dragged by the stator revolving poles or field.
After half a cycle, polarity of the stator poles is reversed whereas the rotor poles could not change their position due to inertia.
Thus, like poles are facing each other and due to force of repulsion a clockwise torque is produced in the rotor.
Hence, the torque produced in a 3-phase synchronous motor is not unidirectional and as such this motor is not self-starting.
However, if SM is rotated by some external means at the start so that it also reverses its polarity as the polarity of stator poles is reversed after half a cycle.
A continuous force of attraction between stator and rotor poles exists.
This is called magnetic locking which drags the rotor poles by the stator revolving field (imaginary poles) and a continuous torque is obtained.
As the rotor poles are dragged by the stator revolving field, hence the rotor rotates at the same speed as that of stator revolving field, i.e., synchronous speed.
Thus, a SM only runs at a constant speed called synchronous speed.