Power Flow in Synchronous Motor

Power Flow in Synchronous Motor:

The figure below gives the details regarding the power flow in synchronous motor.

Torque developed in Motor:

Mechanical power is given by Pm = 2πNsTg/60 where Ns is the synchronous speed and the Tg is the gross torque developed.

Hunting and Damper Winding:

Hunting:

Sudden changes of load on synchronous motors may sometimes set up oscillations that are superimposed upon the normal rotation, resulting in periodic variations of a very low frequency in speed. This effect is known as hunting or phase-swinging. Occasionally, the trouble is aggravated by the motor having a natural period of oscillation approximately equal to the hunting period. When the synchronous motor phase-swings into the unstable region, the motor may fall out of synchronism.

Damper winding:

The tendency of hunting can be minimized by the use of a damper winding. Damper windings are placed in the pole faces. No emfs are induced in the damper bars and no current flows in the damper winding, which is not operative. Whenever any irregularity takes place in the speed of rotation, however, the polar flux moves from side to side of the pole, this movement causing the flux to move backwards and forwards across the damper bars. Emfs are induced in the damper bars forwards across the damper winding. These tend to damp out the superimposed oscillatory motion by absorbing its energy. The damper winding, thus, has no effect upon the normal average speed, it merely tends to damp out the oscillations in the speed, acting as a kind of electrical flywheel. In the case of a three- phase synchronous motor the stator currents set up a rotating mmf rotating at uniform speed and if the rotor is rotating at uniform speed, no emfs are induced in the damper bars.

Synchronous Condenser:

An over excited synchronous motor operates at unity or leading power factor. Generally, in large industrial plants the load power factor will be lagging. The specially designed synchronous motor running at zero load, taking leading current, approximately equal to 90°. When it is connected in parallel with inductive loads to improve power factor, it is known as synchronous condenser. Compared to static capacitor the power factor can improve easily by variation of field excitation of motor. Phasor diagram of a synchronous condenser connected in parallel with an inductive load is given below.

Numerical Problems:

Ex.1 A 3 phase star connected synchronous motor is taking a current of 25 Amps from supply while driving a certain load. Its resistance and synchronous reactances per phase are 0.2 Ω and 2 Ω respectively. Calculate the emf induced in the motor if it is operating at a power factor (i) 0.8 lagging (ii) 0.9 leading.

Ex.2 A 4000 volts 50Hz, 4 pole star connected synchronous motor generates a back emf /phase of 1800 volts. The resistance and synchronous reactance per phase are 2.2 0. and 22 0. respectively. The torque angle is 300 electrical. Calculate (i) resultant stator voltage/phase (ii) stator current/phase (iii) power factor (iv) gross torque developed by the motor.

Ex.3. A 400 volts, 8 kW, 3 phase, 50Hz synchronous motor has negligible resistance and synchronous reactance of 8 0. per phase. Determine the minimum current and the corresponding induced emf for full load condition. Assume efficiency of the motor as 88%.( Aug2001)