Electrical Engineering ⇒ Topic : Effect of load on a synchronous motor

EFFECT OF LOAD ON A SYNCHRONOUS MOTOR

When mechanical load on a D.C. motor or an A. C. motor is increased, the speed decreases.This, is turn, decreases the back or counter e.m.f. (Ed so that the source is able to supply more current to meet the increased load demands. However, this action cannot take place in the synchronous motor for the rotor must run at synchronous speed at all loads.Fig.(a) shows the relative position of a stator and rotor pole at no-load, poles centres are directly in line with each-other.

Fig.(b) represents the relative position of the stator and the rotor poles after mechanical load has been added to the motor. Now there has been a shift of the rotor pole in a direction opposite to that of the stator field flux and the direction of the rotor. It may be kept in mind that there has been no change in speed as the rotor will continue to rotate at synchronous speed. There is only an angular displacement between the centres of the stator and rotor field poles. The angular displacement shown in Fig. (b) is called the 'torque angle'.

                                                             figure  Relative positions of stator and rotor poles

No-load condition-vector diagrams. Fig.(c) shows (properly synchronised to the conditions when the motor the supply) is running on no-load and is having no losses. It is seen that V= E_b, hence their vector differences is zero and so is the armature current. Motor intake is zero, as there is neither load nor losses to be met by it. In other words, the motor just floats.

                                                                                           Fig.(c). No-load (no losses).

Fig.(d) shows the vector diagram when the motor is no no-load but has losses. The vector for Eh falls back by a certain angle act, so that a resultant voltage Er and hence current Io is brought into existence which supplies losses
                                                    

                                                                                  Fig.(d)No-load (with losses

Load condition-vector diagram. When the motor is loaded, it slows down momentarily to adjust itself to the change in load condition, so the rotor pole falls back a little more relative to the stator pole, as shown in Fig.(e). Hence the torque angle increases with the increase in load. Due to increase in load or torque angle a, the resultant votlage E_r across the armature (or stator) circuit increases, and, therefore, current drawn from the supply mains increases. Thus a motor is able to supply increased mechanical load, not by reduction in speed, but by shift in relative positions of the rotor and rotating magnetic field (or stator flux). From Fig.(e) it is obvious that for increasing load with a constant value of back e.m.f. E_b the phase angle 0 increases in lagging direction

                                                                   Fig. 15.(d). Synchronous motor no-load vector diagram

If the angle between stator and rotor pole centres become too great, due to a serious overload then the rotor will pull out of synchronism and operate as an induction motor with the aid of the amortisseur winding. The maximum value of torque which a synchronous motor can develop withaut 'dropping out of synchronism is called the `pull-out torque'. In most synchronous motors this is 150 to 200 per cent of rated torque output.