The motor which always works on the synchronous speed, is known as the synchronous motor.
The synchronous machine can work as an alternator and a motor both in similar way as that of the D.C machine can work as D.C. motor and D.C. generator both. The synchronous motor is that which runs at a constant speed i.e., synchronous speed. The speed depends upon the supply frequency and number of poles.
Synchronous speed = 120 x frequency / Number of poles r.p.m.
i.e.
N=120 x f / P r.p.m.
Working
When the armature is connected to A.C. three phase supply, the magnetic field of the rotating nature is produced. The magnetic field produced by D.C. is the stationary field. Now when the current carrying conductors are placed in the magnetic field. so a torque is developed on the conductors. The armature is wound for A.C. supply producing rotating magnetic flux. The rotor let us consider is wound for D.C. producing a steady flux. Let the first half of the a.C. sine wave is considered, the direction of torque be in anticlockwise, but during the next half cycle the torque will change to clockwise direction because of the change in direction of current in the second half cycle; thus a pulsating torque is developed and the rotor does not accelerate in any of the directions. So the synchronous motor is not a self starting one.
If by any of the external means the rotor is rotated and brought to the synchronous speed and the poles are excited the magnetic interlocking results. The north pole of rotor and south pole of stator will get magnetically locked and so on. The rotor which is excited will now rotate with the speed of rotating magnetic field i.e. the synchronous speed. Thus the motor works on the principle Of magnetic interlocking and the speed will be the constant i.e. the synchronous speed.
Construction. The construction Of the synchronous motor is the same as that of an alternator. It is also of two types the stationary field type and rotating field type as shown in Fig. 20.1. An exciter is also mounted on the same shaft to excite the field winding.
Fig. 20.1 Synchronous motor.
How does the stationary field type synchronous motor run at synchronous speed?
In case of stationary field type A.C. synchronous motor, the d.c. supply is given to the stator and being stationary field construction, the field produced will be stationary in space. Now the three phase supply is given to the rotor through the three sliprings mounted on the same rotor as shown in Fig. 20.2. The winding is connected in star. Now the magnetic field produced by the winding is of rotating in nature at the synchronous speed.
Fig. 20.2. Stationary field type synchronous motor.
There are two magnetic fields, the one of stationary in space and other rotating at synchronous speed now let the rotor is brought to the synchronous speed and then the rotor is excited, it means the relative speed of the magnetic field with respect to rotor is zero, hence the magnetic poles will appear to be stationary in space. At this time the field winding is also excited and these two magnetic fields are interlocked. The poles because of relative speed of rotor and field being zero, are stationary in space, thus the magnetic attraction will interlock both the fields. The rotor poles are stationary only as long as the rotor speed is synchronous speed. So to keep the inter-locking the motor will work on the synchronous speed. Then the external source of accelerating the rotor to synchronous speed is taken off and the motor will automatically run to the synchronous speed.
Explain the behaviour of the synchronous motor on no load and on load conditions. Draw the vector diagram in support of the statements.
Whenever the motor is accelerated and brought to the synchronous speed, the field is excited as a result the magnetic interlocking results to drive the motor to the synchronous speed. The movement the driving source is taken off the rotor retard back by small angle, generally known as the torque angle say (10 as shown in Fig. 20.6. It differ with different motors.
The no load current Of the motor is given by the formula
where
z,
Er The resultant voltage in Volts.
Zs The synchronous impedance in Ohms.
and the power will be, W VL 10 cos per phase.
It is used to meet the losses and keeps the motor in running condition. The behaviour of the motor can be studied on load also.
Motor on load
When the load is put on the motor, the speed remains the same it does not drop down, but the angle of coupling falls as shown in Fig. 20.7 and ultimately the relative shift between the rotor and stator poles increases, but continues to run even on the same synchronous speed, Now because the field excitation is constant, so the magnitude of the back e.m.f. is same and will not change. The angle of coupling will certainly be effected and will change to new value i.e. al. Thus the resultant voltage will change to En and the current to Il but the speed will be same as the synchronous speed.
The power required will be VA p, co*l watts/phase and this power will adjust the motor to meet the load. If the load is further increased. the speed will be constant but the angle of coupling will change to new value (h, the current 12 and power required to drive the load per phase.
There is a limitation beyond which the load cannot be pulled. The synchronism because the force Of attraction between rotor and Stator is not sufficient will not be maintained. The maximum torque is known as the pull out torque beyond which the speed will not be the synchronous speed. The angle of coupling is different with different motors. The maximum torque varies from 150% to 350% of the full load torque.
Fig. 20.7 Vector diagram at different loads
Explain the effects of the varying field excitation of a synchronous motor.
The unique and outstanding characteristic of a synchronous motor is in fact the vide range of operation for power factor. It is possible by the adjustment of the field excitation. The field excitation can be changed in these following two ways:
(a) Under excitation.
(b) Over excitation.
(a) Under excitation condition. Whenever the exciting current is less than the normal exciting current the motor is said to be running under excited. In that condition the generated e.m.f. is less than the Eb.
V = The voltage supplied in Volts
Eb = back e.m.f. with normal field excitation in Volts.
Eh = New back e.m.f. with new excitation in Volt.
Io = No load current in Amp.
l1 = Current at excitation in Amp.
No load power factor
Power factor at new load current
Em = Resultant voltage at no load in Volts.
E = Resultant voltage at new excitation in Volts.
Fig. 20.8 Under excitation synchronous motor.
Thus it is seen that if the excitation is further decreased the current further legs behind or the power factor is decreased as shown in Fig. 20.8. Hence the motor under excited draws the lagging current.
Fig. 20.9. Overexcited synchronous motor.
(b) Over excitation condition. If the load on the motor is constant and the excitation is changed, the e.m.f. induced will change as shown in Fig. 20.9. I.rt us take the example of induced excitation Eb and the voltage fed V volts, the resultant voltage is Ero which results in current 10, which is on lagging side. Now increasing the field excitation, the induced voltage changes from EM) to Eh I and the resultant current from 10 to Il Amp. It results the change of power factor from lagging to leading side. Here different positions are shown and it should be clearly understood that with increasing of the excitation current Start shifting from lagging side to unity and then towards the leading side.
Fig. 20.10. v-curve.
Thus the over excited synchronous motor takes leading current. If the value of exciting current and the load current both are noted and a curve is ploted the curve so obtained is known as the V curve of the synchronous motor as shown in Fig. 20.10. The value of the minimum current will be at unity power factor.
