The procedure of connecting an alternator in parallel with another or with common bus bars to which the number of alternators are already connected is called synchronization of alternator.

Conditions for Synchronization of Alternator

For proper synchronization of alternator, the following conditions must be fulfilled:

  • The terminal voltage of the incoming alternator must be equal to that of bus bars voltage.
  • The speed of the incoming alternator must be such that its frequency is equal to that of bus bars.
  • The phase sequence of incoming alternator must be the same as that of bus bars.

All these conditions must be full filled to ensure that there will be no circulating current between the windings of the alternator already connected to the bus bars and the incoming alternator.
To understand the situation that will come across if the above conditions are not achieved, consider only two single-phase alternators connected to the common bus bar operating in parallel as shown in Figure.

A load is connected to the bus bars. There are two circuits to which current can be supplied by the alternators. One circuit is the external load and the other is local internal circuit i.e. synchronous impedance of the two alternators.
If these conditions are not full filled, a current (circulating current) will flow through local internal circuit of alternators. This circulating current will load two alternators, without supplying any power to the external load. If too high circulating current flows, it may damage the alternators.

Synchronization of Alternator

Here I am describing the ‘two bright and one dark lamp method’ of synchronization of alternators. This method is generally used in colleges to demonstrate this process to students.
Consider an alternator B is to be synchronized. In this case lamp L1 is connected across R – R’, lamp L2 is connected across Y – B’ and lamp L3 is connected across B – Y’ as shown in Figure.

synchronization of alternator image

If the voltages are equal, the frequencies are identical and the phase sequence is correct then the voltage across L1 will be zero and across L2 and L3 will be line voltage.
Under this condition, the lamp L1 will be completely dark and the lamp L2 and L3 will be equally bright. This is the ideal condition for closing the synchronizing switch.
When the frequency of the incoming alternator is different from that of bus bar frequency and the remaining conditions are fulfilled, then the three lamps will flicker alternatively (i.e. one after the other in sequence).
The flickering of the lamps will indicate the difference of speed of the incoming alternator, accordingly the speed is adjusted to minimize the flickering of the lamps.
However, if the phase sequence is not correct all the three lamps will flicker in unison. Then the phase sequence should be corrected by interchanging any two leads of the incoming alternator at the synchronizing switch.
If the voltage of the incoming alternator is not equal to that of bus bar voltages and the other conditions are satisfied, all the lamps will glow with different brightness and will continue to attain the same brightness. The ideal condition can be achieved by adjusting the excitation of the incoming alternator.
Thus when the flickering frequency is minimized, lamp L1 is totally dark and L2 and L3 are equally bright the synchronizing switch should be closed.
This method is only suitable for small low voltage alternators. For large capacity, high voltage alternators, a synchroscope is almost invariably used for synchronizing.

Synchronization of Alternator by Synchroscope

Synchroscope is an instrument used for synchronization of alternators. It indicates whether the incoming machine is running fast or slow. To do the synchronization of alternators by this device,

  • Connect the alternator, synchroscope and other devices as shown in Figure.
  • Check the phase sequence of the incoming machine with the help of a sequence indicator or small induction motor.
Synchronization of Alternator by Synchroscope

To determine the phase sequence with the help of induction motor, connect a small induction motor with bus bars and incoming alternator one by one. If the induction motor is running in the same direction in both cases then the phase sequence is the same.
If it is not correct, interchange any two phases of the incoming alternator at the synchronizing switch.

  • By adjusting the speed of the incoming machine, make the pointer of synchroscope stationery in the vertical position.
  • Measure the voltage of incoming alternator, make it equal to that of the bus bars by adjusting the excitation of the incoming alternator.
  • After satisfying all these conditions, voltmeter will show zero reading.Now close the synchronization switch.

Shifting of Load on Alternators

After closing the synchronizing switch alternator B is connected in parallel with the alternator A. At this instant alternator B is not delivering any load (current) to the bus bars.
The load is shifted from the running alternator A to incoming alternator by increasing the mechanical power input to the prime-mover of alternator B and simultaneously reducing the mechanical power input to the prime-mover of alternator A.
In case of steam machines this can be readily done by opening the steam valve of the alternator B and simultaneously closing the steam valve of alternator A. Thus any load can be shifted to incoming alternator B from alternator A. Generally it is shifted as per their respective ratings.
If alternator A is to be disconnected from the bus bars the process continues till the whole of the load is shifted to the incoming alternator B as indicated by the ammeters and watt-meters in the line circuits. Then the circuit breaker (main switch) and the field breaker (field switch) of alternator A are opened.
The most important point, to be noted, is that the load cannot be shifted from one machine to the other by adjusting the excitation. Once the alternator is connected to the bus-bars, the change in excitation only changes the power factor of the alternator.

Infinite Bus-bars

The large grid systems which are fed by numerous alternators of very large capacity are termed as Infinite Bus-bars.

Control of Bus-bar Voltage

When a number of alternators are running in parallel, the bus-bar voltage can be regulated by regulating the excitation of all the machines. Change of excitation of any one machine will only affect the reactive load delivered by the machine.

Control of Bus-Bar Frequency

When a number of alternators are running in parallel, the bus-bar frequency can be regulated by regulating the power input to all the machines. An increase of power input to any one machine will merely cause it to take more load at the expense of the other machines with the possibility of damage to itself.

Synchronous Motor Objective Type question Answers | MCQ



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#1 The maximum power developed in the synchronous motor will depend on

rotor excitation, maximum value of coupling angle and supply voltage.

#2 A synchronous motor switched on to supply with its field winding shorted on themselves. It will

start as an induction motor

#3 The back EMF set up in the stator of a synchronous motor will depend upon

rotor excitation only

#4 With the increase in the excitation current of synchronous motor the power factor of the motor will


#5 The armature current of a synchronus motor has large values for

both low and high excitation

#6 A 3 phse 400 V, 50 Hz salient pole synchronus motor is fed from an infinite bus bars and is running at no load. Now if the field current of the motor is reduced to zero

the motor will run at synchronous speed

#7 A 3 phase, 400 V, 50 Hz 4 pole synchronous motor has a load angle of 10 degrees electrical. The equivallent mechnical degrees will be

5 degrees

#8 A 3 phase, 400 V, 50 Hz synchronous motor has fixed excitation. The load on the motar is doubbled. The torque angle, ɸ will become nearly

#9 A 3 phase, 400 V, 50 Hz synchronous motor has fixed excitation. The load on the motar is doubbled. The torque angle, ɸ will become nearly

#10 The break down torque of synchronous motor varies as

applied voltage

#11 The name plate of an induction motor reads 3 phase, 400 V, pf 0.8 lagging, 1440 RPM. On similar lines a name plate of synchronous motor should read

3 Phase, 400 V, 50 Hz, 0.8 pf leading, 1500 RPM

#12 In a 3 phase synchronous motor, the magnitude of field flux

remains constant at all loads

#13 A four pole synchronous machine has 48 slots. A coil having one coil side in slot number 1 and the other coil side in slot number 13 will be termed as

full pitch coil

#14 A three phase synchronous motor is running clockwise. In case, the direction of its field current is reversed

the motor will continue to run in the same direction

#15 In a synchronous motor out of the following losses, which one will have the highest proportion ?

Iron losses

#16 When a synchronous motor is connected to an infinite bus, while operating on leading power factor

the excitation voltage will be more than the supply voltage

#17 In a synchronous motor hunting can be minimised

by any of the above method

#18 While starting a synchronus motor by induction motor action, very high EMF is induced in the field. This induced EMF may damage the insulation of the field winding and of the slip rings. This insulation damage can be prevented by

either short-circuiting the field winding by field discharge resistance or splitting the field winding into several sections

#19 Synchronous motors are gererally of

salient pole type machines

#20 The fact that a synchronous motor with salient poles will operate, even if the field current is reduced to zero, can be explained by

magnetization of rotor poles by stator magnetic field

#21 Hunting of a synchronous motor may be due to

any of the above.


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