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Synchronization is an important concept in relation to alternating current.


Electricity generation

Electricity generation requires the connection of large numbers of alternators in parallel and additional alternators must be switched in when demand rises.

Before one alternator is connected in parallel with others, the voltages and frequencies must be identical and they must be synchronised so that they are in phase.

If one machine is slightly out of phase it will pull into step with the others but, if the phase difference is large, there will be heavy cross-currents which can cause voltage fluctuations and, in extreme cases, damage to the machines.

The syncroscope at the top of this set of instruments has a pointer that rotates at a speed proportional to the difference between the system and the generator frequency. When the pointer is stationary, the two sources are synchronized and the generator may be switched onto the system.

Electric motors

Electric motors for alternating current fall broadly into three categories:

  • Universal motors, whose speed is not related to frequency
  • Synchronous motors, whose speed has a precise relationship to frequency, i.e. they run at synchronous speed
  • Asynchronous motors, which run at slightly less than synchronous speed, the exact speed depending on the amount of "slip" between the rotor speed and the speed of the rotating magnetic field. Asynchronous motors typically run with a "slip" of about 4%.

Synchronous speeds

Synchronous speeds for synchronous motors and alternators depend on the number of poles on the machine and the frequency of the supply. In the following table, frequencies are shown in hertz (Hz) and rotational speeds in revolutions per minute (rpm):

No. of poles Speed (rpm) at 50 Hz Speed (rpm) at 60 Hz
2 3,000 3,600
4 1,500 1,800
6 1,000 1,200
8 750 900
10 600 720
12 500


The relationship between the supply frequency, f, the number of poles, p, and the synchronous speed (speed of rotating field), ns is given by:

f = \frac{pn_s}{120\ }.


  • The Electrical Year Book 1937, published by Emmott and Company Limited, Manchester, England, pp 53-57 and 72


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