Electrical Power Systems Technology

Alternating Current Power Systems 153

GENERATOR FREQUENCY

The frequency of the sinusoidal waveforms (sine waves) produced

by an AC generator is usually 60 hertz. One cycle of alternating current is

generated when a conductor makes one complete revolution past a set of

north and south field poles. A speed of 60 revolutions per second (3600

revolutions per minute) must be maintained to produce 60 hertz. The fre-

quency of an AC generator (alternator) may be expressed as:

speed of rotation (rpm) × number of poles per phase

f = ———————————————————————

120

where f is the frequency in hertz.

Sample Problem:

Given: a six-pole three-phase alternator rotates at a speed of 3600

rpm.

Find: the frequency of the alternator.

Solution:

N/3 × rpm

f = ——————

120

6/3 × 3600

—————

120

= 60 Hz

Note that if the number of poles is increased, the speed of rotation may

be reduced while still maintaining a 60-Hz frequency.

HARMONICS

The voltage and current of power lines are often distorted because of

the effect of harmonics. This distortion can be caused by magnetic effects in

transformers, or by power control equipment. Harmonics are frequencies

that are whole number multiples of the power line frequency.