Motor Phase and Frequency

A power system can either be single phase or polyphase. Single phase is typically used in rural areas, small commercial establishments and in household applications. The frequency cycle of single phase peaks and dips in 360°.

A polyphase power system consists of two or more alternating currents of equal frequency and amplitude but offset from each other by a phase angle. Three phase is the most common type of polyphase system. Three phase power is used in commercial and industrial applications. Unlike single phase, the current supply is more even due to the fact that the cycles are offset by 120°. For motors, the advantage of three phase power is simpler construction which requires less maintenance. Also, a more powerful machine can be built into a smaller frame and will generally operate at a higher efficiency than single phase motors of the same rating.

Frequency in a motor can be defined as the number of complete alternations-per-second of an alternating current.

A current is said to have been through one complete cycle when it has gone from zero to maximum, to minimum, and back to zero again. Frequency is the number of these complete cycles over the passage of time and is usually expressed as herz (Hz): one hertz equals one cycle per second (cps).

The diagram below shows the frequency of a three phase motor:

For more about three phase motors, click here.

Frequency Standards

The predominant frequency in North America is 60 Hz. However, 50 hertz systems are common in other countries. Other systems such as 40 and 25 hertz are isolated and relatively few in number. Some manufacturers, such as GE, will allow dual frequency to occur. They will allow 50Hz operation of 60Hz motors, albeit with variable functionality. Typically this means that the motor speed will be equal to 5/6 the nameplate speed.