Difference between three phase and single phase induction motors

Leah Spencer
 


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A single phase induction motors connected to an alternating current does not produce a revolving magnetic field as this needs additional circuits for starting whereas in a three-phase induction motors, the currents in each conductor reach their peak instantaneous values simultaneously in each cycle of the power frequency i. e. , first one, then the second, and finally the third, current reaches its maximum value. A three-phase motor tends to be smaller, cheaper, smoother and efficient enough compared to an equivalent single-phase motor.

Single phase induction motors Vs three phase induction motors

A single phase induction motors is totally different from a three phase induction motors in horsepower, power availability, power conversion, AC frequency, wire usage and also start up.

Horsepower

Single-phase induction motors are commonly available from seven horsepower to a maximum size of manufacture up to 10 horsepower whereas three-phase induction motors are available from 1/2 to 25 or more horsepower.

Available power

Single-phase supplies a power of 240 volts by two 120 volts lines and serves home appliances and lighting whereas a three-phase supplies a power of 208Y/120 by three 120 volts lines and serves farms and factory purposes with some older distribution system, which carry a power with two 240-volt lines and one 208-volt line.

Power conversion

With the help of either a static phase converter or a rotary phase converter, a single-phase AC with 120 volts or split-phase AC with 240 volts may be converted to three-phase AC. Comparatively, a static converter costs less with no moving parts but produces less power whereas rotary converter produces more power with a little high in price.

AC Frequency

Standard frequency of single-phase power systems are either 50 or 60 Hz but a special single-phase power networks may be operated at 16.67 or more to power electric railways whereas the speed of a three-phase power systems is limited by the frequency of the AC voltage to 60 Hz i. e. , 3600 revolutions per minute that is 60 cycles per second x 60 seconds per minute but sometimes a frequency converters are used to increase the maximum speed at which a three-phase motor can run.

Wire usage

Single-phase induction motors are usually connected directly to the supply for starting with just two wires. As some of the three-phase induction motors are large, they need a specific starting arrangements in order to reduce a sudden surge at startup and perhaps to control the acceleration, a start delta starter is used, which requires a neutral supply and so there would be 6 wires or sometimes even have 12 wires brought out from the windings for this purpose to accommodate a range of voltage as well.

Startup

Starting a single-phase motor is usually to split into two phases separated by 90 degrees using a capacitor or inductor in series with the starting winding and is often switched out once a single-phase motor has started. As far as three-phase motors are concerned, it does have a very high starting torque and do not require capacitor or other electronics for starting, which is suitable for use on heavy machinery.

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