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Three phase is a common method of electric power transmission in industrialised countries. It is a type of a polyphase system.

At the power station an electrical generator converts mechanical power into a set of alternating electric currents, one from each electromagnetic coil or winding of the generator. The currents are sinusoidal functions of time, all at the same frequency but with different phases. In a three-phase system the phases are spaced equally, separated from each other by 120° (which is the maximum phase separation possible). The frequency is typically 50 Hz in Europe and 60 Hz in the US.

At the destination, a substation or transformer supplies the power stepped down from the high-voltage transmission line to three sinusoidally varying electric currents of 120 V (in the US) or 230 V (in Europe) alternating current (VAC). This is then delivered to the customer's circuits at a master breaker panel through four conductors. One conductor is the neutral or ground at the power source, the other three lines or phases carrying electrical power to point destinations or supply transformers. Connecting an electrical circuit from one phase to the neutral supplies 120 VAC (or 230 VAC) to the circuit.

The power transmission grid is organised so that each phase carries the same magnitude of current out of the power station; the currents returning from the customers' premises to the power station all share the neutral wire, but the three-phase system ensures that the sum of the returning currents is approximately zero.

Connecting between two phases provides √3 or 173% of the single-phase voltage (208 VAC in US; 400 VAC in Europe) because the out-of-phase waveforms add to provide a higher peak voltage in the resulting waveform.

All three phases are typically used in large industrial motors as this is the most efficient way to transmit large amounts of electrical power. The greatest power demand is when starting the motor.

Some devices are made which create an imitation three-phase from "two-phase" power (termed "220" in the United States; with phase separation of 180°). This is done by creating a third "subphase" between the other two phases, resulting in a phase separation of 180° - 90° = 90°. Many three-phase devices will run on this configuration, but at lower efficiency.

How to test three-phase electrical supply

A three-phase electrical supply consists of three active conductors and an earth ground.

A three-phase induction motor cannot function correctly if its electrical supply is not within certain parameters.

Typical parameters are 208 or 415 volts between phases, 120 or 240 volts from any phase to earth or ground, voltage within 12% of nominal values, and each phase within 5% of each other.

In a typical three-phase induction motor circuit, an appropriate place to test is at the line side of the direct-on-line motor starter.

Figure 1:

      A  B  C        earth/
      O  O  O        ground
       /  /  /
      /  /  /
      O  O  O

Tests should be made between A and B, A and C, B and C, A and earth, B and earth, and C and earth.

Note that listed voltages are for countries which use 120 or 240 volts only!

How to test three-phase pumps

Electricians may not often encounter three phase induction motors used in domestic watering systems.

Procedures to follow to field test these motors and their controls are listed:

Topics including testing motor coil resistance and testing earth fault resistance are covered.

For further information on three phase circuits see:

• Y-delta transform

See also single phase electric power, alternating-current electric power, polyphase systems.