Editing Three-phase electric power (section)

== Phase converters ==
[[Phase converter]]s are used when three-phase equipment needs to be operated on a single-phase power source. <!-- what about the example for operating a large single phase load on a 3ph generator and need to balance it across 3 phases?  -->They are used when three-phase power is not available or cost is not justifiable. Such converters may also allow the frequency to be varied, allowing speed control. Some railway locomotives use a single-phase source to drive three-phase motors fed through an electronic drive.<ref>{{cite journal|date=Oct 2011|title=Speeding-up Conventional Lines and Shinkansen|url=https://www.ejrcf.or.jp/jrtr/jrtr58/pdf/51-60web.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.ejrcf.or.jp/jrtr/jrtr58/pdf/51-60web.pdf |archive-date=2022-10-09 |url-status=live|journal=Japan Railway & Transport Review|volume=58|page=58}}</ref>

A [[rotary phase converter]] is a three-phase motor with special starting arrangements and [[power factor]] correction that produces balanced three-phase voltages. When properly designed, these rotary converters can allow satisfactory operation of a three-phase motor on a single-phase source. In such a device, the energy storage is performed by the [[inertia]] (flywheel effect) of the rotating components. An external flywheel is sometimes found on one or both ends of the shaft.

A three-phase generator can be driven by a single-phase motor. This motor-generator combination can provide a frequency changer function as well as phase conversion, but requires two machines with all their expenses and losses. The motor-generator method can also form an [[uninterruptible power supply]] when used in conjunction with a large flywheel and a battery-powered DC motor; such a combination will deliver nearly constant power compared to the temporary frequency drop experienced with a standby generator set gives until the standby generator kicks in.

Capacitors and [[autotransformer]]s can be used to approximate a three-phase system in a static phase converter, but the voltage and phase angle of the additional phase may only be useful for certain loads.

[[Variable-frequency drive]]s and [[Phase converter#Digital phase converter|digital phase converters]] use power electronic devices to synthesize a balanced three-phase supply from single-phase input power.