Phase converter
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A phase converter is a device that converts power provided as single or multiple phases to a different number of phases. The majority of phase converters are used to produce three phase electrical power from a single-phase source, thus allowing the operation of three-phase equipment at a site that only has single-phase electrical service. Phase converters are used where three-phase service is not available from the utility, or is too costly to install due to a remote location. A utility will generally charge a higher fee for a three-phase service because of the extra equipment for transformers and metering and the extra transmission wire.
Three phase induction motors may operate adequately on an unbalanced supply if not heavily loaded. This allows various imperfect techniques to be used. A single-phase motor can drive a three-phase generator, which will produce a high-quality three-phase source but with high cost for apparatus. Several methods exist to run three-phase motors from a single-phase supply, these can in general be classified as:
- Electronic means of creating three phase where the incoming power is rectified, and the three phase power is synthesized with electronics. Power electronic devices directly produce a three-phase waveform from single-phase power, using a rectifier and inverter combination. This also offers the advantage of variable frequency.
- A digital phase converter uses a rectifier and inverter to create a single voltage with power electronics, which is added to the two legs of the single-phase source to create three-phase power. Unlike a phase converting VFD, it cannot vary the frequency and motor speed since it generates only one leg which must match the voltage and frequency of the single-phase supply. It does have the advantage of a sine-wave output voltage and excellent voltage balance between the phases.
- Rotary phase converters constructed from a three-phase electric motor or generator "idler". These normally require some kind of starting aid and capacitors to improve phase balance and power factor. This is a two motor solution. One motor is not connected to a load and produces the three phase power, the second motor driving the load runs on the power produced.
- Static conversion techniques in which the motor is run at less than full efficiency mainly on two of the legs of the three phase motor. Current is sometimes injected into the third leg with a capacitor or transformer arrangements that provide imperfect phase shift. In these systems the motor must be derated.
- Methods in which the connection of the windings of the motor, normally a wye and or delta configurations, are replaced with novel connections. These techniques are covered in patents of Dr. Otto J. M. Smith, such as 5,545,965. Aug. 13, 1996. “Three Phase Motor Operated From a Single-Phase Power Supply and Phase Converter”.
Each of the above methods has its own set of advantages and drawbacks.
