Phase convertor

A phase converter is a device that converts electric power provided as single phase to multiple phase or vice versa. The majority of phase converters are used to produce three-phase electric 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:

A digital phase converter creates a three-phase power supply from a single-phase supply. A digital signal processor (DSP) is used to control power electronic devices to generate a third voltage, which along with the single voltage from the supply creates a balanced three-phase power supply.

AC power from the utility is converted to DC, then back to AC. The power-switching devices used in this process are insulated-gate bipolar transistors (IGBT).

In one type of digital phase converter the input rectifier consists of IGBTs in series with inductors. The IGBTs are controlled by software in the DSP to draw current from the single-phase line in a sinusoidal fashion, charging capacitors on a constant-voltage DC bus. Because the incoming current is sinusoidal, there are no significant harmonics generated back onto the line as there are with the rectifiers found in most VFDs. The controlled rectifier input allows power factor correction.

The output inverter consists of IGBTs that draw on the power of the DC bus to create an AC voltage. A voltage created by power-switching devices like IGBTs is not sinusoidal. It is a pulse-width modulated (PWM) waveform very high in harmonic distortion. This PWM voltage is then passed through an inductor/capacitor filter system that produces a sine-wave voltage with less than 3% total harmonic distortion (standards for computer grade power allow up to 5% THD). By contrast, VFDs generate a PWM voltage that limits their versatility and makes them unsuitable for many applications. Software in the DSP continually monitors and adjusts this generated voltage to produce a balanced three-phase output at all times. It also provides protective functions by shutting down in case of utility over-voltage and under-voltage or a fault. With the ability to adjust to changing conditions and maintain voltage balance, a digital phase converter can safely and efficiently operate virtually any type of three-phase equipment or any number of multiple loads.

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