Basics About a Phase Converter

What is a Phase Converter?

A wide variety of commercial and industrial electrical equipment requires three-phase power. Electric utilities do not install three-phase power as a matter of course because it costs significantly more than single-phase installation. As an alternative to utility installed three-phase, rotary phase converters, static phase converters and phase converting variable frequency drives (VFD) have been used for decades to generate three-phase power from a single-phase source. 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.

Rotary and Static Phase Converters

Phase converters provide 3-phase power from a 1-phase source, and have been used for decades. The simplest type of old technology phase converter is generically called a static phase converter. This device typically consists of one or more capacitors and a relay to switch between the two capacitors once the motor has come up to speed. These units are comparatively inexpensive. They make use of the idea that a 3-phase motor can be started using a capacitor in series with the third terminal of the motor. It is almost guaranteed that a static phase converter will do a poor job of balancing the voltages on the motor. Unless motors operated on static converters run only for short periods or deliver significantly less than half of their rated output, they will be damaged from overheating.

How Does a Static Converter Work?

The Static Converter is made up of two small components: A voltage sensitive relay and a standard capacitor (Cs) connected to your motor application. The capacitor delays waveforms (or shifts the phase) during the start-up of your motor application. The relay disconnects this start capacitor after the motor has started. From this point, the motor will continue turning on the single phase supply. The performance of such a motor is fairly poor and can be compared to a car motor running on only a few cylinders. Motors operated on a static converter will produce about 50-60% of their name plate power. When you add another low cost run capacitor to the simple design, rated power goes up to around 70% of the motors name plate power. To help with understanding, the Start Capacitor (Cs) is used only to start the motor and then it is switched out completely. The Run Capacitor (Cr) is always in the circuit and is carefully sized to balance the voltages at one load rating (generally around 50% full load). Since Cr is fixed the voltage balancing at either end (0% and 100%) is quite poor. 

The second type of old-technology phase converter is generically called a rotary phase converter. This device consists of a 3-phase motor (usually without external shafts) and a bank of capacitors wired together to act as a single large capacitor. Two of the leads to the motor are connected to the 1-phase power source and the third lead to the motor is connected in series with the capacitor bank to either one of the 1-phase inputs. The output leads from the phase converter are connected across the three motor terminals. Typically the motor used in the phase converter is larger than the loads it is supplying. For example, a rotary converter designed for a 5 kW load might use a 7 kW motor frame. The electrical interaction between the capacitor bank and the free-running phase converter motor generates a voltage on the third motor terminal which approximates the voltage needed for a balanced 3-phase system. However, it usually isn’t a very good approximation. For example, measurements on a 5 kW rotary converter in an actual machine shop installation resulted in line-to-line voltages of 252 V, 244.2 V and 280.5 V, which is about a 12% imbalance in the voltages.

Voltage Imbalance In PercentDerate Motor to These Percentages of the Motor’s Rating

How Does A Rotary Converter Work?

If you add an idle running motor to a static converter, you have a rotary converter. The added motor will compensate for some of the static converter weaknesses and help extend the range of motor sizes and loads. The internal motor is inactive at average load, but works hard when loads don’t match the value of the chosen run capacitor (Cr). Rotary converters are clearly somewhat better than static converters. They can run several motors of different sizes. Large motors will produce up to 90% of their nameplate power, small motors (motor being much smaller than the converters idling motor or pilot motor) a bit more. If the manufacturers oversize these motors, the output symmetry, start capability and capacity will all be increased. This is why manufacturers ask you so many questions about your applications. When in doubt, they will offer a converter with a larger pilot motor or suggest a larger converter altogether.

Digital Phase Converters

Digital phase converters are a recent development in phase converter technology that utilizes proprietary software in a powerful microprocessor to control solid state power switching components. This microprocessor, called a digital signal processor (DSP), monitors the phase conversion process, continually adjusting the input and output modules of the converter to maintain perfectly balanced three-phase power under all load conditions. 

Like rotary and static phase converters, a digital phase converter generates a third voltage, which is added to L1 and L2 of single-phase service to create three-phase power. There the similarity ends. 

A process called double-IGBT conversion generates the third voltage. Double conversion means that 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).

The input module, or rectifier, consists of IGBTs in series with inductors. Operating at a switching

frequency of 10 kHz, the IGBTs are controlled by software in the DSP to draw current from the singlephase 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 crude rectifiers found in most VFDs. The electronic power factor correction on the input module also corrects the power factor of any inductive loads so that the utility sees a system that operates at near unity power factor. The power factor correction makes digital phase converters very efficient and utility friendly. 

The output module, or 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 pulsewidth-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 perfect voltage balance, a digital phase converter can safely and efficiently operate virtually any type of three-phase equipment or any number of multiple loads. The solid state design results in a relatively small package with no moving parts except for small cooling fans. The converters are very efficient, operating at 95-98% efficiency. When the converter is energized with no load, it consumes very little power. Digital phase converters are a patented technology developed by Phase Technologies, LLC, who is the only manufacturer of true digital phase converters.

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