Phase Converters & Power Factor
Phase Converter Efficiency
Installing a Phase Converter
Rotary Phase Converters
Static Phase Converters
VFDs as Phase Converters
     • Harmonic Distortion
Three-Phase Motors
Phase Converters & Voltage Balance
Phase Converter Applications
     • Submersible Pumps
     • Woodworking Equipment
     • Dual Lift Stations
     • Phase Converters & Welders
     • Phase Converters & CNC Machines
     • Phase Converters & Air Compressors
     • Phase Converters & Elevators
     • Phase Converters & Wire EDM
     • Phase Converters & HVAC
Phase Converters & Transformers
     • Step-up Transformers
     • Buck-Boost Transformers
     • Isolation Transformers
Phase Converter Experts
Digital Phase Converters
Regenerative Power
Three-Phase Power
     • Delta vs. Wye Configured Power
Motor Starting Currents

Phase Converters and Power Factor

Power factor is a concept that is not always well understood
and has been largely ignored as it relates to phase converters.
However, it can have a significant impact on the single-phase
service that is supplying a phase converter and its load.


Power Factor Defined

Power factor describes the relationship between real and apparent power of alternating current (AC) power systems.  In an AC power system, current may flow into and back out of a load without delivering any energy.  This reactive current gives rise to “apparent power” which is larger than the actual power consumed. Power factor is expressed as the ratio of the actual power to the apparent power.  A power factor of 1.0, or “unity” is desirable because it means that all the power delivered to the load is “real power” and there is no reactive current component to the load.  As the reactive current and apparent power increase, power factor is expressed as a smaller decimal.
The Impact of Power Factor
A poor power factor is undesirable because even though the power is “apparent”, the current is real and the utility has to generate that current and allow capacity in its lines to carry it.  The typical meter does not measure apparent power which is why the utility may choose to charge a customer a penalty for poor power factor.  And even if the utility doesn’t charge for it, poor power factor can cost a customer by wasting energy through I2R heat losses and overloading the capacity of wires and breakers.

Phase Converters and Power Factor
Rotary phase converters employ a three-phase AC induction motor, and causes a poor “lagging” power factor, therefore, this means that as the current and voltage of the AC power alternate between positive and negative, the peak current lags the peak voltage in time. Inductive loads can cause a poor "lagging" power factor, as  well. Most often a phase converter is powering inductive loads which also have a poor lagging power factor.  When combined with the poor power factor of the rotary

converter, the power factor on the utility line side of the converter becomes very low, often as low as 0.65.  This means that nearly 1/3 of the current flowing in and out of the system is not delivering any real power.  This reactive current can overload the wiring and circuit breakers and if the converter and load are large enough it can overload the step down transformer at the utility service drop and cause the voltage to sag on the utility system.

VFDs and Power Factor
VFDs are sometimes used as phase converters and can create problems related to power factor.  Unlike rotary phase converters where the peak current lags the peak voltage, VFDs will draw current that is in time with the voltage.  The problem arises from the current being drawn in a dramatic spike instead of a smooth sine wave.

This sudden and dramatic current causes harmonic distortion to be introduced into the current.  This can create problems for other electrical equipment on the service, and if the VFD and load are large enough it can overload the step down transformer at the utility service drop.  The distorted nature of the input current is due to the type of rectifier used on VFDs.  Diodes or SCRs are used to “rectify” the incoming AC power, which converts the power to DC by charging capacitors on a DC bus.  VFD manufacturers will sometimes specify the power factor of their drives as unity “displacement” power factor.  This is not the same as true unity power factor and the damaging harmonics will still be present in the input current. 

The addition of input line reactors, which are inductive filters, to the line side of the VFD will mitigate most of the harmonics and are offered as an option by most drive manufacturers.
Digital Phase Converters and Power Factor
Genuine digital phase converters will operate at true unity power factor under all load conditions with almost no distortion of the input current.  This makes them a very efficient, utility-friendly phase converter.  Like a VFD, a digital phase converter rectifies incoming AC power to DC, but the rectifier is of a completely different design.  It utilizes IGBTs (insulated gate bipolar transistors) in series with an inductor on the input rectifier module.  Software in the converter’s microprocessor controls the IGBT so that the current is drawn in a sinusoidal fashion.  This process is known as electronic power factor correction.  So rather than compound the power factor of inductive loads like a rotary converter, a digital phase converter actually corrects the power factor of the load, supplying the reactive current the load needs while the utility sees a load that operates at unity power factor.  The electronic power factor correction can also reduce the impact of motor starting currents, allowing larger motors to be started across-the-line without causing line disturbances on the single-phase line.

A phase converter that operates at true unity power factor is efficient and utility-friendly.  Compared to other converters, it can operate the same load on a smaller utility step-down transformer.  It will not be as likely to overload wiring and circuit breakers and wastes less energy.

More information on Power Factor.