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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 Harmonic Distortion

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For most of the twentieth century, the predominant use of electricity for business and industry was to power motors, lights and heating devices. These uses have little effect on the 60 Hertz (cycles per second) sine waveform of the electricity delivered to them from their utility. They are called linear loads, because the current (amperage) rises and falls in proportion to the voltage wave.

A few industries like steel mills and aluminum smelters used electricity to power arc furnaces, which distorted the waveform, because the current flow was not directly proportional to the voltage. These loads are called non-linear loads.

Non-linear loads cause waveforms that are multiples of the normal 60 Hertz sine wave to be superimposed on the base waveform. These multiples are called harmonics. For example, the second harmonic is a 120 Hertz waveform (2 times 60 Hertz), the third is a 180 Hertz waveform, and so on. The combination of the sine wave with all the harmonics creates a new, non-sinusoidal wave of entirely different shape. The change to the wave is called harmonic distortion.

In the last 20 years, there has been an explosion of microprocessor based

equipment which are also non-linear loads. Examples include computer systems, variable frequency drives, AC/DC converters, electronic ballasts, X-ray machines, MRI equipment and uninterruptible power supplies.

VFDs and Harmonic Distortion

Harmonic distortion of the sine wave voltage may cause problems for electrical equipment, especially for any electronic controls.

Variable frequency drives (VFD) used as phase converters can only be used to power inductive motor loads because of the harmonic distortion of their output voltage.

The output voltage from a VFD is not sinusoidal, but rather a series of pulses which have average values that are sine waves. The switches that control these pulses have to make their on/off transitions very rapidly (in about 0.2 microsecond) for the VFD to operate efficiently. The high frequency components of these pulses travel from the VFD to the load through the connecting wires, which become an electrical transmission line. Transmission line effects are normally not a problem at 60 Hz to the average user because the wavelength of a 60 Hz signal is about

2200 miles (assuming the signal travels at 0.8 x the speed of light in the wires). However, at 5 MHz the wavelength drops to about 180 feet and the effects become important. The electrical impedance of the transmission line is unpredictable but typically has values between a few tens of ohms to a few hundred ohms. On the other hand, the impedance of a motor winding and the VFD is usually just a few ohms. This mismatch between the line impedance and the impedance of the terminations at the motor and the drive causes standing wave patterns to be set up in the line with resultant voltages that can be much larger than the voltage at the drive output.; These standing-wave voltages can damage the wiring, the motor and the drive. If the distance between the VFD and the motor is short (less that 10 feet), there shouldn't be any problem. As the distance approaches 50 feet or more, most VFD manufacturers recommend that output line filters be used on each of the output leads.

Because of the non-sinusoidal voltage produced by a VFD, they should never be used to power anything other than inductive loads such as AC induction motors. If they are used to power CNC machines, they will likely damage the electronic controls in the machinery.

       
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