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.
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.
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
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 Hrmonic Distortion
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.
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.