Variable frequency drives (VFDs)
are designed primarily to control the speed of AC motors, but can be
adapted to function as phase converters.
While a phase converter will supply
a three-phase output at the same frequency as the input voltage from
the power line, a VFD has the ability to create voltages that vary
in frequency. A VFD has an input rectifier (either 4 or 6
semiconductor diodes) which charge up a DC link capacitor. Three
pairs of semiconductor switches are also connected to the DC link
capacitor. These switches generate a pulse-width-modulated (PWM)
voltage for each of the three-phases on the output.
A VFD cannot produce a sinusoidal
output voltage. The inductance of a motor powered by a VFD responds
to the area beneath the curve of a plot of the voltage as a function
of time. So, even though the voltage isn't sinusoidal, if the
on/off times of the switches are chosen correctly then the current
in the leads to the motor can be sinusoidal as long as the
average value of the voltage is sinusoidal. Since the torque
generated by the motor is proportional to the currents and not the
voltages, then to a first approximation the motor behaves as if it
had sinusoidal voltages applied to it.
Problems can arise with VFDs if
they are used to power loads other than motors, if there are
multiple loads on the VFD, if the motor needs to provide braking
action, if the distance between the motor and the VFD is
appreciable, or if the current drawn by the VFD is large compared to
the rating of the utility step-down transformer.
VFDs were not originally designed
to function as phase converters, in fact most VFDs are powered from
a three phase source. When used as phase converters, single-phase
input powers 4 of the 6 diodes on the rectifier, so the drive must
be de-rated. Usually you have to double the size of the drive to
A diode or SCR input rectifier
typically produces large harmonic distortion in the input current.
Table 2 below gives typical values of the harmonic distortion
expressed as a percentage of the fundamental component of the input
current at 60 Hz.
VFD Input Harmonic Content
The harmonic component of the
current will be a problem when the current flowing into the VFD is a
significant portion of the total current load that the step-down
transformer is capable of delivering. If a very large VFD is used
or if multiple smaller VFDs are all attached to the same line then
there may be problems. The relatively large current drawn by the
input circuit of the VFD at the peak of the voltage sine wave can
distort the voltage waveform and cause problems for other users on
the power system. Input line reactors are often used between the
VFD and the power system to help alleviate this problem.
VFDs are designed to
drive a single motor load. The manufacturer's recommendations
usually are that the wires to the motor be solidly connected to the
VFD and that the connections not be
broken under normal operating conditions. That is, one would
not normally install a contactor between a VFD and a motor because
the high voltage and arcing that are a normal part of
the contactor opening and closing can have unpredictable effects on
the semiconductor switches in the VFD and increase the risk of
failure. If multiple loads are connected to a VFD with individual
contactors for each separate load, the VFD may not be able to handle
the current surges which occur when individual loads are switched on
If a VFD were connected to a piece of equipment which
contained three-phase motors as well as other controls, it is very
likely that both the VFD and the equipment would be damaged. For
example, if there were any capacitors in the equipment connected
directly across the VFD outputs, the VFD would have to shut down
immediately or be destroyed by the extremely high currents that
would flow when the output voltage pulses were applied to the
The starting sequence of a VFD is
carefully controlled to avoid damage. When the start button is
pushed, the pulse sequence to the output switches is adjusted so
that the average voltage applied across the motor has a low value,
with low frequency. As the motor starts to spin, the voltage is
allowed to increase and the frequency is increased until the motor
reaches full operational speed. A start at full voltage and max
frequency would overload the output switches. If a VFD is putting
out full voltage at 60 Hz to one motor on its output, and a second
motor is suddenly connected by closing a contactor, then the VFD
will probably either shut down if it can respond to the overload, or
be damaged if it can't.
The circuitry in a VFD does not
allow power to flow from the motor back to the power system, as is
required when the motor acts as a brake. If the application
requires this feature, then one or more braking resistors and
additional switches must be added to the VFD so that this power is
absorbed without destroying either the output switches or the DC
link capacitor. Rotary and static phase converters intrinsically
have the ability to absorb braking currents because two of the wires
to the motor are connected directly to the supply system. A digital
phase converter is able to feed power from the generated phase back
into the power system as well.
The output voltage from a VFD is
not sinusoidal, but rather a PWM voltage. Because of the high
harmonic content of PWM voltages, dangerous voltage rises on the
output can occur if the lead length from the drive to the load
becomes too great. This distance will vary based on the switching
frequency of the drive and the impedance of the electrical system.
Inductive output filters can be installed on the output of the drive
to reduce the harmonics and mitigate the problem of voltage rises.
In general, a drive should not be more than 50 feet from the load
VFDs are especially suited as phase
converters when it is advantageous to vary the speed of the motor
being powered. They are also preferred when the motor load is large
enough to cause line disturbances during across-the-line starting.
A drive can ramp the motor speed up gradually, greatly reducing the
current needed to start the motor.