Variable Frequency Drive Limitations

Variable frequency drives (VFDs) were first developed in the late
1960’s. A VFD is an electronic circuit that controls the speed of an electric
motor by adjusting both the voltage and the frequency applied to the motor.
Prior to VFDs, motor speed was controlled through inefficient voltage
regulators (think light dimmer switch!) or mechanical means – pulleys, gears,
or transmission systems. The original VFDs filled large cabinets and were
unreliable. Today, VFDs are smaller than a shoe box, reliable, and are used in
almost all industrial applications, pumps, fans, conveyors, machining,
compressors, etc.  However, VFD's are not miracle workers and there are important limitations. If you are already using VFDs or are considering their use for the first time, these limitations should be considers.  Learn more facts about VFDs in my paper 5 Basic VFD Facts.

A VFD can make a motor run slower than rated speed, however a major point
of consideration at low speed is cooling. A typical electric motor is cooled by a
fan on its shaft; at low speed the fan moves less air and at some point the
motor will over heat. This is generally not a problem with centrifugal loads
such as fans, pumps, and blowers because the torque required by the load
drops drastically with decreasing speed. In this case, the motor is doing less
work, and there is less waste heat to dispose of.

A VFD can drive a motor faster than its nameplate speed, however, above the
rated speed, the motor looses torque (twisting force). At higher speeds, less
and less torque is possible. The maximum continuous power (speed times
torque) is limited by the motor design, therefore a VFD cannot deliver more
power than which the motor is thermally capable.

A VFD can also make a motor more efficient, but only at reduced load and/or
reduced speed. A drive system will not use less power when the motor is
running at rated speed and rated load. In fact, it uses slightly more due to the
losses within the drive.

A VFD can make a motor reverse without the use of contactors, but it can’t
make the reversal faster than the combination of the motor’s max torque and
the load’s inertia allows.

A VFD can make a motor produce more than its rated torque but only briefly.
The amount of time is limited by either the drive’s overload capacity or the
motor’s thermal capacity. A drive cannot make a motor produce more than its
maximum torque. Max torque or Stall Torque is generally not shown on the
motor’s nameplate. The iron in the motor can only sustain a certain amount of
magnetic flux density, even if iron is driven harder, the flux density will not go
up. The amount of torque a motor creates at the flux limit is the most torque
that can be achieved.