Why Test Meters Have Trouble Reading VFD Output Voltage

A motor prefers sinusoidal power, but a drive’s output voltage is NOT sinusoidal. As shown below, a VFD’s output voltage is a series square pulses which range from wider to thinner to emulate a sinewave.  The following image represents typical voltage output from a VFD.

These square pulses can cause many testers/multimeters to give illogical results. The peak of the pulses is always the same. Certain meters will show little change in voltage as the motor speeds up. The average voltage to the motor is increasing but the meter is reacting mostly to the peak of the pulses.

Only meters with built-in low-pass filters can read VFD output voltage properly.The motor’s strength is a function of the fundamental voltage applied and special meters are required to read the that value properly.

Note: A drive’s output pulses can, in some instances, can cause damage to a motor, especially the bearings. This condition is sometimes called a “compatibility issue.” and will be covered in a forthcoming blog.

To learn more facts about variable frequency drives, download our 5 Basic VFD Fact Guide.

Controlling Shock Hazard from Variable Frequency Drives

Variable frequency drives can present a shock hazard to workers and users.

When a drive is energized but disabled, and the motor is stopped, it’s NOT open…there is no air between the line voltage and the motor leads. To safely work on the motor, the line input to the VFD must be opened. Some newer drives have a feature called “safe torque off,” which also is NOT a contractor. When the “safe torque off ” is active and the motor is stopped, the power connection is NOT open…there is no air between the line voltage and the motor leads. To safely work on the motor, you must open the line input to the VFD. “Safe torque off ” simply interrupts the gate power to the output transistors so that rotation is not possible. The transistor and the diodes still connect the
motor leads to the line voltage, which can cause a shock hazard unless the line input to the VFD is opened.

At Invention House we take great precaution when building and working on VFDs because of these conditions.

Consult the users guide, website FAQ page or the help center of your VFD manufacturer before working on any VFD.

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.