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Load-side solutions for power quality in systems using Variable Frequency Drives

How Variable Frequency Drives (VFDs) Work

VFDs serve as a way of controlling the speed of AC motors by varying the frequency of the power source using pulse width modulation (PWM). This is done by switching the transistors, IGBTs, or thyristors, on and off continuously. While the introduction of VFDs has brought precise control of motors and greater energy savings, it has also introduced new problems that, left unaddressed, can cause downtime.

The Destructive Effects of Power Distortions 

The presence of power distortions between VFDs and motors can damage equipment, jeopardize productivity, and decrease profitability.

Common Mode Waveform

Common Mode

Three-phase utility power produces three smooth sinusoidal waves which at any point average a sum of zero.
This creates an optimal scenario with zero Common Mode.

VFDs give off power in a continuous generation of pulses, which on average achieve a sinewave.
However, the sum at any point is not always zero, which results in damaging common mode. This can cause motors to break down over time.

Peak Voltage WaveformPeak Voltages

The pulse of VFDs is not a clear square pulse. Each rise and fall of the
pulse has an over shoot or transient over voltage. This phenomenon is also known as a “reflected wave.”
These voltage spikes, especially in long cable leads, can reach dangerous levels and damage drives and motors.

Rise Time WaveformRise Time

Modern VFDs utilize IGBT systems that create extremely fast rise times
which can increase motor performance. This can also result in increased motor insulation
heating, which can reduce motor life over time.


Problems that can occur without motor protection filtering:

  • Damage to motor bearings

  • Unexpected ground fault trips

  • Erratic behavior of VFDs and PLCs

  • Premature motor insulation failure

  • Cable damage


Motor Protection is Critical

MTE offers a full line of best-in-class products to protect motors and improve productivity.

Protection against common mode and peak voltage at various cable lengths

RL reactors
(for cable leads up to approximately 300 ft.)
RL reactors are unequaled in absorbing power line disturbances. They are built to
withstand even the most severe power spikes. They reduce nuisance tripping,
reduce harmonic distortion, and minimize long lead effects.

dV Sentry™ filters
(for cable leads up to approximately 1,000 ft.)
The dV Sentry and its patented Triple Defense Core, is proven reduce common mode,
protect against peak voltage, and reduce rise time – all in one unit.

SineWave GuardianSineWave Guardian™ filters
(for cable leads up to approximately 15,000 ft.)
This best-in-class filter delivers unequaled performance in cleaning the PWM waveforms
generated by VFDs. It virtually eliminates high frequency content and voltage peaks,
thereby reducing motor heating to provide extended motor life – and less downtime