High Voltage Spike (DV - DT) and Motor Protection Methods
High Voltage Spike (DV - DT) and Motor Protection Methods
High Voltage Spike (DV - DT) and Motor Protection Methods
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Courtesy TCI
Common mode voltage occurs when the voltages on the three output
lines of a drive do not sum instantaneously to zero. dV/dt lters slow
down the rate of change of PWM switching as seen by the load. This
reduction in the rate of change results in increased capacitive coupling
impedance between bearings and bearing races. This increase in
impedance, in turn, reduces the damaging Common Mode currents.
VFD Panel Buying Guide
(Illustration)
07/04/2016 0
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28/6/2018 High Voltage Spike (dV/dt) and Motor Protection Methods
Courtesy TCI
Note that these graphs are at a VFD carrier frequency of 2.5kHz. There
are very few spikes above 1000V with 70 feet of cable. The spikes are
much worse with 500 feet of cable. Note that these are examples only.
A speci c application may have no problem with voltage spikes at 450 Industry Spotlight: Using
feet, then have signi cant problems at 500 feet, and have no problem VFDs for Irrigation and
at 550 feet. The only way to determine whether there is a problem is to Farming
measure it. Also, remember that general purpose motors can handle 16/02/2015 2
1000V, while VFD rated motors can handle 1600V.
We do not recommend using line reactors. Our experience shows that
they can actually exacerbate the problem in some applications. A dV/dt
lter will usually solve the problem.
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Courtesy TCI
In this example, the carrier frequency has been increased to 8kHz. Note
that the higher frequency exacerbates the voltage problem.
High carrier frequencies may also damage dV/dt lters, so make sure
you stay within the published limits for that particular lter.
Is there Another Way?
Mitsubishi uses a proprietary motor output algorithm called Mitsubishi
Soft PWM and not only reduces motor noise, but also protects from
dV/dt. Other manufacturers may or may not have these capabilities.
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28/6/2018 High Voltage Spike (dV/dt) and Motor Protection Methods
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Mitsubishi’s soft PWM algorithm distributes the motor noise over a
wider range of frequencies, thus reducing the noise in any one
frequency band and resulting in lower dB and a more pleasing sound.
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