How a dV/dT Filter on the Load Side of a VFD can: 1. Reduce motor audible noise 2. Reduce motor motor heating and lower efficiencies 3. Reduce motor winding and bearing damage with long lead lengths

1. Jul. 31 2017
Category: VFD dV/dT Load Filter
The name “Line Reactor” immediately brings to mind that this inductive device is used on the line (input) side
of the VFD. However, that same or de-rated line reactor can often be used on the load (output) side of a VFD
as it delivers power to the motor.
First, we have to understand that the output of a variable frequency drive is not a sine wave but a series of
square DC pulses of varying width. Typically there are 2,000 to 20,000 of these pulses a second. By
controlling the width of these pulses the VFD can simulate a wide variety of frequencies to vary the motor’s
speed. This Pulse Width Modulated (PWM) signal can cause some issues with the system including:
 Increased motor audible noise
 Increased motor heating and lower efficiencies
 Motor winding and bearing damage with long lead lengths
Representation of a VFD PWM output and how it simulates a sine wave.
Typically a 5% line reactor or higher or a dV/dT filter is installed close the output of the VFD to help mitigate
these issues. Some manufacturers will de-rate an input line reactor to be used on the VFD load side due to
higher heating. The HPS Centurion R can be used on both the line and load side of a VFD without de-rating.
Reducing Motor Audible Noise and Heating
The installation of load reactors at the VFD output can help reduce the quick rise time at the edges of the
VFD’s PWM DC square waves. This “rounding” off the square waves better recreates a sine wave at the
motor. This causes an overall improvement in efficiency and potential lowering of the motor’s temperature
rise by 10 to 20 degree C which also increase motor MTBF. In addition, the PWM DC square waves can

2. cause a higher pitched audible whine in a motor. Adding an output line reactor can reduce the audible motor
noise by 3 dB or more.
Motor Winding and Bearing Damage with long lead lengths.
Drive manufactures will typically designate a wire length between the VFD and motor that if exceeded, a
reflected wave phenomenon may occur. In this instance, the output voltage of the drive can be multiplied by
either two or four times the base voltage. This typically occurs somewhere in the motor windings or cable
run. If this occurs it typically manifests itself as:
 Over-Voltage Cable Failures
 Motor Winding Insulation Failure
 Bearing failure:
o The high voltage charge escapes the winding’s insulation but does not cause immediate insulation
failure. One of the most likely paths to ground for the charge is through the windings, across the
bearings and through the motor shaft to the grounded machinery. The arcing through the lubricant on
the motor bearings causes tiny pits to form. If enough pits form, bearing failure will occur.
While a line reactor may also help mitigate the issue, a dV/dT filter will better lower the top voltage of the
reflected wave, especially at longer lead lengths. If using a line reactor, a minimum of 5% is recommended,
higher impedances can be achieved by wiring 2 reactors in series. Typically adding a dV/dT filter to the
output of a VFD will increase the rise time of the edge of the DC pulses and mitigate issues with reflected
waves. A dV/dT filter combines a line reactor with an RC filter board that acts as a snubber circuit. If the
VFD to motor distances exceeds 1000’, other technologies such as sine wave filters may need to be considered.