Motor Protection

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Motors controlled by VFDs or servo drives require additional attention to avoid overvoltage spikes, known as dV/dt. Voltage wave reflection is a function of the voltage rise time (dV/dt) and the length of the motor cables. This phenomenon creates additional overvoltage spikes which cause premature degradation and failure to the motor insulation.

The challenge for OEMs, system integrators and distributors is to ensure the installed motors are well protected from overvoltage. Markets using VFDs have adopted a special motor, better known as, a motor rated VFD or inverter duty motor.

The motor rated VFD construction can change significantly based on the manufacturer. However following the National Electrical Manufacturer’s Association (NEMA), the greatest difference between a standard motor and an inverter duty motor is the winding insulation.

For example, a nominal 480 Vac drive using a standard grade motor should maintain performance and function with peak voltage up to 1000V. For inverter duty rated motors the acceptable peak voltage is typically 1500V.

Typical example of dV/dt measured on the motor with cable length 50m (150ft)


To better understand what causes motor failure and unforeseen challenges, it is best to first understand how a VFD is assembled. VFDs are made up of three major parts:

  • The rectifier - takes incoming AC power and converts it to DC power.
  • The DC link - several capacitors used for energy storage from the output of the rectifier.
  • The inverter - produces 2-20 kHz signal used to generate the output waveform to the motor using pulse width modulation (PWM)

PWM is a technique which generates the width of a pulse based on modulation signal information. Due to this technique, the dV/dt presents a significant concern.

Typical example of VFD layout

The Solution

Enerdoor has developed the motor protection series to protect motors from harmful overvoltage and dV/dt spikes generated by the drive’s output.

This is particularly useful for applications with variable frequency drives and servo drives. Enerdoor solutions include: common mode and differential mode chokes, sine wave filters and snubbers; all of which are designed to work with various carrier frequencies, output frequencies, and applications.

Specific Solutions

Sine Wave Filters: This series reduces the effect of the PWM by converting the drive’s output to a true sine waveform,eliminating dV/dt.
The FIN915SFH model is used with fundamental frequencies up to 25kHz. 

Typical example of a waveform between the VFD and motor using an Enerdoor sin wave filter FIN915SFH


Enerdoor snubber FIN47SNB is a unique solution to reduce common mode and differential mode noise. The snubber is used in parallel to the system and is an ideal solution for clients in need of improving the reliability of winding insulation and bearings.

Typical measurement of dV/dt on the motor side of VFD with cable length of 100m (300ft)

Typical measurement of dV/dt on the motor side of VFD with cable

length of 100m (300ft) with Enerdoor snubber FIN47SNB installed


The Enerdoor Motor Protection series reduces harmful dV/dt generated by variable frequency drives imposed onto the motor. Motor protection devices are designed to work in various applications of switching frequencies and frequency outputs.

This series carries CE and UL approvals and offers a current range from 3 to 1000A. Enerdoor motor protection includes common mode and differential mode inductance, sine wave filters and snubbers. Unique features include: high linearity vs frequency and current, very low operating temperatures, and compact dimensions.

The FIN960F high frequency inductance is a unique solution used for synchronous spindle motor applications. This line works with very low temperatures and frequency output up to 2 kHz while allowing the motor to operate at a low temperature.

The FIN905SF and FIN915SFH sine wave filters reduce the PWM effect, convert the PWM to a sine wave and eliminate dV/dt. These lines work with applications in open or closed loop feedback. The FIN905SF works with frequency output up to 70 Hz. The FIN915SFH line is used with fundamental frequencies up to 2 kHz while maintaining a very low application temperature.

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