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Electric motor items to consider when adding a VFD to your system


So you are thinking about adding a variable frequency drive (VFD) to an electric motor in your plant. Great! Variable Frequency Drives are great controls that allow you to adjust the speed of the machine at the push of a button or program it into your automation platform.

But what could this do to your motor? There are a variety of factors to consider before adding a drive to an electric motor, such as:

Speed Range

Motors are designed to operate at a certain speed. If they are designed to operate on A VFD at certain speed ratios, those are listed (Dependent on torque type) on the nameplate. These motors have been tested by the manufacturer and are nameplated to be acceptable for use in those situations. This does not mean a motor that is not namplated for these ratios will not work, you just may want to consider if it designed to operate at a different speed or not.

Bearing Failures on VFD Controlled Electric Motors


Shaft currents can have a destructive effect on electric motor bearings. This article, with information from our friends at AEGIS Shaft Grounding Rings, is intended to inform professionals about these voltages and their effects on bearings in electric motors.

Because of the high-speed switching frequencies in PWM inverters, variable frequency drives induce shaft currents in AC motors. The switching frequencies of insulated-gate bipolar transistors (IGBT) used in these drives produce voltages on the motor shaft during normal operation through parasitic capacitance between the stator and rotor. These voltages, which can register 10-40 volts peak, are easily measured by touching an oscilloscope probe to the shaft while the motor is running. (Reference: NEMA MG1 Section


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