VFDs and EDM

What is EDM?

EDM is an acronym for Electrical Discharge Machining and is one mode of damage that is done to bearings where high frequency bearing currents are caused to flow.

What causes EDM with VFDs?

The prime source of EDM is the steep rising/falling voltage edges of the PWM waveforms. Reducing the dv/dt and the di/dt will reduce the EDM without any alteration to the motor design or connection. The high frequency component of the voltage which causes the EDM is known as “Common Mode Voltage”. Some motor designs are more prone to EDM than others, and some PWM waveforms are also more prone to causing EDM than others.

Mechanisms:

There are essentially four mechanisms for the creation of shaft voltage that can cause EDM, these are:

Capacitive Coupling

Inductive Coupling

Frame Voltage

External Coupled (Electrostatic)

Capacitive Coupling

Capacitive coupling is due to the capacitance between the stator windings and the rotor and is a function of

the exposed area of the stator winding and the gap between the stator winding and the rotor. In some designs, the slots in the stator are quite open to enable the windings to be fitted into the stator and in other designs, the slot openings are smaller.

The capacitive coupling between the stator and the rotor causes the shaft voltage to raise and lower relative to the motor frame such that the voltage at each end of the shaft is equal. There is no current flow along the length of the shaft.

A grounding brush across one bearing will provide a low impedance discharge path and prevent the voltage across both bearings rising to a level that will initiate an insulation breakdown.

Inductive Coupling

Inductive coupling is due to high frequency currents flowing around the stator and inducing a current to flow in the rotor shaft. This induces a differential voltage between the ends of the rotor shaft. A grounding brush across one bearing will typically increase the voltage across the other bearing and cause an earlier onset of EDM on the other bearing unless the opposite bearing is an insulated bearing.

Frame Voltage

Frame Voltage is the voltage developed on the frame of the motor due to the common mode voltage from the VFD.

If the motor is insulated from all other conductive paths, then the frame voltage will not have a major impact on EDM.

If the motor shaft is electrically connected to other conductive paths, then a high frame voltage can cause a high “bearing” voltage if the shaft voltage is held to ground by the driven load. For example, a motor driving a pump where the inlet/outlet of the pump is well grounded by conductive piping, perhaps connected to underground pipes etc. This can result in severe EDM both in the motor, (typically drive end) and in the pump. An insulated coupling will overcome some of this problem. If the frame of the motor and the frame of the pump are well bonded, the issue will be reduced. A drive end grounding brush on the motor shaft will dramatically reduce this problem.

External Coupling

Electrostatic problems occur where the motor is driving insulated materials with frictional surfaces. Grounding brushes will  reduce the voltage across the bearings and reduce EDM issues due to electrostatic problems.

Solutions

If the mechanism of a particular installation is not determined, then the fitting of a shaft grounding brush at the Drive End and an insulated Bearing at the Non Drive End is a very safe way of reducing the potential for EDM issues under all mechanisms.
Shaft Brushes at both ends will increase inductive shaft currents and should not be used.