To reduce shaft voltages caused by capacitive coupling between the stator winding and the rotor, add a shaft grounding brush. This will provide a current path for any current coupled to the rotor and prevent bearing voltages from causing a voltage breakdown across the bearings.
The shaft grounding brush is fitted at the drive end on the outside of the motor frame. The best grounding brushes make a large area of contact to the shaft and bolt directly to the stator frame.
When current is induced into the motor shaft inductively, current flows axially along the length of the shaft causing a voltage drop along the length of the shaft. If there are bearings at eanch end of the shaft, and the voltage along the shaft length (between the bearings) is high enough to cause the insulating lubricant to break down, we will have electrical discharges through the bearings.
If a shaft brush is fitted at one end, then the voltage to cause a breakdown is halved as there is only one bearing in the circuit. Without the shaft brush, we have two bearings in series.
To prevent a bearing discharge, we need to break the circuitthrough the shaft, round vvia the shaft brush, through the stator and back via the non drive end bearing. The way to break the circuit, is to replace the non drive end bearing with an insulated bearing, but there are several variations in the insulated bearing that can be used.
1. External coated. The outer surface of the bearing is coated with a ceramic film to provide an insulation barrier.
2. Internal caoated. The inner surface of the bearing is ceramic coated.
3. Hybrid Bearing. The bearing is assembled using ceramic insulated balls.
The insulated bearing with the outer surface coated is the easiest bearing to fit and is least prone to damage, but it has the highest surface area so offers the greatest capacitance in series with the circuit path. This will allow high frequency currents to flow and of course, the induced currents are all high frequency and have been either capacitively or inductively coupled. A high capacitance bearing may only reduce the current flow by a small margin.
The insulated bearing with the coating on the inner surface offers a much reduced capacitance and therfore a much improved EDM reduction. This bearing is much harder to fit without causing damage because the ceramic coating has to be driven over a long length of motor shaft.
The lowest capacitance is offered by the hybrid insulated bearing and while it is the more expensive bearing, it is the best at reducing the EDM problems.
The common mode filter takes the form of a igh inductance high frequency ring around the three phase conductors at the output of the VFD. The earth path must not pass through this ring. Commonly in industry, theis ring is referred to as a ferrite ring, ans while a ferrite ring can be used, there are ammorphous iron rings that are much more effective. These common mode filters are effective at reducing EDM and EMC as well.
There are many theories about earthing straps and separate earth paths, but the common mode filter is probably one of the most effective non invasive means of reducing EDM.
Sinewave filters on the output of the VFD will reduce the dv/dt and therefore reduce the capacitive currents in the motor. This will also reduce the EDM in the bearings.
There are waveform generation strategies where synchronous switching is employed to minimise the induced voltage amplitudes. These strategies can minimise both the EMC and EDM issues.
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