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Motors that are operated with a variable frequency drive (VFD) are more susceptible to bearing failure. In standard generated sine waves, the voltage is balanced; this means that the amplitudes of the wave form on both the positive and negative sides are equal. In PWM (pulse wave modulated) generated sine waves, produced by VFD's, the voltages are not balanced. This unbalance can cause voltage potential in the motor structure, leading to voltage passing between the rotor and stator to ground through the motor bearings causing damage such as:
As the voltage arcs between the bearings and the housing, it leaves microscopic weld splatters. These splatters make the bearings' rolling elements; ball or roller, and internal raceways rough; which could lead to the bearing failure.
Parasitic capacitance builds between the motor stator and rotor irons; resulting in a buildup of voltage, which is passed into the motor shaft. This current moves through the shaft looking for the easiest path to ground, that path usually happens to be through the bearings.
The shaft grounding ring is placed near either end of the shaft (see Figure 2). The carbon fiber brushes attached to the ring contact the shaft. The brushes now become the easiest path to ground for current running through the shaft. The current no longer passes through the bearings, which eliminates bearing failure caused by the unbalanced PWM wave.
There are several designs of shaft grounding rings. While they provide the same protection, each of them offers a unique mounting options specific for your need:
Splits rings come in two pieces allowing for insulation on a motor shaft that it still connected to the driven equipment without removing the pulley or coupling. These rings can either be mounted with screws, clips or with conductive epoxy.
These rings are installed flush with the motor using conductive epoxy, clips, stand-off brackets or bolt through. The Epoxy needs 10 minutes to dry after installation. This line (method of installation) of rings is often installed when motors are in a shop for repair or in the field prior to installation on the driven equipment.
Rings can be press fit installed if the motor is in a shop. The motor end bracket is bored out to a 0.0005"-0.0015" interference fit.
When using a variable frequency drive to operate a motor it is important to check the manufactures motor warranty information. All motor manufacturers will void bearing failure warranty claims if bearing protection is not used when operated on a VFD. See specific motor manufactures disclaimers.
Our engineers recommend that motors being operated with a VFD be tested for shaft voltage amplitude and frequency. The motor should be installed in its running condition, and should be tested through the entire RPM/frequency range and load. This will help determine the best corrective action to implement if excessive shaft current is present. Larger motors (generally 400 frame size and up: 404/405T) with excessive shaft current may need the additional protection from an insulated bearing, or installing a shaft grounding ring at each bearing, or using iPro extra capacity rings.
Shaft grounding rings are only a small cost in comparison to the initial purchase price of the motor and VFD, and drastically less when downtime and replacement costs are considered. They are easy to install and are warrantied for the life of the motor. Anytime a motor is being operated with a VFD you should consider purchasing a shaft grounding ring and other protection if required.