Unexpected bearing failures in electric motors can cause downtime, lost production, repair costs, extra manpower expense and other negative consequences.

One cause of these bearing failures can be electrical erosion. This is internal bearing damage due to induced currents in the motors under normal running conditions. This type of damage is more likely to occur in motors driven by a Variable Speed Drives (frequency drives, inverters).

The VFD induces electrical currents that may increase until they find discharge paths to earth via the motor frame. The electric currents pass through the bearing and when the voltage reaches a level to overcome the lubricant film layer in the bearing, electrical discharge occurs. However, not all motors driven by a VFD suffer from electrical erosion, although the potential risk is always present.

SKF has a new detection device, the Electrical Discharge Dectector (EDD) Pen (order TKED 1), that will find these electrical currents before they create problems. With just a few minutes, you can easily train any person within your facility to use this uncomplicated instrument.

Once detected, the damage can be prevented. SKF has two ways: insulated bearings: INSOCOAT, which will solve most of the problems, or Hybrid bearings – ceramic balls, which will solve all of the problems.

It is difficult and expensive to find the problems without the new EDD pen. You can dismount the bearing and analyze it. This is usually after it has failed. Another way is to use an oscilloscope to detect electrical discharges, which requires wiring and sensors and a good knowledge on electrical analysis issues, as well as an oscilloscope and trained user. Neither is as easy or as cost-effective as the EDD Pen.

The EDD Pen detects electrical discharges (or pulses) in bearings of electrical motors on a chosen time base (10 seconds, 30 seconds or indefinite). It then displays a number on its LED screen. The number displayed is the number of discharges detected and counted by the EDD Pen in a given time frame. The more pulses counted, the greater the risk of electrical erosion and lubricant degradation occurrence. The LED backlit screen allows for use in dark environments.

In general, depending on motor type and duty, the tester can be up to 30 cm (12 inches) away from the motor to obtain a representative reading.

Like all condition monitoring equipment used for predictive maintenance, the EDD Pen should be used in a consistent way. Before every measurement, make sure that the EDD Pen is at the same distance and direction from the bearing, and check that you measure using the same time base. Like all condition monitoring measurements of electric motors, you can only compare when the running conditions are similar. To get the best results, use the EDD Pen to compare your motor in the same running conditions over time. This means that it is not wise to compare results if your motor is running at different speeds or loads.

You should also not compare an inspection of your motor that has just started and is cold with one that has been running some time and has reached its operating temperature. To compare inspections of your motor running under different conditions will add little value to your predictive maintenance program and will cause confusion in interpreting the results.

So, if you suspect that your electric motors are vulnerable to unexpected bearing failures, the answer is predictive maintenance. With the new SKF EDD Pen, you can determine the condition of machinery while it is in operation. Talk to your IBT Sales Rep to learn more or write to us.