The below video is a 15 minute segment of an hour long presentation given by Todd Hatfield of HECO at the 2017 Reliability, Process, and Maintenance (RPM) Symposium. This presentation discusses common preventable causes of electric motor failures. This section of the presentation discusses bearing related failures as bearings account for about 51% of all electric motor failures.
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 220.127.116.11)
Shaft circulating currents are caused from "dissymmetries" in the magnetic paths through the stator and rotor iron. As the rotor rotates within the stator and as the stators magnetic field rotates, small differences in the magnetic "reluctance" of the core parts generate small voltages between the ends of the shaft.
This is our third and final article in a series of articles on ball bearings in electric motors. You are encouraged to view our prior articles at the links below:
This article picks up where those left off... What else should you consider when thinking about electric motor ball bearings? What else could go wrong? Well, there is a number of items that need to still be considered, but let's look at a few of the major items.
Before reading this article, please first read our article on Ball Bearing types and options. That is the first article in this series of articles on electric motor ball bearings. This article is focusing on some factors to consider when greasing ball bearing motors. There is a variety of useful information in this article, but please be sure to check with the manufacturer of the motor on their recommended greasing frequency, amount of grease, and type of lubricant to use. This article is speaking in generic terms and utilizing "rules of thumb" on greasing bearing in electric motors. This is for educational purposes only, you should check with the manufacturer of your motor before implementing any form of a electric motor greasing program.