Protecting Motors from Shaft Currents

There are many reasons why electric motors fail, resulting in unplanned and expensive system downtime. Maintenance issues like over-lubrication, unchecked intense vibrations, and moisture exposure are some of the culprits. Sometimes motors break down due to insulation failure after long periods of overheating or prolonged exposure to dirt, grease or other environmental factors. Other motors stop working due to electrical overload because many motors aren’t designed to deal with excess current safely. But that’s where Baldor Motors come in.

When AC motors are operated by variable frequency drives (VFDs)[1], those drives use pulse width modulation (PWM) to control the speed of the motor. These motors use a capacitor to induce voltages onto the shaft of the motor which can then discharge in the motor’s bearings. This could cause electrical discharge machining (EDM) with fluting, frosting and pitting damage to the bearings. This bearing damage can mean both repair costs and equipment downtime.

Additionally, VFD-operated electric motors tend to have an unbalanced voltage condition where the power to the motor isn’t supplied in a smooth sine wave, but instead in a series of positive and negative pulses. The voltage is either zero volts, positive, or negative with rapid switching between pulses.

Sometimes, motors can wear out anywhere from three to six months after installation. Due to this recurring problem and the damage inflicted on motor bearings, some design engineers are specifying bearing protection requirements for motors operated by VFDs.

More about EDM

In many industrial applications, EDM is a desired process used to intentionally shape metal. However, in the case of motors, EDM can wreak havoc on motor bearings. According to the AEGIS Bearing Protection Handbook, the high frequency switching speed of insulated-gate bipolar transistors produces voltages that can arc through the motor’s bearings. The Handbook states: During virtually every VFD switching cycle, induced shaft voltage discharges from the motor’s shaft to the frame via the bearings, leaving a small fusion crater (fret) in the bearing race. When this event happens, temperatures are hot enough to melt the 52100 vacuum degassed bearing steel and severely damage or burn the bearing lubrication.

How do we protect motors from these currents?

The need to prevent premature bearing failure is high. Fortunately, there are solutions to the problem. Baldor Super-E^â motors offer the AEGIS^â bearing protection ring, a shaft grounding device mounted to the frame of the motor.

Features of the Baldor-RELIANCE® Severe Duty XT Motor

• 1 – 100 HP 56 thru 405T
• AEGIS® bearing protection ring installed internally
• Choice of TEFC or open drip proof enclosures
• Meet or exceed NEMA Premium® efficiencies for LEED and FEMP buildings
• Class H insulation system - meets NEMA MG 1 Part 31.4.4.2
• 1.15 service factor with Class B temperature rise at rated horsepower (sine wave)
• Heavy duty frames with cast endplates, suitable for mounting in any position
• Dynamically balanced rotors for reduced vibration and quiet operation
• Ball bearings with Mobil Polyrex® EM grease
• Other speeds, horsepower ratings, and voltages available as custom motors
• AEGIS® SGR guarantees the prevention of bearing damage caused by drive-induced currents
• Additional off-the-shelf inventory available for quick shipment through our Mod Express® department
• Three-year limited product warranty

Features of the Baldor premium efficient Super-E® motors

• 230/460
• Three phase
• C-Face
• TEFC
• Foot mounted
• Internal AEGIS® bearing protection ring
• 1 thru 50 Hp
• NEMA 56 thru 405T

If you need to replace an electric motor or add another one, please talk to your local manufacturer’s rep about the benefits of using Baldor Motors with your VFDs.

[1] https://www.est-aegis.com/datasheets/AEGISHandbook_English.pdf

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