Bearing currents in modern drive systems (1)

What are bearing currents and why are they a problem?

Some new drive installations can have their motor bearings failing only a few months after start-up.This can be due to bearing currents induced in the motor shaft and discharged over the bearings.
Modern motor design and manufacturing practices have nearly eliminated bearing failures under normal circumstances, but the rapid switching in modern drive systems generate high frequency currents that can damage bearings.
When these currents find the path to earth over the bearings, metal transfer between the ball and races occurs.This is known as electric discharge machining or EDM.

What is the source of bearing currents?

Bearing currents are due to the existence of a common mode voltage in the drive system. A typical three-phase sinusoidal power supply is balanced and symmetrical under normal conditions. Normally, the neutral is therefore at zero volts. But with the PWM switched three-phase power supply, perfect balance between phases cannot be achieved instantaneously.This creates a potential between the inverter output and earth which will force currents through stray impedances in the motor cables and motor windings.This is known as common mode current.

How are high frequency bearing currents generated?

• In large motors, above 100kW (frame sizes IEC 315 and up), high frequency bearing currents are induced in the motor shaft due to asymmetrical flux distribution in the motor.Voltage pulses fed by the inverter contain such high frequencies, that the leakage capacitances of the motor winding provide paths for currents to leak to earth.
  This induces a voltage between the shaft ends. If the induced voltage is high enough to overcome the impedance of the oil film of the bearings, a circulating type of high frequency bearing current occurs.


• When leakage current returns to the inverter via the earthing circuit, it tends to seek the paths with the lowest impedance. If the motor shaft is earthed via the driven machine, a part of the leakage current can flow through the bearings, shaft and driven machinery back to the inverter.This type of bearing current is referred to as shaft earthing current and is caused by poor stator grounding.
  
• In small motors, less than 30kW, due to the relative sizes of internal stray capacitances, the internal division of the common mode voltage may be such that it causes shaft voltages high enough to create high frequency bearing current pulses.This can happen in installations where the shaft is not earthed via the driven machinery.