Drive systems in modern times: bearing current
Bearing currents: what are they and why can they be problematic?
A few months after starting up, some new drive installations can have their motor bearings fail. Bearing currents can be induced in the motor shaft and discharged over the bearings to cause this.
Under normal circumstances, modern motor design and manufacturing practices have virtually eliminated bearing failures, but rapid switching in modern drive systems generates high frequency currents that can damage bearings. The metal transfer between the ball and races occurs when these currents find their way to earth over the bearings. This process is called electric discharge machining (EDM).
Modern drive systems can damage bearings if they do not have adequate earthing during rapid switching.
Where do bearing currents come from?
A common-mode voltage in the drive system is responsible for bearing currents. Under normal conditions, a three-phase sinusoidal power supply is balanced and symmetrical. Therefore, the neutral is normally at zero volts. The PWM switched three-phase power supply, however, cannot achieve perfect phase balance instantly. Due to the potential created between the inverter output and the earth, stray impedances in the motor cables and motor windings will cause currents to flow. This is referred to as a common mode current.
What causes high frequency bearing currents?
Motors over 100kW (frame sizes IEC 315 and above) experience high frequency bearing currents due to asymmetrical flux distribution in the motor shaft. Due to the high frequency of the pulses fed by the inverter, the leakage capacitances of the motor winding provide paths for the currents to leak to earth. This produces a voltage between the ends of the shaft. A circulating type of high frequency bearing current occurs if the induced voltage is high enough to overcome the impedance of the oil film of the bearings.
Through the earthing circuit, leakage current tends to return to the inverter via the paths with the lowest impedance. By earthing the motor shaft via the driven machine, part of the leakage current can flow back to the inverter through the bearings, shaft, and driven machinery. The shaft earthing current is caused by poor stator grounding and is referred to as bearing current.
For motors under 30kW, the internal division of the common mode voltage may be such that high shaft voltages create high frequency bearing current pulses due to the relative size of stray capacitances within the motor. In installations where the shaft is not earthed via the driven machinery, this can happen.
Finding the bearing damage
Bearing damage typically occurs after a few months of use, although the time frame varies depending on the drive set-up. Since multiple parallel inverter-motor circuits result in a lower impedance, sectional drives are more susceptible to bearing current damage than stand-alone ones.
If a bearing is damaged, a loud running noise can be heard, or continuous vibration monitoring can be used to detect the damage. If the bearing is damaged, it should be replaced before it causes a loss of production. You can determine if the rolling tracks of a bearing are fluted or frosted, and the rolling elements have a grey, dull finish.
How to prevent bearing damage
High frequency bearing currents are affected by three methods: proper cabling and earthing; breaking bearing current loops; and damping the high frequency common mode current.
- Motor cables should be symmetrical multicore. To avoid carrying currents at fundamental frequency, the protective earth conductor must be symmetrical. Conductors surrounding all the phase leads or cables that contain three phase leads and three earth conductors can achieve symmetry.
- Create a short, low-impedance path for the common mode current to return to earth conductors on the inverter. In order to achieve this, shielded motor cables are the best and easiest option. Shields need to be continuous and of a good conducting material, such as copper or aluminium, with 360° termination at both ends.
- Install high frequency bonding connections between the installation and known earth reference points to equalize the potential of affected items. Use braided copper straps 50-100mm wide; flat conductors have a lower inductance than round wires.
- By isolating the non-drive end bearing, insulating the shaft from the frame, or by using an insulated coupling between the motor and the driven machinery, the bearing current loop is broken.
- Adding impedance to the common mode loop will dampen the high frequency common mode current.
The carrier frequency of many inverters can be adjusted between 1 kHz and 10 kHz. Reducing the carrier frequency to 1-3 kHz can provide some short-term relief in cases of bearing damage.