Electric motors controlled by drives are at risk of destructive bearing currents causing premature bearing failure. No one argues this, but opinions vary on how to assess that risk – What can you do, what can you measure, to find out whether a motor is at risk of premature failure? In this two part series, we’ll look at the options.
Bearing Current and Bearing Voltage
Unfortunately, you can’t directly measure bearing current in the field. Measuring it requires special equipment and serious alterations to the motor, and is unfeasible outside a research lab. You could measure bearing voltage instead: Whenever current arcs through a bearing, the voltage (electric potential difference) across the bearing suddenly drops. By “suddenly,” we mean over the course of under 20 nanoseconds. For comparison, modern drives have switching speeds between tens and hundreds of nanoseconds (see figure).
Unfortunately, it’s also impractical to directly measure bearing voltage. So what can you measure?
Measuring Shaft Voltage
The shaft voltage – between the motor shaft and frame – is an excellent measure of the bearing voltage, because the shaft is in metal-metal contact with the inner bearing race, and the frame contacts the outer race. And luckily, it can be easily measured with an oscilloscope. The shaft voltage changes too rapidly to be accurately measured by a multimeter – we recommend a scope with at least 100 MHz bandwidth.
When a shaft voltage reading shows a relatively slow buildup and then a sudden collapse, that is the smoking gun for capacitive discharge bearing current, or EDM current, which occurs when the rotor first gets capacitively charged by the common mode voltage in the motor windings, and then discharges through a bearing.
The Damage Done
You can even compare the damage done by discharges with shaft voltage readings. The energy stored in a capacitor is
E = ½ CV2,
where C is the bearing’s capacitance and V is the shaft voltage when the discharge occurs. This equation says that a discharge at 20V releases four times as much energy as a discharge at 10V.
Most of this energy is released into the bearing as heat, and although the amount of energy in one discharge is small, it is released across a tiny area. That small area heats up to thousands of degrees – hot enough to melt steel and create the microscopic pits that are the hallmark of bearing current damage.
Detecting Other Bearing Currents
EDM current is a risk for all unprotected motors on VFDs. There are two other drive-induced bearing currents, with different mechanisms, but they can also be detected by shaft voltage measurements. These other bearing currents are more destructive, because they can flow continuously after the first arc, whereas shaft voltage discharge current only arcs once and then stops until the rotor gets capacitively recharged.
Rotor ground current, in motors without good high frequency bonding to the drive, can be detected by measuring the shaft voltage at one end. And high frequency circulating current, in larger motors (over 100 hp/75 kW), can be detected by simultaneously measuring shaft voltage at both ends of the motor, as shown in the figure at the right. The shaft voltage goes negative at one end (top), and positive at the other. At 2 divisions from the left, the voltage at the negative end arcs through the bearing and returns to zero, but the voltage at the opposite end remains positive.
The Bottom Line
Measuring shaft voltage and examining the waveform will tell you whether current is arcing through the bearings. In Part Two, we will look at two other methods of assessing bearing current in motors on drives, and see how they compare to shaft voltage testing.