The damage that shaft currents can do to bearings in electric motors and connected equipment has been studied for close to a century (Table 1). But the underlying causes of shaft currents weren’t understood well until recently, so solutions that worked in one case often failed in another. While there’s still much to learn, the information and materials now available make it possible to develop effective solutions.
Although the phenomenon has been called shaft voltages, circulating currents, bearing currents and circulating voltages, the term shaft currents is the most common. That's because current, not voltage, does the damage that leads to premature bearing failure.
A basic principle of electricity is that voltage will be induced in a conductor that passes through a magnetic field. The same thing happens as the shaft, rotor and various internal parts pass through the magnetic field that travels through an electric motor. This interaction induces a voltage/current in the frame-shaft-bearing path that is the root cause of shaft currents.
It’s not the voltage that damages a bearing, but rather the current. Fuses fail because the current, not the voltage, is too high. But, we don’t have a practical way to measure the current through the shaft, so we measure the magnitude of the voltage instead.
Magnetic dissymmetry causes circulating currents
Early on, shaft currents in electric motors were caused by magnetic dissymmetry (gaps in the iron, such as segmented laminations used to build stator cores larger than about 35 in. (900 mm in diameter), uneven air gap, circulating currents in the parallel circuits of a three-phase winding, or variations between bolt-in DC poles. Because the electromagnetic field in the stator rotates around the stator bore, those dissymmetries are a source of induced voltage in the frame. Through-bolts in rotors are another source, but more about them later.
The current that magnetic dissymmetry induces “circulates” from the frame through one bearing, along the shaft, through the other bearing and back to the frame. The practical solution to this problem is to break the circuit by insulating the opposite drive end (ODE) bearing. Just as turning off a light switch stops the current flow through a light bulb, insulating the ODE bearing interrupts the circuit through the bearings (Figure 1).
Figure 1. An insulated bearing prevents circulating currents from destroying the bearings.
Another solution is to install a shaft grounding brush in parallel with the ODE bearing (Figure 2). Although this doesn’t stop the current, it diverts some of it from the bearing to a parallel path (Figure 3). The lower the resistance of the brush-shaft interface compared to the resistance through the bearing, the more current will be diverted from the ODE bearing. Although a grounding brush is effective for reducing bearing failures on the ODE, it does nothing to reduce the current through the drive end (DE) bearing. Because the DE bearing is usually larger than the ODE bearing, though, it can better withstand shaft currents.
Figure 2. Mounting a grounding brush reroutes only some of the circulating current.
Figure 3. Graphicical representation of parallel path of grounding brush.
Note that brush resistance is a critical part of this solution. Not just any carbon brush will do. The special grounding brushes for this purpose have extremely low resistance. The key to success is to provide a path that’s much lower resistance than the path through the bearing to divert as much current as possible. If the resistance through the brush/shaft path is equal the bearing/shaft path, half of the current will pass through the bearing. Bearing life would be extended, but the problem would still exist.
Another problem with the grounding brush solution is that resistance across the brush/shaft interface increases as the shaft becomes dirty or corrodes. That diverts more current back through the bearing.
Although it’s current that damages bearings, manufacturers don’t publish current-carrying capacities for bearings. Even if they did, there’s no practical way to measure the current passing through the shaft. That means we must rely on measuring shaft voltages to determine if harmful shaft currents are present.
The guideline many manufacturers use is 100 mV maximum for ball bearings and 200 mV for sleeve bearings. (NEMA MG-1 part 126.96.36.199 suggests a limit of 300 mV measured end-to-end on the shaft. But it’s impractical to make the measurement with the motor in service.) With shaft currents caused by magnetic dissymmetry, it was rare to measure shaft voltages greater than a few volts.
Variable-frequency drives up the ante
Enter the pulse-width-modulated variable-frequency drive (PWM VFD). When VFDs started to become popular, shaft currents became a major issue. For quite a while, service centers tried to solve the problem by insulating the ODE bearing or adding a grounding brush. Unfortunately, each of these so-called tried and true solutions yielded mixed results. Users disagreed on which was best, and many reported less than satisfactory results with either method. A belt-and-suspenders approach seemed to work best, but it wasn't clear why.
Longer cable runs worsened the problem, as did poor grounding connections, earning certain drives reputations as motor killers. Higher switching frequencies (20 kHz) also caused more bearing problems than slower 5 kHz drive settings, but there was no clear line above which to expect trouble.