Shaft grounding

Keep motors alive on variable-frequency drives.

By Adam Willwerth

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Already common in HVAC, pumping and industrial automation systems, the use of variable-frequency drives (VFDs) is growing. VFDs are smaller and more powerful, more reliable, easier to program and less expensive than ever. But, VFD/motor systems must be designed for reliability and trouble-free operation to keep the energy savings, which can reach 20% or more, from being wiped out by a system failure.

VFDs can induce electric currents on motor shafts that ruin bearings, shorten motor life and diminish system reliability. One way to mitigate the effects of these currents is to ground the motor shaft to protect bearings and eliminate expensive repairs.

Energy-saving potential

The VFD converts line power to direct current and filters it to smooth the waveform. A pulse-width-modulation inverter turns the DC back to AC, but now in variable form. The typical output frequency, also called the carrier or switch frequency, is between 2 kHz and 12 kHz.

Short of dismantling the motor, there are two ways to check for bearing damage from induced shaft currents.

– Adam Willwerth

VFDs that drive motors directly in constant-torque applications avoid using any more power than necessary. With encoder feedback, a VFD also can control motor speed.

Regardless of the application, engineers who select the VFD should understand the entire system, including the possible current paths. A pulse-width-modulated waveform has high-frequency components (harmonics) that are coupled capacitively to the motor shaft and can discharge through the bearings. Even inverter-duty motors are vulnerable to bearing failure from VFD-induced currents.

These currents can cause pitting, fusion craters and “fluting” (Figure 1). This electrical-discharge machining leads to bearing noise, premature bearing failure and motor failure. There is considerable evidence to prove that VFD-induced bearing damage is a large and growing problem. Consider:

    Figure 1. Fluting is a problem that sparking inside a bearing can promote.
    Figure 1. Fluting is a problem that sparking inside a bearing can promote. 
  • Most motor bearings are designed to last 100,000 hours, yet motors controlled by VFDs can fail within one month (720 hours).
  • An HVAC contractor recently reported that every 30 hp to 60 hp fan motor he installed in a large building project failed within a year (two within six months). Repair costs totaled more than $110,000.
  • Several large pulp and paper companies noted that VFD-controlled AC motors typically fail within six months because of bearing damage.
  • The largest motor manufacturer in the United States has cited eliminating drive-related motor failures as its No. 1 engineering challenge.
  • Almost a dozen Internet blogs focus on VFD-induced shaft currents.

Other problems appear if the motor isn’t designed for use with a VFD, or if the motor or VFD are mismatched to the load. For example, when maintaining constant torque, a motor loses efficiency and runs hotter at lower speeds (but hotter still when controlled by a VFD). If the motor is operated at less than 30% of maximum speed, it might need extra cooling or thermal protection.

Similarly, a VFD-controlled motor’s torque may drop more quickly at lower speeds than when using pure sine-wave power. For constant-torque loads, a VFD should be rated for 60 seconds at 150% of the load. A VFD’s current rating also limits the load-acceleration rate.

The cable connecting a VFD with a motor shouldn’t exceed 50 ft. total length to avoid standing waves that meet at the motor terminals, in effect doubling the voltage to the motor. If a longer cable is required, use additional line filtering to protect the motor and other nearby sensitive equipment from harmonic content and radio-frequency interference (RFI). RFI also can be reduced by enclosing motor leads in a rigid conduit. Regardless of length, the cable between a VFD and the motor it regulates can be enclosed in a corrugated aluminum sheath or another kind of grounded, low-impedance shielding.

VFDs might not be appropriate for pumps that maintain high pressure. During periods of low flow, the motor might not be able to slow down enough while maintaining pressure. VFDs that shunt excess energy from the DC bus can be used in systems that require dynamic braking.

Look at bearing damage

Figure 2. An oscilloscope trace reveals the presence of pulsing shaft currents.
Figure 2. An oscilloscope trace reveals the presence of pulsing shaft currents.

Short of dismantling the motor, there are two ways to check for bearing damage from induced shaft currents: measuring either voltage or vibration to look for energy spikes in the range of 2 kHz to 4 kHz. Both methods require special equipment and experienced personnel to conduct tests and analyze the results. Both are best used early on to establish a baseline and to monitor trends later. Neither method is foolproof.

By the time a vibration test confirms bearing damage, it usually has reached the “fluting” stage. Likewise, the main benefit of a voltage test might be the relief it provides when the results indicate no bearing damage. If a baseline voltage measurement is taken when a VFD is installed, successive tests might provide early warning of harmful current loops. But, there are many variables — predicting bearing damage is not an exact science.

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