Pinpoint Funny Motor-Ground Currents
EP Editorial Staff | May 14, 2018
By Howard W. Penrose, Ph.D., CMRP aka ‘Motor Doc’
Unusual bearing or winding failures in across-the-line motors, improper operation of variable-frequency-drives (VFDs), control issues, and other unusual electrical, electronic, and mechanical issues can be the result of modern technology. Add in special grounding cases such as HRG or LRG, or ungrounded or floating systems, and things really get interesting.
With increased application of electronics, parasitic voltage and current in the ground system can be an issue. Yet, when investigating faults, how often do we check ground current? And what value are we seeking?
Unfortunately, there are no standards in place for ground current—just general guidelines based upon expected leakage current through motor-system insulation. The guidelines are used by some ground-fault-circuit-interruption-solution vendors to set trip values. What you should expect to see are values below kVA/1,000 A of the circuit. Example: In a 480-V system with a peak operating current of 500 A, the leakage current should remain below 0.24 A (240 mA). Some causes for having higher ground current include:
• unshielded cables with unbalanced voltage
• other electronic ‘noisy’ ground systems
• leakage due to contamination
• VFDs with unshielded cables
• electro-magnetic induction
• unbalanced voltage
• ground faults
• ground loops
• soft starts.
Winding faults can occur in these instances because, along with ground currents, there will be a corresponding voltage potential on the motor’s frame. When voltage also contains impulses, the voltage-to-ground reference across the insulation system, regardless of how clean the sine wave is to the windings, will also see those impulses. Such impulses act as if they are part of the motor voltage and can harm the insulation system. You may also find fluting or frosting on the inner and outer races of across-the-line-motor bearings due to shaft-current potential. This unexpected finding was determined to be the cause of common-mode ground voltage and current in the split bearings shown in the photo.
You can check for such conditions by measuring the motor ground using an ammeter. Always investigate conditions in which the current exceeds 1 A. An oscilloscope can be used to evaluate voltage across the motor, including identification of any high-frequency noise or fast-rise-time impulses. Once the condition is identified, investigate to ensure that:
• shielded cables are grounded at both ends
• grounds are part of a solid cable or located away from:
• individual phases
• ground loops
• or other conditions, as identified above.
Overall, ground conditions definitely can cause unusual reliability problems in motors and electronic systems. Although not often included in failure investigations, identification of such conditions is relatively straightforward. Chasing down the cause can be more challenging. EP
Howard Penrose, Ph.D., CMRP, is founder and president of MotorDoc LLC, Lombard, IL (motordoc.com). Among other things, he also is current chair of the Society for Reliability and Maintenance Professionals, Atlanta (smrp.org). Contact him directly at email@example.com.