Prevent shaft current from causing motor damage
Although many problems could occur in dc motors, a major issue that can be rooted out and corrected is shaft current. Shaft current is an issue that could result in the motor failing, damage to the bearings, and the possibility for extended downtime while repairs take place. The issue of shaft currents has been a problem for years, but it is becoming an increasing problem because variable frequency drives (VFDs) are being used more frequently.
The primary issue associated with bearing currents is electrically induced bearing damage (EIBD). Due to the current that is flowing through the bearing, it is possible for pitting, arcing and fluting to take place, resulting in premature failure of the bearing, leading to extensive downtime and repair costs. Although it is less likely to occur, it is also possible for shaft current to damage a flexible coupling if it is connected to the motor drive end shaft and the shaft is conductive.
Most dc motors operate on three-phase power. The sum of the three phases was designed to be at 0 V, but when they are operated with a VFD, the power reaching the motor is not in the form of a smooth sine wave, it is a series of pulses. As a result, there is never a balance of power and the pulses could lead to shaft current.
The voltage looks for a path to ground, reaching the bearings and resulting in pitting and other problems in the bearing race. Eventually, it could lead to many pits in the bearing race, which further leads to fluting and additional damage to the race, along with bearing degradation and eventually, motor failure.
Testing dc motors for shaft current
Consider testing dc motors for shaft current as part of a preventive maintenance program. Any of the motors capable of producing shaft current can be tested on-site and analyzed for any potential problems that could occur.
Since it is possible to catch those problems before they become a larger issue, repairs can often be recommended and carried out on-site. As part of the testing process, the motor shaft is probed at an exposed area, gathering the shaft voltage data to be analyzed on site. Recommendations can then be given for a course of action to take that would result in the greatest benefit to your facility.
Shaft current solutions
There are a variety of solutions available to deal with shaft current. One or a combination of the following are useful countermeasures:
Faraday shield: One approach to the shaft current problem may be an electrostatically shielded induction motor. Reduced voltage from the insulation stops bearing damage.
Grounding brush: Installing a grounding device, typically made of copper, on the shaft provides a route for any shaft current to travel away from the bearings.
Shaft grounding ring: Similar to a grounding brush, but uses conductive microfibers that lead shaft current to ground.
Insulated bearings: Shaft current through the bearings is eliminated, but insulated bearings do not prevent current from occurring. Since the current needs to go somewhere, there may be damage to other equipment.
Shielded cable: Installing shielded cable significantly improves high-frequency grounding with a low impedance path between the motor and the VFD. Testing for shaft voltage is only one part of a complete preventative maintenance program.
By having such a program in place, companies are more likely to experience fewer problems with unexpected downtime and equipment failure.