City shares repair costs
Coordinating the remediation with Biby was Water Treatment Systems Superintendent Don Koci. To keep the plant running, they shut down and shipped only one or two motors at a time to IEMCO.
The shop sent a technician to Hutchinson to measure shaft voltages. After repairs, a shop representative came to the plant for the re-installation and startup of each reconditioned motor, to confirm that voltages were below the level at which they could damage bearings.
After the first 10 damaged motors had been refurbished, Koci sent the remaining seven directly to the shop at the city’s expense. To some degree, this was for preventive maintenance, since bearing damage was less advanced in some of these motors than in others.
Today, all 17 motors are running with no problems.
Grounding rings weren’t on the market when Hutchinson’s plant was designed. Since then, PEC has adjusted motor specifications so that:
1) All new motors that will be connected to VFDs must be equipped with shaft grounding rings.
2) Shaft grounding devices must be factory-installed or installed by a reputable motor shop with expertise in the proper installation of the devices.
3) If the shaft grounding devices are not factory-installed, a third party shall be engaged to test the installation to ensure no damaging shaft currents are present.
4) A warranty against VFD-induced bearing damage or failure for the life of the motor is required.
“If you install the rings beforehand, you avoid these problems altogether,” says Biby. “The [$1,500-$2,000] investment is insignificant when you look at the total cost of a large project.”
What to watch out for
The elephant in the room is the growing awareness that motors could be built to withstand shaft currents in the first place. A few manufacturers have recently made the AEGIS bearing ring, which is manufactured by Electro Static Technology of Mechanic Falls, Maine, standard on certain models. However, retrofitting remains the most common way to prevent bearing damage.
Sadly, some failed motors were originally marketed as “inverter-rated,” “inverter-duty,” or “inverter-ready” models. Specifiers didn’t understand that most had extra insulation to protect the windings, but nothing to protect the bearings.
Systematic plan for a long-term solution
Virtually all VFD-driven motors are vulnerable to bearing damage. To make the savings generated by VFDs sustainable, an effective long-term method of shaft grounding is essential. Although a ring safely bleeds damaging currents to ground, vertical pump motors need something more.
Carriers should be electrically isolated by disconnecting the motor from the pump shaft electrically (though not mechanically). In addition to protecting the motor’s thrust bearing from electrical damage, this keeps shaft currents from jumping to the bearings of the pump itself, or to the bearings of a gearbox, tachometer, encoder, etc.
“It’s the combination that does it,” says Wilkins. “The grounding ring does a great job, but the ring in a vertical motor is competing for the current that exists in the possible path of the thrust bearing. In vertical applications the thrust bearing can be a lower-impedance path to ground because of the Hertzian point contact of the thrust bearing and the load that it’s placed under. So you have to eliminate that current path via insulation on the carrier.”
Many motor manufacturers and repair shops use carriers fabricated from inferior metals, inappropriate coatings, or application protocols that fail to provide long-lasting protection. IEMCO uses a tightly controlled flame-spray welding procedure and grinds each newly coated carrier to very tight tolerances. The finished carrier has a hardness of Rockwell 50C and provides a resistance of more than 1 gigohm at 1,000 volts.
“It’s a process that we feel we have perfected,” says Wilkins. “After we’ve added the grounding ring and upgraded the carrier, the motor is truly inverter-ready.”
—Adam Willwerth is sales and marketing manager for Electro Static Technology (www.est-aegis.com) of Mechanic Falls, Maine.