Timberline Blog

Electrical Fluting Phenomenon

While performing routine preventive maintenance at a customer’s location, my technician Reuben noticed the sound of what seemed to be a failed bearing noise coming from the 20 HP supply fan motor on a Trane Voyager 3 packaged roof top unit. He removed the belts from the blower section, checked the bearings and determined that they were pitted. He test fired the motor and noticed that he was reading 5 VAC from the motor shaft to ground, which ended up being the root cause of the failure.

The phenomenon Reuben experienced is called electrical fluting. This occurs when current is passed through the motor bearing instead of a grounded source. PWM (Pulse Width Modulation) drive switching frequencies result in an undesirable motor shaft current. The side effect of this is bearing damage within the motor through pitting and fluting. Variable frequency drives (VFD’s) use PWM to create different frequencies, which make motors able to run at varying speeds resulting in energy savings. This is a great technology, but it comes with a cost.

Upon determining this, I quickly provided the customer a proposal for a new high efficiency Baldor Super E motor along with a shaft grounding device. This motor was chosen due to the quality, efficiency and its Xcel Energy rebate potential. The shaft grounding device was field installed on the new motor to re-direct the current directly to ground rather than through the bearing. This inexpensive, intelligent solution eliminated the potential of fluting in the future.

Thanks to Reuben’s attention to detail, and our service teams’ quick response, our customer was able to have this motor changed at a convenient time prior to permanent failure. A month or so later, my customer received a rebate of $850 from Xcel Energy.