Fulfilling the promise of variable frequency drives

By Adam Willwerth

Figure 1

A Closer Look At Bearing Damage

Every VFD-controlled AC motor develops a parasitic capacitance between the stator and rotor.  Short of dismantling the motor, there are two main ways to check for bearing damage from induced shaft currents — measuring vibration and measuring voltage.  Both require special equipment and experienced personnel to conduct tests and analyze the results.  Both are best used to establish a baseline early on, so that trends can be monitored later.  Neither method is foolproof.

By the time vibration tests confirm bearing damage by identifying particular energy spikes in the range of 2 to 4 kHz, the damage has usually reached the “fluting” stage (Figure 4).  Likewise, the main benefit of voltage tests may be the relief they provide when the results indicate no bearing damage.  If a baseline voltage measurement is taken right after a VFD has been installed, successive tests may provide early warning of harmful current loops, but there are many variables — predicting bearing damage is not an exact science.
Induced shaft currents, which are sometimes called common mode voltage, can be measured by touching an oscilloscope probe to the shaft while the motor is running [Figure 1].

Figure 2

These voltages repeatedly build up on the rotor to a certain threshold, then discharge in short bursts along the path of least resistance, which all too often runs through the motor’s bearings to the frame (ground).  Serious bearing damage is thought to be more likely in systems that operate with high carrier frequencies, a constant speed, or inadequate grounding.

A high carrier frequency of course means a high discharge rate.  For this reason it is advisable to purchase a VFD that permits fine tuning of the carrier frequency in increments no larger than 1 kHz.  In general, it is advisable to keep the frequency as low as possible, and no higher than 6 kHz.

There is some debate as to whether constant-speed operation makes VFD-controlled motors more vulnerable to electrical bearing damage. Obviously, the question is moot since VFDs are seldom used in applications that require constant speed.

Figure 3

There is no doubt that inadequate grounding significantly increases the possibility of electrical bearing damage in VFD-driven motors.  Viewed under a scanning electron microscope, a new bearing race wall is a relatively smooth surface [Figure 2].  As the motor runs, tracks eventually form where ball bearings contact the wall.  With no electrical discharge, the wall is marked by nothing but this mechanical wear.  Without proper grounding, VFD-induced electrical discharges can quickly scar the race wall.
 
During virtually every VFD cycle, these induced currents discharge from the motor shaft to the frame via the bearings, leaving small fusion craters in ball bearings and the bearing race wall. These discharges are so frequent that before long the entire bearing race becomes riddled with pits known as frosting [Figure 3]. The damage eventually leads to noisy bearings, but by the time such noise is noticeable, bearing failure is often imminent.  Since many of today’s motors have sealed bearings to keep out dirt and other contaminants, electrical damage has become the most common cause of bearing failure in VFD-controlled AC motors.

Figure 4

In a phenomenon called fluting [Figure 4], the operational frequency of the VFD causes concentrated pitting at regular intervals along the bearing race wall, forming washboard-like ridges.  Fluting can cause excessive noise and vibration.  In an HVAC system, the noise may be magnified and transmitted throughout the entire building via ductwork.

Adam Willwerth is Development Manager for Electro Static Technology, 31 Winterbrook Road, Mechanic Falls, ME 04256-5724, TEL: (207) 998-5140, FAX: (207) 998-5143, www.est-aegis.com.

Click here to read Part 1 and Part 3 of Shaft Grounding