The VFD (variable frequency drive) has become the "go to" technology for plant engineers seeking an energy efficient solution for controlling electric motor speed and torque. As this informative article from ABB points out, a VFD can reduce energy consumption by as much as 60% by reducing the amount of energy drawn by a motor. Small reductions in speed result in significant energy savings.

Related Blog: Variable Frequency Drives (VFD) Frequently Asked Questions (FAQ)

Centrifugal pumps and fans running at 80% consume just 50% of the energy as compared to full speed operation. The same motor running at 1/2 speed drops energy consumption to 1/8. Savings like that are hard to ignore. VFDs also provide other multiple benefits including:

• Reduced wear and tear
  
• High reliability
  
• Efficient process control to .01% motor speed tolerance
  
• Manage load changes in temperature, pressure, and force
  

For plant managers, the VFD is the fundamental technology for an energy-efficient plant upgrade, but as any engineer can tell you, "There ain't no such thing as a free lunch." For every gain, there is always a trade-off. With the variable frequency drive, that trade-off comes in the form of parasitic capacitance. In this article, the experts at Mader Electric will explain the solutions available to ensure that you can avoid the pitfalls of parasitic capacitance to maximize system uptime and hang on to the energy savings from your VFD installations.

Parasitic Capacitance Hazards

The high-speed frequency switching capability of insulated gate bipolar transistors (IGBT) are the heart of PWM (pulse width modulation) inverters of today's VFD technology. This rapid switching performance enables the precise control which makes the VFD so valuable. The VFD provides a soft-start capability to extend motor life by reducing mechanical stresses caused by full voltage startups. The VFD can also prevent motor overheating in cyclic load conditions. Lower maintenance costs, longer life-cycle, and increased uptime are the payoffs for VFD upgrades.

While high-speed switching technology continues to advance over time, the output waveform is still far from a perfect sine wave. As a result, rapid switching can induce shaft currents in AC motor shafts. This phenomenon is known as "parasitic capacitance". It's important to note that the VFD output is not a true AC wave but a series of voltage pulses generated from a DC bus. During normal operations, parasitic capacitance voltages build up between the stator and rotor and seek the path of least resistance to discharge. Unfortunately, that path is usually via the motor bearings.

Fusion Craters and Bearing Damage

When shaft voltages discharge through bearings they can trigger arcs similar to those used in electrical welding. Thousands of tiny arcs create "fusion craters" in the bearing race, small pits which release microscopic metal fragments that cause, frosting, fluting, and premature failure of bearings. Since parasitic capacitance occurs continuously during normal motor operations, bearings designed for a 100,000-hour life-cycle can fail within 720 hours, or just one month, in motors controlled by VFDs when the parasitic capacitance problem is not addressed. When the protective dielectric property of the bearing's grease is overwhelmed by high shaft voltages, repair and replacement costs can easily wipe out any energy savings accrued by a VFD upgrade. Fortunately, there are solutions available to address the hazards of parasitic capacitance in VFD controlled motors.

Protecting Bearings in VFD Controlled Motors

Insulated bearings with a ceramic or non-conductive resin shield are one solution but high cost is a factor. Grounding brushes in contact with the motor shaft offer an economical technique to provide a low-impedance path to ground.

The best solution to protecting against parasitic capacitance discharge is to provide the lowest path to ground to divert shaft voltages away from the bearings with multiple discharge points, which brings us to the Shaft Grounding Ring. Many motor manufacturers stock standard motors with internal shaft grounding rings (SGR) installed for this purpose. SGRs can also be added internally at an electric motor repair facility or externally in the field. The SGR operates on the principle of ionization to enhance the electron transfer rate for the super-efficient discharge of the high-frequency shaft currents created by the VFD.  

About Mader Electric

At Mader Electric we have the highly trained technical professionals to keep your motor and control systems up and running efficiently and profitably. Reliable motor and control operation is our priority, and we are experts at VFD installation and programming, SCADA, U.L. 508A custom control panels, replacement controls for OEM and industrial users, as well as process and machinery automation. When you're ready to optimize your plant operations please don't hesitate to contact us.