White Paper:

AC Drive

CurrentShield Technology

Motors & Drives for Cleanroom Applications

Introduction

Cleanroom technology has matured dramatically during the past decade because of its importance to the semiconductor industry. Semiconductor manufacturing requires stringent air-quality standards to ensure the elimination of small airborne contaminants.

Fans, powered by electric motors and variable-speed drives, are critical to moving air through cleanrooms according to user specifications. Control flexibility and energy efficiency are provided by the drives.

PWM drive switching frequencies, as well as unique cleanroom operating characteristics, however, sometimes result in undesirable motor shaft currents, a side effect that causes bearing damage within the motor through pitting and fluting. The audible noise generated by the damaged bearings is unacceptable in the cleanroom environment and motor failure soon follows.

A highly effective method for preventing such cleanroom motor damage is to pair a PWM inverter with a motor specifically designed to attenuate shaft voltages and eliminate damaging bearing currents.

Baldor's patented CurrentShield technology uses this approach. A new line of Reliance electrostatic shielded induction motors, paired with a line of standard PWM inverters, has demonstrated positive results in attenuating motor bearing currents. This solution has been proven to prevent premature failure of motor bearings in cleanroom environments.

Typical cleanroom in the semiconductor industry.

The idea of developing a motor that protects its bearings from damage caused by stray shaft-borne currents is not new. Various methods for insulating bearings from motor shaft currents have been in use for many years, especially on motors that are connected to a variable-speed supply source.

Baldor began studying the shaft-borne current phenomenon in 1994. The result is CurrentShield technology, which allows electrostatic-shielding of induction motors. This is a superior way of preventing potential bearing damage caused by electrostatic discharge to the motor shaft.

Baldor's current focus with CurrentShield technology is on relatively small motors, from 1 horsepower through 50 horsepower, and at low voltages. Cleanrooms require large numbers of relatively small motors, mounted directly in the air stream of the system's air handlers. Because of the large number of motors and their position in the air stream, the noise of individual motors must be minimized. Lower-speed motors (600 to 1200 RPM) produce less audible noise, are direct-driven for total lower installed cost, and are usually preferred to achieve lower fan speeds. Motor manufacturers make significant efforts to provide precision balance and precision bearings - beyond the specifications of standard industrial motors for cleanroom applications.

When motor bearings become damaged with pitting and fluting (see figure 1) as a result of stray motor shaft currents, the noise in the bearings quickly becomes unacceptable in the cleanroom environment.

Cleanroom-Duty motor and drive packages with CurrentShield technology.

With CurrentShield technology, Baldor has developed a PowerMatched motor and drive package that is superior to those currently offered to the industry by other manufacturers. Other manufacturers are using AC motors and drives with mechanical shaft grounding kits, insulated bearings, or special conductive grease. The goal of our new cleanroom-duty motor family is to eliminate motor shaft currents, thereby protecting bearings, extending motor life, and solving the problem at a lower cost and more reliably than these other methods.

Another important benefit of CurrentShield technology is that it lasts for the life of the motor winding and the shield requires no maintenance.

Figure 1
Surface roughness of a bearing race due to electrical fluting. The pitting and fluting causes unacceptable audible noise and eventual motor falure.

Motor voltage peak and dv/dt limits, NEMA MG1-1993, Part 30, Figure 30-5 (NEMA 1993a, 1993b). NEMA Standards To help deal with the effects of fast-switching IGBTs on motors, NEMA (National Electrical Manufacturers Association) has defined a standard for motor insulation system capability. NEMA standard MG1 Part 31.40.4.2 attempts to define dv/dt or change in voltage over time as well as maximum peak voltage. It is described as a 0.1 micro second rise from 10% to 90% of steady state voltage with a 1600-volt maximum peak. The standard is a start toward helping motor manufacturers to define motor insulation integrity. It also has an indirect relationship to the motor shaft bearing current problem. Electro-magnetic Interference (EMI) generates undesired effects that induce current into parts of the motor and surrounding areas where they may be disruptive. EMI includes motor shaft currents and reflected waves. This white paper addresses the effects of motor shaft currents and solutions to the problem

Cleanroom Applications

Significant growth of the semiconductor industry during the last decade has led to advances in cleanroom technology. Simply put, cleanrooms are designed to eliminate airborne contamination through insulation and thoroughly filtered air. This environment improves both product quality and output. First used as surgical facilities, cleanrooms are now vital to the semiconductor, pharmaceuticals, and high-tech instrumentation industries, to name a few, and are used as research and development facilities as well.

Cleanrooms keep contaminants that could cause circuits and critical equipment to fail while being manufactured, or even in production years later, out of the air. Fans are the key components in the air handling systems of cleanrooms. As these systems have become more sophisticated, variable-speed drives have become crucial to providing motor-control flexibility and energy efficiency.

In a typical cleanroom application, hundreds of fans operate around the clock to keep air moving through the cleanroom after it has been processed by filters that remove nearly all contaminants. The fans ensure that columns of air move from the ceiling to the floor at the same speed, eliminating variations in air currents, as well as recirculating and filtering the air. Due to the extremely stringent indoor air quality (IAQ) requirements in cleanrooms, make-up air must be mixed with recycled air on a continual basis. Variable-speed motor control adjusts make-up and recirculation fan-speed to maintain consistency.

If a fan fails, the variable-speed motors increase fan output from other fans until the system is operating at 100% again - maintaining air quality and flow.

Motor Shaft Currents

Semiconductor industry experts agree that variable-speed control of motors in cleanroom air-handling systems save significant amounts of money due to improved efficiencies and better process yields.

These benefits aside, AC drives with fast-switching insulated gate bipolar transistors (IGBTs) create new application issues to consider. Variable-speed drives produce many switching pulses that result in common mode noise. (See figure 2.) Common mode noise is a result of faster rise times (dv/dt) and higher carrier frequencies - the rate at which pulses are generated from the drive to the motor. Rise time, or dv/dt, is defined as change in voltage per change in time.

Common mode noise produced by variable-speed drives often results in undesirable motor shaft currents, a side effect that causes bearing damage through pitting and fluting of the bearing race (illustrated in figure 1). The audible noise generated by the damaged bearings is unacceptable in the cleanroom environment and motor failure soon follows.

Figure 2
Common mode equivalent model.

Figure 3 Motor shaft currents pass to the bearing and discharge with enough energy to pit the ball bearing and the bearing race.

When the motor shaft is turning, the bearing grease insulates the bearing balls and the bearing race. The motor bearings and the race act as two capacitors. The stator and the rotor generate charge accumulation through capacitive coupling that is electrostatically induced into the motor shaft. This current then passes through the motor shaft to the bearings and discharges from the balls with sufficient energy to pit the bearing race (see figure 3).

Variable-speed drive applications in which motor speed is frequently changing are immune to the effects of motor shaft current because the motor shaft current is distributed across the entire surface of both the bearing and the race. Bearing wear is even and gradual. The motor shaft current problem appears in cleanroom applications because the fan motors run for months on end at the same speed. This means that the discharge appears on the same location of the bearing and race, causing pitting and fluting.

In cleanroom applications motor speeds are varied infrequently. Once the air-handling system is set up and running, the speed of the motor/drive combination is changed only occasionally to account for increased particle count in the cleanroom filters.

Current Solutions to Bearing Pitting and Fluting

To minimize or eliminate the effects of motor shaft currents, motor manufacturers rely on a number of different methods for grounding or insulating the shaft. These methods must be effective when the shaft is rotating at its normal operating speed.

1. Shaft Grounding Kit - With this method of diverting motor shaft currents away from the bearings, a shaft grounding kit is installed with a conductive grounding brush that is in contact with the shaft. The brush conducts shaft currents to the motor's ground. Eventual oxide buildup on the shaft reduces the brush's effectiveness. If the shaft gets covered with dirt or grease, the brush may fail to pick up the currents and sparking occurs across the brush or the bearing and race. Regular maintenance is required to help ensure that this method is effective. In addition, there is no easy way to tell if the brush is working.

2. Conductive Grease - This method entails using special conductive grease that conducts the motor shaft currents through the grease to the motor's ground. But there are concerns that the process that makes the grease conductive interferes with the performance of the grease as a lubricant. Also, the conductive agents in the grease eventually separate and the grease loses its performance capability as a shaft protector as well as a lubricant. If the grease isn't changed regularly, a very costly operation in cleanroom applications, shaft currents build up and transfer to the bearings. There is a hidden danger to this method because separation in the grease can't be visually detected - the grease may appear to be effective, when in fact it's not.

3. Insulated Bearings - By isolating the bearings with insulation, it is possible to prevent motor shaft currents from entering the bearing race and damaging the bearing balls. There are different approaches to insulating the bearings. Some motor manufacturers cover the bearing seats with glass-impregnated tape; others coat the entire bearing housing with ceramic material; some even coat the bearings themselves. The face of the insulator may eventually coat with dirt, grease, or water, which are conductors. Currents generated by the inverter will eventually find a weakness in the insulation. The question is what is the dialectic strength of these materials? When motor shaft currents exceed the voltage threshold of the insulation it begins to breakdown and the current flows through the material.

Grounding kits, conductive grease, and insulated bearings are alternatives that do work well in some applications. They do have the limitations noted above and, they still allow motor shaft currents to exist. What these methods have in common is that they are passive in nature. They fail to eliminate motor shaft currents at the source; they simply attempt to keep the currents away from the bearing and race. Ultimately, to eliminate the common mode condition generated by the variable-speed drive is the answer.

The CurrentShield Solution

Baldor's CurrentShield is a new, different, and better solution for addressing the motor shaft current issue in cleanroom applications. CurrentShield is the industry's first electrostatic shielded induction motor. CurrentShield technology dispenses with shaft current at the source through the use of a conductive material that creates a Faraday Shield (see Faraday Cage Effect). The Faraday shield, or cage effect, means that the electrical charge on a conductor sits on its outer surface. In this case, the shield is placed between stator windings and the rotor (see figure 4), forming a conductive tube in the motor, leading the current to the ground. Therefore, no electrostatic field is present within the conductor.

Figure 4
Reliance Cleanroom-Duty motors with CurrentShield technology. An electrostatic field is placed between the stator windings and the rotor - eliminating motor shaft current discharge.

Figure 5
Shaft current meets NEMA standards, dv/dt bearing currents attenuated by a factor of 30, and there is no current discharge.

The resulting electrostatic shield in the motor eliminates current discharge between the stator and the rotor.

Figure 6 Electric motors, controlled by variable-speed drives are critical to maintaining air quality in cleanrooms.

There are significant advantages provided by CurrentShield technology over shaft grounding, conductive grease, or insulating the bearings. The CurrentShield is part of the motor, permanently installed and requires no maintenance. This solution can be applied to any motor where bearing currents are an application concern.

• CurrentShield Versus Shaft Grounding - CurrentShield motor technology eliminates shaft voltage from occurring, reduces installed cost, eliminates dust generated by the carbon brush, lessens life cycle cost because no maintenance of the brush or brush holder is required, eliminates environmental contamination, and provides a single-source solution.
• CurrentShield Versus Insulated Bearings - CurrentShield eliminates shaft voltage, which still occurs with insulated bearings, provides a better overall motor insulation system so the motor can operate over long lead lengths, and incorporates a system solution rather than a component solution for improved reliability. Although this solution requires an additional process in the motor's manufacturing cycle, the base motor is a standard, proven product.

While conducting research and testing different materials and designs, Baldor considered three different types of electrostatic shield alternatives:

• Conductive Foil Tape
• Conductive Slot Stick Covers
• Conductive Coating

While conductive foil tape and conductive slot stick covers were effective in eliminating motor shaft currents, neither the materials nor the manufacturing process was as practical as using a conductive coating. Figure 5 illustrates the effectiveness of CurrentShield technology. These new CurrentShield cleanroom-duty motors completely eliminate bearing current.

The real test of a cleanroom-duty solution is to see how it performs under actual application conditions. Baldor has developed a set of tests to simulate cleanroom semiconductor fabrication load and run speeds to provide motor useres with data that clearly shows the effectivness of CurrentShield motor/drive packages. These test results, performed in both Baldor labs and at fabrication plants, prove that a matched cleanroom-duty motor and variable-speed drive is the optimal solution for attenuating motor shaft currents under cleanroom operating conditions.

Baldor's CurrentShield technology pairs the new Reliance Cleanroom-Duty motor detailed above with a Reliance GV3000/SE drive. This package has been proven to help reduce bearing damage due to shaft voltage levels, extending motor life and reducing maintenance costs.

Conclusion

As this white paper has illustrated, the idea for developing motors that protect their bearings from stray shaft-borne currents is not new. Although various methods of providing this protection are available, their effectiveness is debatable. CurrentShield technology, resulting in electrostatic-shielded induction motors, is a superior way of preventing bearing damage from electrostatic discharge to the motor shaft.

Baldor's current focus with CurrentShield technology is on relatively small motors (1 - 50 horsepower) because of continuing problems that the semiconductor industry is experiencing with air handling systems in cleanrooms, but the technology can be applied to most inverter-driven induction motors.

Cleanrooms require large numbers of relatively small horsepower motors, mounted directly in the air stream of the system's air handlers. Fans, powered by electric motors and variable-speed drives, are critical to moving air through cleanrooms according to user specifications. The non-sinusoidal output currents produced by variable-speed drives, however, often result in the undesirable motor shaft currents addressed in this white paper.

CurrentShield technology provides cleanroom motor users with a new, superior approach for dealing with the motor shaft current phenomenon. The goal of CurrentShield technology is to eliminate motor shaft currents, thereby protecting bearings, extending motor life and efficiency, and solving this problem at a lower cost and more reliably than the shaft grounding kits, conductive grease, and insulated bearings offered by other manufacturers. We are so confident in this new technology that we back this motor/drive package with a five-year limited parts and labor warranty. If the drive/motor package fails from motor bearing currents within this period, we will replace it absolutely free.

Document D-7723

This material is not intended to provide operational instructions. Appropriate Reliance Industrial Company instruction manuals and precautions should be studied prior to installation, operation, or maintenance of equipment.