Many of today's adjustable- speed drives use transistors that switch at high frequencies for more responsive control of ac induction motors and to reduce audible motor noise. However, there's a catch. These high frequencies can generate motor shaft currents that damage the shaft support bearings.
Several methods are available to insulate bearings and protect them from such shaft currents. They work well in some situations, but usually become less reliable with time. A new class of motors goes one step further – adding an electrostatic shield to prevent these harmful currents from ever reaching the shaft.
Turning on the juice
Adjustable-speed PWM drives with insulated gate bipolar transistors (IGBTs) generate as many as 20,000 switching pulses per second to produce a smooth sinusoidal power source to the motor. This high switching rate produces a very short turn-on time or voltage-rise time, about 1 μsec. Voltage-rise time is also sometimes expressed as the change in voltage per change in time (dv/dt), as switching occurs.
The combination of fast switching and short voltage rise time causes a condition called common mode noise, in which the motor stator induces an electrical charge to the rotor. The electrical current in the rotor then passes to the motor shaft and into the support bearings. At this point, it discharges across the grease-filled gap between the balls and outer race of the bearing, often producing pitting and fluting in the race. The damaged bearings generate an unpleasant audible noise, and they soon fail.
Shocks in clean rooms
Electrically damaged bearings usually show up in drive-powered motors that, for application reasons, run at or near constant-speed. Because the speed doesn't vary much, electrical discharges occur in a regular pattern on the bearing's rolling elements (balls) and outer race. These repeated discharges cause accumulative damage to the race. Motors that frequently change speed are immune to such damage because the current spikes occur in a distributed pattern over a larger area of the bearing. Therefore, surface wear is even and gradual.
Bearing damage is also more common in direct drive applications – where the motor drives the load directly rather than through a belt and pulleys. A belted arrangement creates a side load on the motor that pulls the bearing balls against the outer race on one side, letting current flow freely between balls and race rather than by discharging.
These conditions (constant speed and direct drives) are commonly found in clean rooms used for manufacturing semiconductors, pharmaceuticals, and high-tech instruments. Clean rooms require lots of ventilating fans, sometimes hundreds, to circulate air after filters have removed contaminants. Such fans typically operate around the clock at near-constant speed to maintain uniform air flow.
Each fan arrangement consists of a low-voltage (575 V or less), adjustable- speed IGBT drive connected to an ac motor, which directly connects to the fan. These adjustable- speed drives help in moving air through clean rooms according to stringent air quality specifications.
Shield steps in
Because of the clean-room problems, a new method was developed to protect motor bearings from electrical currents. This method pairs an electrostatic shielded induction motor (ESIM) with a PWM inverter drive. Within an ESIM motor, a thin electrostatic shield between the stator and rotor eliminates the potential for shaft voltage and current at its source rather than directing current away from the bearings or insulating the bearings as with other protection methods.
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A conductive material in the shield creates a cage effect, called a Faraday Shield, which means that an electrical charge on a conductor sits on its outer surface. The shield, located between the stator windings and rotor, forms a conductive tube in the motor that directs the current to ground. As a result, there is no electrical charge between the stator and rotor, hence no shaft current.
By eliminating shaft voltage or currents, this shielding technology protects motor bearings, extending their life and efficiency more reliably than other methods such as shaft grounding or insulated bearings. The shield lasts as long as the motor windings and requires no maintenance. It can be built into virtually any new motor, although the added cost would be impractical for small motors, generally under 3 hp.
To date, electrostatic shields have been applied mostly to motors for air-handling systems in clean rooms. This includes 3 to 150-hp motors that operate at constant speeds and low voltages. Many such motors are low-speed versions (600 to 1,200 rpm) that drive fans directly and produce less audible noise. However, electrostatic shields can be applied to induction motors above 150 hp and those that run up to 3,600 rpm.
Sometimes it isn't practical to buy new motors that shield bearings against higher voltages. For example, you may need a temporary fix until a new production line is installed, and the cost of new motors isn't justified. Or maybe you have a lot of fractional horsepower motors for which shielding is too expensive. If so, you'll need to rely on one of several methods for protecting the bearings, such as grounding the shaft, insulating the bearings, or filtering out high voltages.
Shaft grounding. With this method, a brush (usually carbon) contacts the shaft and conducts current to the motor's ground, diverting it away from the bearings.
If dirt or grease collects on the shaft, the brush may fail to pick up the current, and sparking will occur across the brush or bearing. Therefore, regular cleaning is required.
Insulated bearings. You can prevent current from entering bearings by insulating them. Different approaches include covering the bearing seats with glass-impregnated tape, coating the entire bearing housing with ceramic material, and coating the bearing races.
Regardless of the method, insulation has some drawbacks. For example, the material may eventually be covered with dirt, grease, or water, all of which conduct electricity. Also, currents generated by the drive may find a weakness in the insulation. When motor shaft current exceeds the voltage threshold (dielectric strength) of the insulation, it begins to break down and let the current flow through to the bearing. Once this occurs, you need to replace the insulation.
Hybrid bearings that contain nonconducting ceramic balls offer a promising and more reliable insulating technique. These ceramic balls are more expensive than their metal counterparts, and haven't been used much for this purpose, however they're immune to the shortcomings of other insulation methods.
Conductive grease. This method entails using a special bearing grease that conducts shaft current to the motor's ground and prevents discharging. It's a simple method and works well as a temporary fix.
However, the conductive properties of the grease may interfere with its lubricating characteristics. Also, the conductive agents in the grease eventually separate and the grease loses its ability to protect against shaft currents as well. The grease must be changed regularly (a costly operation in clean room applications) to prevent shaft currents from transferring to the bearings.
Shaft grounding, insulated bearings, and conductive grease work well in some applications. However, these methods generally require regular maintenance to keep them working effectively.
Reactors and filters. Another technique, used mainly to protect motor insulation, involves placing either reactors or filters between the drive and motor. These devices either minimize voltage peaks or slow the rate of voltage rise, which keeps reflected waves (peak voltages) below levels that would destroy motor insulation. This technique also reduces shaft currents and bearing damage, although it's difficult to gage the extent to which this happens.
However, a reactor introduces a voltage drop that may reduce the motor's ability to generate its rated torque. Filters range widely in cost and some types need to be carefully matched to the application.
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NEMA insulation standards
Fast switching IGBTs have another undesirable side effect besides shaft currents. They generate voltage reflections, also called reflected waves, with high peak voltages that damage motor insulation. This is especially true where long power cables connect the drives and motors. The limiting length is generally considered to be 200 to 300 ft, although it can be much less depending on the drive design and carrier frequency.
To help deal with this effect on motors, the National Electrical Manufacturers Association (NEMA) has developed a standard (MG1 Part 126.96.36.199) for motor insulation capability. This standard is based on established limits for the change in voltage over time (dv/dt of not less than 0.1 μsec rise time) and the peak voltage (1,600- V maximum peak).
The standard is a start toward helping motor manufacturers define the insulation integrity needed to prevent insulation damage due to reflected waves. Because fast-switching IGBTs can cause both insulation and bearing damage, their rising popularity has led motor manufacturers to look at ways to minimize both conditions.
What's up with carrier frequency?
Ahigh carrier frequency (fast switching) offers two benefits. First, ac drives simulate line power by using pulse-width modulation. With higher frequency, the drive generates more pulses, producing a smoother sine wave to more closely simulate line power. Second, motors connected to ac drives produce audible noise at a tone close to the carrier frequency. Frequencies above 3 kHz make this noise less discernible to humans; above 10 kHz, it can't be heard at all.
IGBT drives operate at carrier frequencies up to 16 (or even 20) kHz. However, higher frequencies are better only up to a point. They tend to reduce heat losses in motors, but increase them in drives. Also, they generate higher voltage spikes, which may damage motor insulation.
A 1994 study, conducted in Belgium and published by IEEE, found that:
• Motor heat losses decrease at carrier frequencies up to 10 kHz, then start to increase again.
• Drive heat losses increase with higher frequency.
• Audible noise drops substantially up to 6 kHz, and above that the noise level doesn't decrease much.
Because of these tradeoffs, the study recommended 6 kHz as an optimum carrier frequency for the best balance of low losses and low noise. As a result of these and other studies, the push for higher frequencies has now eased considerably. Today, drives that operate in a 6 to 10 kHz range are becoming the most popular.
Tom Lowery is custom/configured product line manager, responsible for design and application of adjustable-speed drives and motors, for Reliance Electric, Cleveland, Ohio.