Clean-room-duty motor and drive packages include electrostatically shielded motors ranging from 3 to 150 hp and low-voltage (up to 575 V) adjustable-speed PWM drives. Motor operating speeds range from 720 to 3,600 rpm.

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

Electrostatic shielded induction motors (ESIM) for clean rooms use a thin layer or tube of conductive material between the stator windings and the rotor. This tube conducts the current to ground, acting as a shield that stops current from entering the rotor and shaft.

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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