Cast-iron frames are more rigid and their bases are machined flatter than NEMA alternatives, resulting in easier alignment, lower vibration and longer bearing life than a steel band motor with a weld-on base.

Cast housings have cooling fins and generally dissipate heat more effectively than steel band motors.

IEEE 841 motors include extra corrosion proofing, features such as epoxy paint, neoprene gaskets, grease fittings and reliefs, and corrosion-resistant hardware to extend their life.

Winding insulation systems are designed to withstand the voltage spikes of pulse-width modulated (PWM) adjustable speed drives

commonly used in pump and fan applications.

Test data are supplied with that motor, including measurement of winding resistance, no-load readings of current, power and nominal speed at rated voltage and frequency, mechanical vibration check and high-potential test of the winding for each motor.

Bearing isolators often are used on motors when additional protection from dust, liquids and slurries is needed. As these isolators rotate, they expel liquids and keep them from entering the bearing cavities.

A labyrinth device normally affords optimal protection whether the motor is stationary or running. Most motor manufacturers install labyrinth seals on the output shaft of IEEE 841-compliant motors, and several manufacturers install them on the fan end as well.

Bearing failures result from improper lubrication. Often, too much or too little grease is used, or greases are not compatible with each other. Contamination is another leading cause of bearing failures. Bearing fluting problems from conduction of electrical current have also led to failure, particularly in motors with PWM adjustable-speed drives.

Why energy conservation?

Electric motor-driven industrial equipment consumes 63 percent of the energy used in U.S. industry, according to the 1998 DOE report. This energy consumption could be reduced by as much as 18 percent if companies were to adopt "proven efficiency technologies and practices." Much of the savings would come from installing new premium-efficient motors in place of older, less-efficient designs.

If you were to survey motors used in U.S. industry today, according to DOE data, only about 10 percent would be found to comply with the minimum efficiency levels mandated by the EPAct. Motors with significantly higher efficiency levels are available for even more energy savings. These high efficiency motors are built to comply with the NEMA Premium efficiency standard as defined in NEMA MG1 part 12.60.

Besides reducing energy costs, these motors offer many additional benefits. Even though IEEE 841-2001 does not specify that motors be built to NEMA Premium efficiency levels, many manufacturers build 841 motors to NEMA standards. NEMA Premium efficiencies are specified for two-, four-, and six-pole low-voltage (<600 volts) motors 1 hp - 500 hp, and also for medium voltage (5,000 volts or less) motors 250 hp - 500 hp.

Premium-efficient motors are more efficient at full load because of reduced internal losses. Even greater efficiency can be attained when machinery is operated at less than full load.

Almost all of the additional benefits are the result of premium-efficiency motors' lower temperature rise. Cooler-running motors allow the motor's insulation to last longer, increasing motor life. For every 10 Degrees C decrease in the motor's operating temperature, its insulation system life doubles. Bearing and lubrication life also are increased.

Because they run cooler and the electrical design uses more active material than other motors, premium-efficient motors are more tolerant of both low and high voltages. Remember that low voltages are likely to occur as utilities try to avoid blackouts by taking voltage reductions (brownouts).

IEEE 841 motors and premium-efficient motors are rated for adjustable-speed use with an inverter that can provide significant energy savings. On a pump or fan application, the use of an inverter to adjust the motor speed might save more than 50 percent of the energy used for the same motor operated at full speed when the flow is controlled by a valve or damper. These motors are termed "inverter-ready" and are provided in either TEFC or ODP enclosures.

Because internal motor losses are reduced, a smaller cooling fan can be specified. This fan provides quieter operation than that used on a standard motor.

Figure 3. Power Factor Penalties At Reduced Loads

As motor load drops below 75 percent, the power factor falls off precipitously.

Application considerations

Even with these advantages, there are a few factors to consider when applying premium efficient motors.

To lower motor losses, the motor slip is reduced. A premium-efficient motor might run 1,790 revolutions per minute (rpm) full load, but an older motor might reach only 1,760 rpm. In a constant-torque application such as on a conveyor, this generally is not a problem. But on a variable-torque application such as a centrifugal pump or fan, the load increases by the cube of the speed increase. Therefore, a 1 percent increase in speed equals a 3 percent increase in load, drawing more current. You might want to consult the appropriate fan and pump curves when replacing motors on variable-torque applications.

Many premium motors produce less starting torque than older motors. Although they are required to produce a minimum starting torque defined by NEMA, the older motors might produce much more than those minimum values. Therefore, a new motor might not start a load requiring high starting torque.

Premium-efficient motors generally require higher starting currents than older motors. The high inrush current might cause nuisance trips on starters. You might need to change heaters and oversize IEC-type starters.