Motors matched to load profiles lead to high-efficiency service

Pulp & Paper, Aug 1995 by Cole, Richard, Thome, Terry

1. Measure the motor's rpm and compare with nameplate full-load rpm.

2. Measure the motor's amperage and compare with nameplate amps.

Note that for both measurements, motor input voltage must be checked at the same time to make sure the motor is receiving full-rated voltage. Motor performance varies with the square of the voltage change, so even a small voltage variation, such as a mild brownout, can make a big difference in the measurements. If voltage is chronically erratic or off nominal at the measuring site, consult the motor manufacturer for assistance in determining performance variations during voltage fluctuations.

If rpm and amperage measurements taken under full voltage fall close to their respective nameplate values, the motor is running at or near rated load. If they are not close to nameplate values, these measurements can be used with standard amp/watt/horsepower charts to approximate the motor's horsepower output at the time of measurement. It is not a bad idea to graphically plot the motor's duty cycle to show how much of that cycle keeps the motor above 75% of rated load. The savings that result from premium-efficiency motors will vary according to how much of the motor's operating time is spent above 75% of rated load.

Load horsepower is most reliably determined when the motor can be temporarily replaced with a calibrated motor--one for which the amp/watt/speed profile has been documented--but the expense of doing this often makes it impractical. Measurements also should be accompanied by notation describing the motor's operating environment, including ambient temperature, airborne dust or falling particulate, and water or other liquids. As with the duty cycle, it should indicate whether environmental factors are continuous or periodic.

SYSTEM CONSIDERATIONS. Due to the humid and often corrosive ambients, low-voltage conditions and overloading typical of pulp and paper operations, it is advisable to routinely consider severe-duty models when switching to premium-efficiency motors. However, it should be noted that their construction--adding to the already larger dimensions of premium-efficiency--designs as explained earlier--might make these replacements too big for the space occupied by the existing motor. Where motors will be installed vertically, as is common with mixer, agitator, and pump motors, special bearings still must be specified to support the motor's weight on the shaft, and either bearing isolators or slingers should be employed to prevent liquids from migrating up the shaft and into the motor.

While centrifugal pump fans and blowers that run 24 hours/day are among the top candidates for changeover, premium-efficiency motors tend to run a bit faster than their standard-efficiency counterparts. When applied to "diminishing-torque" loads--where the load horsepower changes by the cube of the speed change--this small increase in speed will demand a large increase in motor output horsepower, in turn causing the motor to draw more power and lose part or all of its energy-saving benefit. In belt-driven applications of this type, motor rpm can be compensated by variable-pitch sheaves to adjust the fan or pump back to specified speed. Slowing the load rpm too much, however, might cause the motor to run at less than 50% of rated load. Direct-drive loads typically require additional engineering evaluation to assure energy-saving benefits.