Motors

One of the largest uses of electricity is to drive motors of all types. In the past these have been considered very efficient devices - operating at approximately 75-90% efficiency. However, modern designs can achieve even higher efficiencies (85-95%) and provide attractive economic value. Evaluating how much a high efficiency motor will save is not as easy as it sounds. Both the motor and the driven device must be understood. How often will it operate and at what load?

Remember that high efficiency motors may only achieve a savings at the nominal rating or full load. During periods of light load, efficiency drops off quite dramatically as shown in graph below, possibly as low as 40 or 50%. Therefore, savings calculations have to consider both full and part load operation. The average efficiency for most moderate sized motors in most industries is between 60 and 75%. Energy efficient motors generally raise this by several percent, which can be quite economic.

A parallel consideration is the proper sizing of motors. Oversized motors will tend to run at poor efficiencies and at a lower than desirable power factor when lightly loaded. Where the motor needs to be sized for relatively infrequent peak power loads but will generally operate at much lower loads, a variable speed drive may be appropriate. These are becoming quite popular in fan and pump applications, with significant energy savings potentials. However, be especially careful to train operational personnel in their use and troubleshooting, or they will simply be put in "bypass" thereby nulling any potential energy savings.

Here are two additional ways to keep efficiency up:

1. Maintain supply voltage. An induction motor is capable of operating within a variation of plus or minus 10% of rated voltage, but the performance characteristics are not necessarily the same as those when the motor is operated at rated voltage.
2. Maintain voltage balance. Voltage balance of a three-phase power supply to a motor is essential for efficient operation. A voltage imbalance of just 2.5 to 3% can increase system losses by approximately 25%. Accordingly, single-phase loads taken from a three-phase power supply should be allocated carefully so voltage imbalance will not exceed 1%.

And furthermore . . .

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