When the subject of efficiency comes up, motor users get this advice probably more often than any other: “Get rid of those oversized motors. Replace them with motors rated closer to the actual load horsepower, and you’ll save energy.”
On the surface, that seems to make sense. We’re used to thinking of a “mismatch” as bad. Suiting components to each other, having their ratings alike, must be better, somehow.
But motor efficiency doesn’t behave that way. In general, it has a maximum or peak value for any motor (whether standard or “energy efficient”) at about 3/4 of the nameplate horsepower rating.
Why is this true? Because of basic physics. Any motor has five internal losses. Core or iron loss is essentially the same at all loads. Two other large losses (in the stator and rotor conductors) drop off rapidly with decreasing load. Meeting performance goals, both at full load and during starting, leads to ratios between those losses that cause efficiency to be practically “flat” from half load to full load, with a peak value somewhere between. Trying to shift that peak to some specific load point — such as at “85% load,” or at full load — can result in extremely special, costly designs. Sometimes it’s not even possible.
Because of that relationship, an oversized motor is often a more efficient way to drive the load than a smaller motor more nearly matching the required horsepower. As an example, consider a typical 20 hp 1800 rpm “energy efficient” TEFC motor design. Published efficiency for this machine is 89.5% at 20 hp; at 3/4 load, which is 15 hp, published efficiency is 90.2% — nearly a full percentage point higher. The same manufacturer’s 15 hp energy efficient motor has full-load efficiency of 88.5%. Suppose the load actually requires only 15 hp. Here’s the comparison:
20 hp motor, 15 hp load: motor efficiency At 15 hp = 90.2%
15 hp motor, 15 hp load: motor efficiency At 15 hp = 88.5%
The choice won’t always be that clear cut. And the larger motor will have a lower power factor at the reduced load. But don’t assume efficiency will be improved by “matching the motor to the load.” Often the reverse will be true.
Operators and electricians often assume that higher motor current means lower motor efficiency. But in examples like this one, a higher current for the larger motor at the 15 hp load is caused by a lower power factor. That doesn’t affect the relative efficiency or the relative energy usage for the two motors.
If the larger motor’s lower power factor presents a problem, external capacitors on the circuit usually furnish an easy solution. Efficiency is not easily changeable; power factor is.
Richard L. Nailen, P. E.