“Power factor controller” is a confusing term. The device isn’t really intended to govern power factor as such. Rather, it electronically adjusts motor voltage to suit motor load, using the motor power factor as an indicator of that load. The device works. It is no scam. But it seldom pays off.
We call these “Nola devices,” after NASA’s Frank Nola who made the invention in 1975. Sixty manufacturers were once in the market, many of whom still exist.
When a motor runs fully loaded, its internal power loss is mostly “copper loss” caused by current flow through the windings. If motor voltage is lowered, current must go up to supply the same shaft output; copper loss goes up too, and efficiency tends to drop.
In contrast, a lightly loaded motor’s internal loss is mostly magnetic loss in the core iron. That drops rapidly when voltage is lowered. Little shaft output is needed, so the reduction does no harm. Net losses go down, and efficiency rises. Total loss can be cut 10%, 20%, even 50% — but the actual watts involved are quite low compared to full-load operation. That’s what a Nola device does — reduces motor voltage when load is light.
Ideal Nola controller applications are industrial sewing machines, office machines, and some machine tools, where motors can’t readily be shut off frequently yet must idle most of the time. Unless shaft output averages 1/3 of rated power, or less, the Nola device normally isn’t cost effective.
Repeated tests confirm that for three-phase motors of all sizes. The loss picture is different, and the savings greater, for small single phase motors. But such motors use so little power anyway that the total savings is also small. Besides, typical appliance motors, as in a refrigerator, normally don’t run at little or no load. They’re either fully loaded — when the Nola controller has negligible effect — or they’re shut off (and you can’t save any more energy than that).
A simple example illustrates the economics: Assume a 1/4 hp single phase load with motor efficiency of 40%. Assume this condition exists 1500 hours a year. If the Nola device were to save half of the motor losses, with energy at 13.6 cents a kilowatt hour, the annual savings would be:
0.746 (1/4) (0.5/0.4) (0.6) (1500) ($0.136) = $28.53
In reality, since most appliance motors are much smaller and run fewer hours, this would probably be in the $2 to $3 annual range. A controller costing $50 to $75, which is typical, wouldn’t repay its cost for decades. As for putting that controller on the power saw in your basement, operating 20 or 30 hours annually — forget it.
Richard L. Nailen, P. E