QUESTION: "Why do my new 3 speed motors not slow down when I change the speed switch's setting?"

ANSWER: The motor's lowest horsepower rating exceeds the application load. The "3-speed" motors you purchased are really rated to overcome different loads, and not necessarily yield 3 different speeds. Consequently, they will only slow down if the load they are subjected to matches or exceeds the rated capacity of the "speed" (wire lead tap) selected. Actually, these are not so much 3-speed motors as, instead, motors tapped to yield multiple horsepower outputs. Without being subject to at least the highest load for which the motor is capable of overcoming, the motor will not appear to be slowing down.

Example: Your original fan coil motor was rated 1/25, 1/40 and 1/50th horsepower. But, since bigger almost always seems better, your maintenance manager decided to purchase replacement motors having ratings of 1/10, 1/15 and 1/25. When energized at high speed (usually the black wire and white common), the motor easily overcomes the static pressure load of the coil at its rated RPM. Unfortunately, output at subsequently lower horsepower ratings (speed taps) also exceed that required to draw the designed volume (CFM) of air through the coils. Without a properly matched load (achieved by engineering the motor HP @ rated RPM to that of the blower wheel's physical size, the coil length, depth and fin spacing), a motor's speed will never appear to slow down. And now your phone is ringing, because there's no speed change, and the new motor is, "too noisy."

SOLUTION: Match the horsepower rating of each "speed" tapping from your old motor, to that of your replacement. This is seldom easy, as most old motors never listed hp ratings on their labels, or it's been warn away. So, in lieu of horsepower ratings, consider matching at least the high-speed ampere rating. As long as you are replacing motors with like-for-like designs (shaded pole with a shaded pole, or PSC with a PSC), this amperage method seems to work quite well.

If all else fails, and you can't locate a properly rated multiple speed motor, consider an inexpensive variable speed control switch to replace the existing OFF-H-M-L rotary switch.

David Whitnack

Johnstone Supply of Philadelphia

4700-B Wissahickon Ave.

Philadelphia, PA 19144

Voice 800-321-4822

Fax 800-595-4822

E-mail

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