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Efficiency of controlling a fan with a wall speed control vs integral speed control

iLikeDirt | Posted in Energy Efficiency and Durability on

Imagine a fan with a 75 CFM/watt rating on high and 150 CFM/watt rating on low. This fan is set to high speed with the pull chain and the speed is controlled using a wall-mounted speed control switch. At the maximum speed, it’ll be producing 75 CFM/watt. But at low speed, will it continue to produce 75 CFM/watt, or will it instead produce the 150 CFM/watt that it would produce if it was set to low speed using the pull chain?

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  1. charlie_sullivan | | #1

    In a standard ceiling fan, the motor is a single-phase induction motor, and the wallbox speed control would reduce the voltage supplied without changing the frequency. That's a poor way to control the speed of the motor, as the mismatch between the frequency and the rotation speed increases as you slow it down, and efficiency suffers. So the wall box is likely to result in lower low-speed efficiency than the pull chain, which typically changes the way the poles in the motor are wired.

    The efficiency with the wallbox control is not likely to be simply 75 or 150 CFM/W. It will almost surely be worse than 150 CFM/W, and probably closer to 75, but it could be higher or lower than 75.

    But if you look at the energy star listings, the top 100 or so models use ECM (brushless DC) or inverter controlled motors, and have 5 to 10 X better efficiency. Any of those also have remote controls and won't work with a regular wallbox control.

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