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We use ceiling and table fans in home which are can be set to low or high speeds using regulators. I want to ask that does it uses or consumes same amount of electricity at different speeds? Here, "same amount of electricity" means exactly I want to know that, my electricity bill will be different for different speeds or will it be same. I know that, when it has less speed, less electricity flows through it, but my friend told me that, when it is at less speed, the remaining electricity get wasted at regulator, so total electricity consumed by fan + regulator is same, so same electricity bill will be generated regardless on which speed we use it. So is my fried correct or wrong?

image of ceiling fan with regulator

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What type of motor? What type of regulator? Have you tried measuring the power? –  EnergyNumbers Oct 29 '12 at 8:06
@EnergyNumbers no, i haven't tried measuring the power. And I also don't know what kind of motor and type of regulator. But I'm talking about the normal ceiling fans in home (that run on 230 v, here in India) and normal regulators providing 1 to 5 different levels of speed. I will try to add images in my question, so you will get better idea. –  android developer Oct 29 '12 at 9:06

3 Answers 3

up vote 1 down vote accepted

It depends on the fan, but I'd guess the majority of domestic fans will use less power at lower speeds.

I can state with authority that the fan in my car (a Ford Focus) uses roughly the same power regardless of speed because I've just had to replace the ballast resistor that is uses to control fan speed. When you select a lower speed the fan dissipates power as heat in the ballast resistor so the speed setting makes little difference to the power drawn.

I can't be sure about domestic fans, but in the car fan the heat dissipated in the ballast resistor is very noticable and indeed theresistor gets too hot to touch. The fan on my desk does not get hot when used at a lower speed, so I think it's very likely it doesn't simply dissipate power to lower the speed and therefore it will use less power at lower speeds. It's probably significant that the car fan is DC while domestic fans are AC. It's much easier to control power in AC circuits because you can use a thyristor or something similar to control the power delivery in a lossless way.

The only way to be sure is, as EnergyNumbers suggests, to measure the power drawn. A simple power meter like this one is all you need. Unfortunately I'm working away from home this week otherwise I could measure the power drawn by my own fan and give you a definitive answer. However I'm sure there must be some fan owning, power meter armed, experimental physicists reading this :-)

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thank you very much for your answer. I noticed that, when my fan is on highest speed, no heat generated but as I reduce the speed from 5 to 1, heat is generated at regulator, at it's maximum for lowest speed. –  android developer Oct 29 '12 at 9:21
It sounds as if your fan is using a ballast resistor, though even a switching regulator will generate a bit of heat. I think you need to measure the power drawn by the fan to be sure. –  John Rennie Oct 29 '12 at 9:24

I have checked the power consumption for various speed of fan.

It consumes $80 W$ at full speed $45 W$ at second speed and subsequently reduced at lower speed.

This I have measured from the Digital Electrical Meter.

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No. Presume the fan is in series with the regulator. Presume the fan when run at a lower speed draws less current. Presume the mains voltage stays the same.

Then the total current is lower at lower speed, and so the total power used is also lower.

Now, if you skip some of these presumptions, you could set up some screwy system, say, put a lightbulb in series with the fan, and wire the regulator in parallel with it so at lowest speed the fan is shorted out and the bulb gets full power, that would draw more power at low speed, but that is an absurd thing to do.

I sense your friend does not like running the fan at lower speed, believing that the motor is most efficient at it's rated speed, and that is probably correct. However, the efficiency of the fan in making you comfortable at any particular speed is an intangible.

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protected by Qmechanic May 9 at 13:37

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