We use ceiling and table fans in home which are can be set to low or high speeds

Question

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?

Explanation / Answer

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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