I have a ceiling fan in the sitting room and in the winter, I run it backwards at the slowest speed. This is to circulate the warm air from the single radiator.
My question is - Does a ceiling fan use more electricity at full speed than at slow speed. My thought is that it might be slowed down by adding some resistance into the system. Or it might be pulsed, and therefore use a lot less.
Normally, it’s simply done by switching motor windings, or more accurately, switching “taps” along the length of one winding, so your guess about resistance is pretty good, but probably not quite as you imagined.
This pic shows the general idea. High speed uses the least amount of winding, but will draw the most current due to decreased resistance vs the low speed, which uses the entire winding. The L1 and L2 terminals are the incoming 120 volt power. This particular diagram is for a setup where you’d choose the desired speed more or less permanently by attaching the L2 wire to the tap, but a ceiling fan’s speed switch would sit in the “attach to desired speed” area and rotate through the taps.
Newer fans with remote controls usually toss this all out the window as they use the motor equivalent of a light dimmer.
So the one in my sitting room which has a pull switch will be more economical at low speed. The one in the bedroom with a remote uses the same at any speed…?
I am talking Kw hours here.
Not quite on topic, but my electrician neighbour says you should always start a fan on high, then turn it down, to avoid straining the motor. I have no idea if he is a good electrician, though he does have his tickets.
In the spectrum of household appliance energy use, ceiling fans are down near the bottom at around 75 watts, which is less than two cents per hour, assuming you’re paying twenty cents per kWh.
The variable speed fan will use less power at lower speeds as well, but a trivial amount of additional power is needed for the electronics to drive it and the remote control receiver.
Cieling fans comprise a multipole induction motor with high rotor resistance, run nearly stalled. The speed is controlled by varying the torque. There are two stator windings. One is supplied with the full AC line voltage. A second quadrature winding is supplied with a phase shifted signal via a run capacitor.
In a multispeed fan, the run capacitor is comprised of several sections. For example a 3 and a 6 uF section. in this example, the 3 uF section would be used for the low speed, the 6 for medium, and the two placed in parallel (9uF) for high. This is very common, and the same switch as used in three-way table lamps does the speed selection job.
Increasing the capacitance increases the current in the quadrature winding which increases the torque. The fan blades load the motor well below synchronous speed, so the speed is determined by the torque.
Alternatively, the speed can be controlled by voltage reduction of the supply via a triac chopper circuit, similar to a light dimmer. Unlike a dimmer, these usually have the high speed next to the off position, thus providing a good kick to get the fan moving initially.
The triac speed control method tends to be noisy due to harmonic content of the supplied waveform.
It may well be true as my fans are designed to start on high and then reduce to off in steps. (keep pulling the cord).
Seems I got the answer I wanted - thanks guys.
My understanding is that the amount of current necessary to run a fan on low might not be enough to overcome the initial friction (stiction?) of starting. If you run current through the motor but it doesn’t actually start moving it will burn up.
My electric fan has lights that show what speed it is running and when you first turn it on, even if set on low, it goes to medium until it is moving then goes back down to low.
