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Really basic question that I've struggled with for a while. In low-cost fans, a series of mechanical buttons select fan speed. I assume each button is connected to a specific resistor which limits the current/divides the voltage to the fan motor, depending on how they are arranged.

My question is, is it irrelevant to the overall power used by the appliance whether the fan is operating on low or high speed? My assumption is that the energy is simply wasted to heat the resistor on the low speed setting.

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These fans use shaded pole motors which are a type of asynchronous motor.

Speed can be controlled by varying winding current.

This isn't done with actual resistors, but by switching between several windings. The lowest speed setting corresponds to the longest length of wire in the windings, thus the highest resistance. Highest speed corresponds to lowest resistance (shortest winding). This can also be done by switches connecting windings in series, parallel, or a combination.

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(source)

Also, more winding turns mean higher inductance, thus higer impedance, which reduces current without wasting power in winding resistance.

Since higher impedance results in lower current, the fan does not draw constant power from mains, and lower speeds do use less power.

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    \$\begingroup\$ As a side note, when the motor fails, it is often the winding between the common and the high connection. By disconnecting medium, and connecting high directly to common, you can get the fan to work for a short time, albeit faster than high, probably overheating the motor, and certainly not UL approved. The current runs only through the windings between the high connection and the low connection. I've extended the "fan life" by additional weeks doing this. \$\endgroup\$
    – Keeta - reinstate Monica
    Commented Aug 27, 2019 at 18:51
  • \$\begingroup\$ Plus, from a purely physical point of view, force, and thus the power needed to propel a fan faster goes up quadratically with speed. So, most definitively even a hypothetical "wasteful" fan that burns energy in a restistor turning slower would likely be more energy-efficient. \$\endgroup\$
    – Damon
    Commented Aug 27, 2019 at 20:17
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Resistors can be used to control motor speed an you described, but they must dissipate a lot of heat. The preferred speed control method is to switch the value of capacitor that is connected in series with the auxiliary winding of the single-phase motor. See Speed control for PSC induction motor As illustrated in that question, it is a natural characteristics of a fan that it requires less torque to operate at lower speeds. That mean that less power is used, since power is torque multiplied by speed. An increased proportion of that power is dissipated in the rotor of the motor, but less total power used and less power is dissipated in the rotor as compared to full-speed operation.

Shaded-pole Motors

Another answer describes much the same performance for shaded-pole motors. Shaded pole motors tend to be used for smaller fans such as exhaust fans for bathrooms and stove hoods. Window fans and pedestal fans are more likely to use the capacitor-run or permanent-split-capacitor (PSC) motor described above.

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  • \$\begingroup\$ I just assumed they were controlled wastefully using resistors, good to know there's more it. Thanks for your response. \$\endgroup\$
    – Chorlton2080
    Commented Aug 27, 2019 at 13:25

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