I have heard that the traditional resistor-based fan speed regulators are inefficient. In fact, I have noticed that such regulators tend to get hot when the fan is set to low speed. However, does that mean that they save no power at all at low speeds, or is it just that power is saved but the amount of power saved is far less than ideal? I was expecting that since power P = V / R^2, the overall power consumption of the fan plus regulator system would be comparatively lower at lower speeds.
[Physics] Do resistor-based fan regulators save no power at all
electrical engineeringelectrical-resistanceelectronicspower
Best Answer
With no resistance, the full voltage is applied to the fan, and you get mechanical work done, at whatever efficiency the fan itself is capable of. Never minding the fan itself, so far as the electrical aspect goes, you could say it's 100% efficient.
With resistance in the system, for example about equal to the resistance of the fan, you have less current flowing through the system. Half as much. Same voltage. So the system is using half the power. The fan, now one part of a voltage divider circuit, is getting half the voltage, and getting half the current. It runs slower, doing less, doing about 1/4 the work. 1/4 of the fan action divided by 1/2 power going into the system means that the system is 50% efficient.
Imagine a lot of resistance in the system, megohms. You'll have only a trickle of current, say a microamp. The fan barely moves. The system will be using very little power, but most of that little power is just making the resistor hot. Or rather, a small fraction of a degree warmer than ambient temperature. The system efficiency is close to zero.