Can two cooling fans be wired in series?

110V fans are different to DC fans, they use induction motors while the latter uses a permanent magnet motor with an oscillator driving the coils.

You can generally safely over-voltage most fans slightly without damaging them, because they won't draw a proportionally higher current and the cooling effect of the fan going faster will help prevent it from overheating. I've once tested one of those 3.3V CPU fans to destruction and it took well over 12V before it somked, I wouldn't worry, I think they'll be fine if you connect them in series, even if the loads are unbalanced, I even suspect that they're designed for operation up to 14V, most 12V apliances are.

RODALCO said:

In series both fans of equal power rating will draw exactly the same current.
When conditions change, e.g. one fan stops the stopped fan may draw more current,and voltage drops across it. The running fan will run faster because it's voltage is increased.

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Actually note how dependant current draw is on the inflow and outflow conditions. If you block the inlet to a fan it draws less current and actually speeds up because there's less air loading down the blades.

Slow

I thought it slows down because it has to fight the higher air pressure on the fan output from trying to go backwards through the fan. (or am I assuming there is an open space on the input side of the fan to produce a vacuum.).

Nvm.

dknguyen said:

I thought it slows down because it has to fight the higher air pressure on the fan output from trying to go backwards through the fan. (or am I assuming there is an open space on the input side of the fan to produce a vacuum.).

Nvm.

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It's the same effect as putting your hand over your vacuum cleaner hose. The motor speeds up because the load has been reduced.

I am implementing a simple design for a project of mine that solves just that problem. It can be assembled with readily available parts and seems to work a treat. Simulated efficiency is around 90%.