Read this before connecting two fans in series

This post is created in response to various people on the internet trying to/suggesting others to put two fans in series in order to power them from a higher voltage power supply.

In short, don't.

This idea comes from the voltage divider rule. When two resistors of the same impedance are connected in series, the voltage across each is half the voltage across both of them.

However, the impedance of a DC-powered fan is not constant. While I'm sure of the previous sentence, I am not sure of the causes and the extent of their effects so don't quote me on this.

One reason is that the current through to motor coil needs to alternate, so this is either done by a brush (commutator) or an inverter (for brushless fans). And when the current changes direction, there will be a short duration when no current flows, which translates into high impedance,

Which translates into a fan receiving more voltage which translates into that fan spoiling if you exposed it to voltages greater than what the fan can handle.

Well this is the theory, but as you know people on the internet say a lot of things that aren't true, so people probably wouldn't believe me when I tell them otherwise with no evidence to support. So I got the evidence.

The two fans to be the lambs for testing. They are 12V DC fans to be connected to a power supply set to 12V. So according to the voltage divider rule each fan should get 6V each.

 

One fan connected. Nothing interesting about the voltage measured across the fan, which should be the case if your power supply is at least halfway decent.

The connection for two fans in series.

Measured voltage across the second fan.

As you can see, the voltage peaks go over 9V, even though the average voltage is about half of the power supply voltage. The fans are 12V ones so they are okay, but I wouldn't want 6V fans to be exposed to this.

And this is the voltage across the second fan when the first fan is seized or stuck.

When an electric motor is stationary, the back EMF is zero and it draws more current. (This is why motors burn out when seized driving a heavy load.) Nowadays there are protections in place, easy to implement thanks to the electronic power supply on the fan itself - this Globe fan measures 0.30A when stopped and 0.17A during normal operation.

Put in another way, the impedance of the fan is lowered when it is stopped. So when you put a seized fan and a normal-operating fan in series, the normal-operating fan with a higher impedance will get more of the voltage, which is what you see above. At >8V average, even if the spikes don't kill a 6V fan, the >30% overvoltage will.

Conclusion

Do not connect two fans in series to use them with a higher-voltage power supply that they otherwise cannot handle with each fan on their own e.g. connecting two 12V fans in series and powering them from a 24V supply - you will still expose the fans to voltages higher than what the fans can handle.

However, if your plan is to run the fans at a voltage lower than rated e.g. two 12V fans in series with a 12V supply so each fan gets 6V average each, you can do that.

The fan starts at ~3.4V. This is a 12V rated fan.