# Small induction motors in series

#### jack beresford

Joined May 17, 2015
2
Hi People

I have a pair of 240VAC, single phase, 125mA cooling fans that I wish to use in a window above my workbench as exhaust fans.

Now, run singly on 240V, they are quite noisy, and I wondered what I could do to cut their speed down a tad. I thought of a power resistor, to cut the voltage, and thus increase the slip, but then it would surely be more efficient to wire these two in series. That would halve the current to 62.5mA and slow them surely, but if there is too much slip, are they not likely to heat up and let out the sacred smoke? I suppose the way to go is to try, but if someone could shed a bit of light I would be grateful.

One thing I'm unsure of is the voltage drop across each motor, compared with the voltage drop across just one. Am I right in assuming that zero voltage drop implies infinite current (theoretically)? And 240V drop implies infinite resistance? Perhaps this can't be calculated and must be measured?

Cheers, Jack

Sent from my iPad

#### strantor

Joined Oct 3, 2010
5,528

Joined Jul 18, 2013
22,823
At 125ma, I doubt if they are going to burn out, You can often stall the small shading ring type fans without undue harm, apart from a temperature rise that is not usually enough to burn them out.
Try them in series and watch the temp.
Max.

#### ian field

Joined Oct 27, 2012
6,539
At 125ma, I doubt if they are going to burn out, You can often stall the small shading ring type fans without undue harm, apart from a temperature rise that is not usually enough to burn them out.
Try them in series and watch the temp.
Max.
In the past I've done it with 110V fans on 230V, but fan bearings are not created equal - and go downhill from there. One of the fans got to needing a push to get going and the other run too fast. Over time it got slower and slower, then started going again when the other burned out!

What I'd suggest is; look for a mains transformer with the highest secondary voltage you can find, wire the primary as normal and connect one end of the secondary to the live, phase the winding so its "bucking" - that is the secondary voltage is subtracted from the mains voltage fed to the fan.

Usually these types of fans are designed to be as quiet as they can make them, you don't have to halve the voltage to make them tolerable. A drop of about 30 - 45V ish should do it.

#### jack beresford

Joined May 17, 2015
2
Thanks guys
I've wired them up in series (had to do some frame mods first) and they run fine. Just a little slower than singly and a lot quieter.
I will have to wait until I've painted the mounting board before I can give them an extended run. If I have any probs I will report back. If you don't hear from me, all has gone well. Thanks again, cheers....

#### ian field

Joined Oct 27, 2012
6,539
Thanks guys
I've wired them up in series (had to do some frame mods first) and they run fine. Just a little slower than singly and a lot quieter.
I will have to wait until I've painted the mounting board before I can give them an extended run. If I have any probs I will report back. If you don't hear from me, all has gone well. Thanks again, cheers....
While I certainly remember doing series 110V fans for 230V mains, my memory is a bit flaky on whether I ever tried slowwing 220V fans by series operation. I just have that nagging suspicion that you'll find them not shifting enough air when summer really gets going.