110 volt Microwave Fan with three terminals

[vigsgb]

I am new and recently ripped apart a microwave and yes I did discharge the capacitor prior to ripping into it.   Nope I am not planning on messing around with the magnetron or high voltage transformer.   With that out of the way I have looked at the cooling fan and cannot find any information on how to hook it up...  I see three terminals and decided to just stick mains power into two of the terminals and yep it fired up...but the windings got very hot and quickly.  So I was wondering how do I tell which terminals on the fan to hook up?  The fan is a 120 volt 60 Hz...  Can't find any specs online.

[SeanB]

Use the 2 outermost as connections. The fan often has a winding used to run the turntable motor, which typically runs on 24VAC or 43VAC. Check DC resistance and use the 2 with the highest resistance between them. That will be the outermost ones in most cases. If you look carefully one will have 2 wires connected that runs into the winding. Do not use that one.

[fcb]

It's possible it's a 2 phase AC motor, and the 'motor run' capacitor (this creates the phase shift for the second coil) is elsewhere - although I can't think what benefit there would be to using this type of motor in something this small.

Can you post a pic?

[SeanB]

Never seen that in a microwave, only a shaded pole unit. The turntable is invariably driven off the fan supply though to cut copper cost in it.

[fcb]

Yeah SeanB, I can't think why they would either - I haven't had too many microwaves apart though.

[vigsgb]

I see that the middle connection does indeed have two wires going into the windings and the two outter terminals have the most resistance.    After hooking up the mains to the outter connection it no longer heats up and I get 24 volts a.c. from the center terminal.   Thanks much...  never thought it was to run something else.  I will take a few pictures and post.

[Zero999]

Wow, I never thought they would integrate a transformer and motor into the same part.

I've opened a few microwaves in the past and have never seen this before.

Please note that the 24VAC is not isolated from the mains so all connections to it should be properly insulated.

[vigsgb]

Wow, I never thought they would integrate a transformer and motor into the same part.

I've opened a few microwaves in the past and have never seen this before.

Please note that the 24VAC is not isolated from the mains so all connections to it should be properly insulated.

Good point I did notice that when testing I was able to get the 24 volts a.c. when I touched the meter to mains ground.

[SeanB]

Turntable motor draws a low current, and winding it to full mains uses a lot of copper. As the losses in the gear train are more than the power it delivers using a lower loss in the motor is good, and using an existing core and winding just makes good sense.