This is a typical microwave oven fan these days, has a tap to run 21V ac turntable motor.

 

Just to confirm, this photo clearly shows the tap going to the middle terminal of an OEM-1011X1.

 

I was wondering if they use it as an autotransformer to power something else with a lower voltage?

Many times those motors are indeed used as autotransformers, often to provide a quite low voltage for a pilot light. So applying 120 volts to the six volt winding will indeed be destructive. AND, the windings are insulated but the insulation ic clear and colorless.
You may be able to see which terminal is the tap by inspection, to see which one has two wires connected to it, or if not, then an ohm meter. The 120 volt line connects across the highest resistance connection.

 

There is a third terminal in the photo. Was anything connected to that before you removed it from the oven?

Does that third terminal have a wire from the motor winding soldered to it?

I'm wondering if the motor winding is tapped & that the 120v rating applies to the outer windings?

Having said that, the physical position of the terminals doesn't always correspond with the position of the tap. Perhaps the winding is tapped, but is brought out to the unused terminal?

Good questions PeeSeeBee,

The third terminal was not connected to anything, only the first two via the female connector.

 

Just a guess here, but my guess is that it may have been used as a way for the control board to determine if the fan was running or not. It MIGHT have been a protection for the whole microwave oven. But this is just a guess.

I also agree, resistance testing should be done before any more voltages are applied.

I do have one comment that is not guesswork - and that's using that fan to blow away smoke from soldering. First, I believe the air movement is going to be too fast and harsh. If you blow the fan AT your work you'll likely cool your iron before you can accomplish the soldering work you want to do. Blowing away, as I have a small 12 volt fan doing, will gently attract the smoke and blow it away from my face. Otherwise without the fan, no matter where I sit, the smoke always seems to find my face. Sort of like barbecuing.

When i connected the fan it did not blow very fast, it was actually very nice consistent flow. I was thinking about installing it in such a way that it will blow between my face and the working surface. I did not want to use a 12v dc because I will have to step down the 120v and I am still new to that area.

 

I found this comment also:
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.

Will give that a try, thank you thedoc8

 

This is a typical microwave oven fan these days, has a tap to run 21V ac turntable motor.View attachment 167283 View attachment 167284

Your motor is labeled as 240V. I don't understand, why would they put 120v or 240v on something that can only take max 24v.

 

Your motor is labeled as 240V. I don't understand, why would they put 120v or 240v on something that can only take max 24v.

As I and others have said, the motor works as an auto transformer when fed with 240v there is 21v across the smaller winding, the outer ends are fed with 240vac.
Max.

 

If you are absolutely sure that there was nothing connected to the other outermost terminal, then the only thing I can think of is that the fan wasn't fed straight from the mains while inside the microwave. I don't know why that would be the case. The only way to find out for sure is if we had the model number & could find the schematic, but you don't have that information.

To run it from 120v I think we are unanimous in saying that you should use the outer connections.

If your motor winding does have (say) a 24v tap then maybe you connected 120v across the 96v winding & that's why it lasted around 5 seconds before blowing. If you had connected 120v across the 24v winding, I would have thought it would have blown within a second or 2, if not straight away.

 

Your motor is labeled as 240V. I don't understand, why would they put 120v or 240v on something that can only take max 24v.

As Max says, the 21v or 24v (or whatever low voltage it is tapped at), is an output derived from the mains voltage input.

For example, if you had 1000 turns on the winding & tapped it at 500 turns, you would measure 60v from the tap to one end of the winding & 60v from the tap to the other end of the winding (with120v across the ends).

If you tapped it at 250 turns, you would measure 30v from the tap to one end of the winding & 90v from the tap to the other end of the winding.

 

If you are absolutely sure that there was nothing connected to the other outermost terminal, then the only thing I can think of is that the fan wasn't fed straight from the mains while inside the microwave. I don't know why that would be the case. The only way to find out for sure is if we had the model number & could find the schematic, but you don't have that information.

To run it from 120v I think we are unanimous in saying that you should use the outer connections.

If your motor winding does have (say) a 24v tap then maybe you connected 120v across the 96v winding & that's why it lasted around 5 seconds before blowing. If you had connected 120v across the 24v winding, I would have thought it would have blown within a second or 2, if not straight away.

I still say measure the resistance of the motor between the various pins and apply the 120 volts across the highest resistance pair. How hard can that be?? Just use a reasonable resistance range. One that gives a reading in the middle third of the scale, if is an analog meter.

 

Personally if faced with an unknown such as this, I would test just to confirm whether the smaller winding was additive or subtraction.
Max.

 

Personally if faced with an unknown such as this, I would test just to confirm whether the smaller winding was additive or subtraction.
Max.

The smaller voltage winding is a tap, because anything else takes more wire.

 

I still say measure the resistance of the motor between the various pins and apply the 120 volts across the highest resistance pair. How hard can that be?? Just use a reasonable resistance range. One that gives a reading in the middle third of the scale, if is an analog meter.

The motor with 3 taps is gone. The other motors that i have only have two taps. I measured the resistance and it's: 42.1.

 

The motor with 3 taps is gone. The other motors that i have only have two taps. I measured the resistance and it's: 42.1.

42.1 ohms is a reasonable DC resistance for a small motor, the impedance at 60Hz will be quite a bit higher. And it may even be that the three terminal motor could be repaired, if the need existed. But at least something was learned by a few folks and the practice of using a motor as a transformer is revealed. I recall that I own a reel to reel tape machine that uses the drive motor as a power transformer for the tube electronics portion. I believe that it was an AMPRO brand machine. While the performance was adequate and the mechanical design was good, using the motor as a transformer is to me a sign of cheapness to the extreme.