View Full Version : OT, electric motor reversal?

10-17-2009, 06:22 PM
I have a Modine propane "Hot Dog" heater for a greenhouse. Didn't use it last winter due to fuel costs, but want to fire it up now. I found out the exhaust fan motor was frozen up and I couldn't get it loose enough to run, try as I did.

So, I bought a replacement, same make/model, I think. The brand is FASCO, and the replacement is an exact copy.

But what I didn't check is the rotation. Now that I've installed the replacement motor, the "air proving switch" doesn't make, and consequently, the heater won't go pass it that stage.

So my question is, shouldn't the fan rotate toward the outlet? This is a squirrel cage fan within a housing, in a "D" shape. The outlet is at the top, and it seems to me the rotation should turn toward it, I'm pretty sure water pumps do that.

I tried reversing the polarity, but it didn't change rotation. Is there a way to do it, or am I stuck with a motor with the wrong rotation? Am I right about the fan rotation?

10-17-2009, 07:51 PM
You're correct, the fan should rotate toward the outlet as in the diagram:


J Tiers
10-17-2009, 09:58 PM
many small fan motors are shaded pole, in which case the rotation is fixed, unless you reverse the core assembly.

Others are "PSC", which have a capacitor but no switch. Those will rotate reverse if the capacitor is in series with the wrong winding. Assuming the windings are same, which may or may not be true, connecting the cap in series with the other winding will reverse it. if you get both ends of teh capaciotor winding, you can reverse the connection, which will also reverse the motor.

A split phase, or capacitor start motor is reversed by reversing the polarity of teh start winding. Instructions are typically on the plate, or under the wiring cover.

Without knowing what kind of motor, or what it looks like, we may be giving totally wrong info. You must judge, or show pictures

10-17-2009, 10:04 PM
One phase motor (w/ start switch), reverse the two start winding leads.
Three phase motor just reverse two of the three wires coming into the motor.

The drawing on the motor will tell you the wire colors.

10-17-2009, 10:16 PM
Most smaller fan motors are shaded pole as Jerry said. They have the advantage of not burning up if stalled. Depending on the design of the actual fan housing and the rake angle of the blades it may not make much difference which way it rotates as far as efficiency is concerned.

Have you looked at the motor? It may be double ended and installed backward.

10-17-2009, 11:15 PM
You can turn those type fans by hand in both directions and it is very easy to see which direction produces the maximum amount of air. Then you will know for sure which way the fan needs to turn.

Unless the motor is designed to be reversable you are stuck with a motor that runs only in one direction.

Here is a cool old timers trick. If the motor is connected to the fan with a V belt you can twist the V belt into a figure 8 and put it on the pullies or shivs. This makes the 2 pullies run in oposite directions without changing motor rotation.

Don Young
10-17-2009, 11:28 PM
Squirrel cage blowers can have the vanes slanted in either direction depending on the design so you can not determine proper rotation that way. However, the rotation should be toward the outlet as you show. If your motor permits it is sometimes possible to remove the stator and turn it 180 degrees to reverse the motor.

10-18-2009, 12:02 PM
After reading the posts here, I know the motor is turning backward from what I need, and there is no wire switching to reverse it. The shaft is in a dedicated configuration, so I can't turn the motor end for end.

HOWEVER, I took the old one, and found I can get the end plate off of one end (the other is welded), and I can remove the winding/magnet assembly. So, if I turn this winding assembly end for end and reinstall it, with the shaft in the same configuration, will that reverse the rotation?

Many thanks for the help.


10-18-2009, 12:08 PM
Yes, that will work.

J Tiers
10-18-2009, 01:15 PM
if there is no way to put the entire assembly on the other side of the scroll, that may be your only option

10-18-2009, 05:52 PM
I took the new motor apart and turned the windings end for end and re-installed it. It was a little tricky getting the wires fished back out of the casing, but I got it OK.

I got the whole thing back together, the motor rotation was reversed, the pressure switch made just fine (I could tell the air flow was right just spinning it by hand) and the heater works fine.

I learned another great thing here today, and I thank all that posted. This place is the best.

Don Young
10-18-2009, 11:25 PM
Glad you got it working!

dr pepper
10-19-2009, 05:23 AM
Never heard that one before, glad it worked.
Usually there is on a single phase motor, 4 wires a, az, z and t, reversing a and az without changing any of the other wires reverses the motor.
A lot of smaller ones though only have 3 wires which I thought untill I heard your trick couldnt be reversed without digging in to the winding connections.

J Tiers
10-19-2009, 10:15 AM
Lots of motors in teh past had a nice little cast cover to put over the end of the shaft that you were NOT using. That way you could put it on either way and get either direction. Most seem to have been on saws, for some reason.

I can't suppose it was cheaper than bringing out another wire, but.....

dr pepper
10-23-2009, 03:53 AM
I tried that not so long back, a motor I was looking at did indeed have a metal cover on the back, only when I removed it I found that it was where the centrifugal switch was housed.