Did not know you could do this with a small motor.

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drg

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Last week had a request at work to fix post mounted fan that was blowing backwards.......

It was a simple 1 phase fractional horse dayton fan around 30" diameter mounted on a bldg. column and it was blowing air in the wrong direction from what it was suppose to be .

I was puzzled but a guy came over shut the fan off and when it came to a stop he put a pencil in between the fan guard and spun the blades the opposite rotation hit the switch and it worked the right way !!

I said o.k. you guys know how to fix it and left ............seems like something is odd here , certainly I don't understand how the motor works this way but it does.

This fan is in the blgd. I work in and if there is something to fix I'll do it but there is nothing we can figure out ..........we really don't want to replace it but sooner or later someone will stick something in there and get it caught or possibly hurt and then mounds of paperwork and headaches.

Any ideas here?
 
A bad or weak cap will let that motor start in either direction. A bad pole in the motor would do the same thing. I'd go for the cap first, and visually check the end switch.
 
Years ago, in what was probably Popular Mechanics magazine, there was an article about a guy who used two of the same size motor, one wired for each direction, and the two pullied together (plus a pully for the load - I don't remember what it was.)

He could start either motor first, which would select which direction the two (and thus, the machine) would run. Then, because the centrifugally-actuated start-winding switch would open, he would energize the second motor, adding its power to the first.

I have reversed shaded-pole motors by disassembling them and flipping the winding and pole piece.
 
080610-2250 EST

drg:

If you truely have a single phase motor ( this excludes a capacitor run motor ), then the running direction is determined by the direction that you start the motor turning.

A single phase motor is like a swing where you put energy into the mechanical system in synchronized pulses. In the single phase motor you only have a bidirectional oscillating magnetic field.

In contrast a three phase motor has a rotating magnetic field of approximately constant amplitude. This runs much smoother than a single phase motor.

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Is there ambiant air flow in the wrong direction causing the fan to spin backwards before the fan in question is turned on? If it is free spinning fast enough this is not suprising that it will continue to run backward when energized.
 
A typical fan motor that has been described is normally a shaded pole or permanent split cpacitor type. they both have shading coils which are fixed location single wrap coils about the size of a # 10 wire. When current is applied the shading coil "draws" the rotor towards the shading coil and the motor rotates in the direction or towards the shading coil. It acts like a big magnet and starts the rotor turning . The rotor then just continues in that t rotation . A capacitor only increases the current momentarily creating more of a phase shift and acts as a kick start for the rotor. The position of the shading coil relative to the winding coils still is the determining factor in rotation. All the cpacitor does is give the motor more starting torque. The reason the pencil trick works is because once rotation is established the motor will continue in that direction. The pencil is like the shading coil in that it pushes the rotor a bit to get the load rotating.
 
Capacitor start motors typically 1/4 and up have 2 windings, start and run. The start winding is connected across the run and again acts as a swing if tou will to pull the rotor in the direction that the coils are connected relative to the run winding. As the rotor gets up to speed the high current and thus hi torque of the start winding is disconnected by means of a centrifugal switch whis opens the start and allows the run to still be connected . Rottation is reversed in this type of motor by switching the connection in the start winding (5 and 8 in some motors and red and black in others althought here are other variations these are the 2 most common).This winding reconnection changes the phase relationship of the start and run (Polarity of the start coils. ) Hope this helps you understand how these 2 types of motors operate.
 
080611-0624 EST

The only way to have single phase induction or synchronous motor self-start is to have some means to approximate a rotating magnetic field during the start time. This requires some form of a second coil and a current that is phase shifted relative to the main coil. In effect for starting, at least, a 2 phase motor.

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stew said:
A typical fan motor that has been described is normally a shaded pole or permanent split cpacitor type. they both have shading coils which are fixed location single wrap coils about the size of a # 10 wire.
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Sorta. The shaded-pole motor has the shading, or shunt coils, but the split-cap motor uses a cap in series with one winding group to 'synthesize' a second phase using the capacitive leading current.

When current is applied the shading coil "draws" the rotor towards the shading coil and the motor rotates in the direction or towards the shading coil. It acts like a big magnet and starts the rotor turning . The rotor then just continues in that t rotation.
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Sorta again. The shading coil acts as an inductor that slows the rate of flux change in the shaded section of each pole piece. The unshaded part of the pole face gains and loses magnetism faster than the shaded part, mimicing a 4-pole motor with only two poles.

A capacitor only increases the current momentarily creating more of a phase shift and acts as a kick start for the rotor.
All the cpacitor does is give the motor more starting torque.
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It doesn't work because of increased current, it works by causing one winding (or set of windings) to reach max flux earlier in each half-cycle than the other winding(s).

A centrifugally-operated switch opens the start winding once the motor reaches a certain percentqage of full speed. Your clothes dryer uses a similar setup with the start button you have to hold until you hear the click.

A cap-run motor doesn't drop out the capacitively-connected winding, probably for greater torque. A cap-start/cap-run motor has three windings: one for starting, one for continuous phase shifting, and one main running winding.

The position of the shading coil relative to the winding coils still is the determining factor in rotation.
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That's absolutely true, but even some small shaded-pole motors can be spun in reverse. My microwave turntable doesn't always turn in the same direction.


Added: I see you do grok this in your second post. My apologies.
 
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Thanks for the replies everyone, No there is not really a ambient air flow that would be causing the fan to even slightly turn when off .......

Also there are quite a few of these fans scattered about the building that do not have this problem........what I will do tonight ( if I remember ) is take a couple fans that are not a problem and spin them in both direction and turn them on and see what happens, will be interseting to see the results.

Oh, buy the way these are not "capacitor start" motors.

I'll post back with results and specifics of the actual motors that I do this test on.......sounds kind of crude and funny but it is one way to learn and keep busy at work through the night.
 
LarryFine said:
That's absolutely true, but even some small shaded-pole motors can be spun in reverse. My microwave turntable doesn't always turn in the same direction.
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I don't think that is a mistake, I think that is intentional, mine alternates directions each time it starts.
 
iwire said:
I don't think that is a mistake, I think that is intentional, mine alternates directions each time it starts.
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iwire said:
If I recall Stew's background is in winding and repairing motors. :cool:
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(That's why Bob has such a high post count!:wink:)

Okay, Stew, how would one construct a small motor to alternate each start?

I think it's totally random - mine does rotate one way around 80% of the time.
 
LarryFine said:
how would one construct a small motor to alternate each start?
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Why do you assume its an AC shaded pole motor? :smile:

It could just as easily be a DC motor and the controls simply alternate it?

Heck even the Greenlee "Smart Bender" uses a DC motor. :cool:
 
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