Reverse polarity of starting winding - AC motor

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Old 09-23-19, 02:32 PM
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Reverse polarity of starting winding - AC motor

Hi guys,

I have an 1ph 240V (NZ voltage) AC motor running a chainsaw sharpening stone that I needed to get spinning in both directions (one way from factory).

I successfully split the starting winding and running winding and brought 2 new wires out of the cage, now 4 in total. Initially I had the polarity of the starting winding reversing on a 2-pos DPDT rocker switch which worked great. I don't know why I did it but I guess I had to test what would happen when someone inevitably did and slowwwly rolled the rocker switch while it was running, which must have joined one pole before the other or both at once, and cooked the motor windings.

Anyway. Dug through the coils found the burnt end, ran new wires again, and now am using a 3-pos DPDT switch which has 'OFF' in the middle, so now of course this can't happen.

What can happen and is question one, the main winding can be energised with the 3-pos switch in 'off' and the motor just hums but doesn't start. In this state is the main winding at stall current? Would it cook itself if left like this for too long?

What also happens is when the starting winding polarity is reversed while running, it is not enough to overcome the inertia of the armature with the main winding driving it, so it doesn't reverse, just the hum/note changes slightly. Is this running the start winding at stall current also?

It makes a lot of sense having a centrifugal switch but it doesn't, and I'm working with what I've got!

Thanks in advance!
Josh
 
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Old 09-23-19, 02:44 PM
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What can happen and is question one, the main winding can be energised with the 3-pos switch in 'off' and the motor just hums but doesn't start. In this state is the main winding at stall current? Would it cook itself if left like this for too long?
Not sure about stall current but burn up for sure of left on too long.
 
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Old 09-23-19, 03:20 PM
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Please provide a picture or information on motor nameplate. Also provide a schematic of the motor/ switch arrangement you created.
 
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Old 09-23-19, 03:30 PM
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You cannot reverse the polarity of a motor when it's running.
 
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Old 09-23-19, 03:59 PM
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You can certainly reverse the polarity of one of the coils, it just doesn't do anything as far as I can see. I have seen guys start these things in reverse with no circuit alteration, using a drill to speed the armature up past where the starting winding will be able to counter it. I've never seen any harm come of it after reports of hours of use - though they may have centrifugal switches isolating the starter winding I'm not sure.

They push the bur over the wrong side of the tooth when sharpening one side if they spin in one direction, so the drill is a common workaround, or you have to modify it like I have. Stupid that the manufactures haven't clicked...

There is no motor plate, just this tagged onto the units frame. The sketch below is what I've done.
 
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Old 09-23-19, 04:09 PM
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Your circuit is correct.
You can START the motor in either direction but you can't change directions once it's running.
 
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Old 09-23-19, 10:48 PM
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And if its started with the winding off?

Or if the winding is swapped while running? Will damage occur in either case?
 
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Old 09-24-19, 04:14 AM
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Motors may survive the strangest misuse, but do not have to. It may go well once or twice, but...
 
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Old 09-24-19, 09:38 PM
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The run winding is always running at full current. The capacitor in your start winding is like a switch. It allows the start winding to be energized for a split second and that's what determines the spin direction. Any longer then that and the start winding would fry. That split second is not enough to change the direction of the motor once up to speed. Quite possibly nothing would happen if you tried reversing it while running or the capacitor could explode.
 
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Old 09-26-19, 06:36 AM
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The rotation direction switch will burn its contacts after a few operations of switching current direction through the aux winding with the rotor rotating in the opposite direction for the current direction through the aux winding. I also don't think motor rated torque and speed are obtainable with this condition. If the time to stop the motor before changing direction impacts the production, I suggest you look into getting 3 phase motors or look into some other kind of single phase motor control.
 
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