AC Motor Switch Wiring

Thread Starter

SJESolutions

Joined Feb 2, 2021
3
I am but a lowly MechE trying to solve what should be a basic electrical issue:

I have a 120VAC snow blower. No parts are available. The switch went bad. It is a DPDT trigger- and there are 6 terminals. The wiring is as such:

Please see the pic attached- this is the cover to the switch...Wiring.jpg
The plug is white and black. The white is on terminal 3, the black on terminal 6. The black to the motor is on terminal 2, the white to the motor is on terminal 1. There is a blue to the motor on terminal 3 (with the white). The red to the motor is on terminal 4 and terminal 4 is jumped to terminal 5. It appears the 4,2,6 pole does not throw when the button is depressed. I need to replace this switch- what kind of switch do I need? Just a single throw- is that possible? I cannot imagine the use of the
 

dendad

Joined Feb 20, 2016
3,914
It is a DPDT, Double Pole Double Throw switch. It is handy having the switch circuit on the case.
The switch aplies power to the motor when operated, and likely a load or short on the motor in the off position to stop it rotating quickly.
They are readily available, just make sure you get one with the same or larger current rating.
Are you after an exact replacement or just a functional one?
 

Thread Starter

SJESolutions

Joined Feb 2, 2021
3
Thanks- the weird thing is that the one pole (line) does not appear to throw (there is no cam to move it) but the contact is burned and pitted. The other pole looks like it used to throw (but the cam is melted). There is continuity between the two winding (I assume there are two), so not likely a motor issue. I want it to work- so I don't need a direct replacement (and I could not find one online).
 

dendad

Joined Feb 20, 2016
3,914
Just about any 240VAC 10A or 15A DPDT switch will work. Just ensure you get the connections correct.
If in doubt, get a qualified person tp help.
Here are just a couple....
DPDT_2.pngDPDT_1.png
Make sure the switch has a mains rating, and is not just for 12V.
Some switches are ON-OFF-ON types, but what you need is just OFF-ON.
Do you have a multimeter to check the connections?
 

Thread Starter

SJESolutions

Joined Feb 2, 2021
3
OK- thanks. I have a multimeter. I can connect the same as the oem switch. I will try a standard DPDT and update. I doubt it will work (because when the line throws to off nothing will happen). After that a new unit for $185! I just prefer to fix than throw away.
 

MisterBill2

Joined Jan 23, 2018
9,304
My guess is that the switch is set up to stop the snowblower motion quickly by short circuiting the motor when it is switched off. They do that on electric mowers and it shortens the motor life a lot, so I always remove that function. All you really need is a double pole switch, because it is better to open both sides of the line for a powered device. The difference is now it will coast when you switch it off, so DO NOT put your hands in the blower while it is coasting.
Hey folks, I had forgotten to put a "not" in place.
 
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MisterBill2

Joined Jan 23, 2018
9,304
Lawn mowers of mine have all had brush type motors. BUT an induction motor will brake hard if the line terminals are shorted while it is spinning. I made a 1750 RPM half-hose motor tumble over doing that one timme, just to prove it would. Of course, that was a no-load condition.
 

Tonyr1084

Joined Sep 24, 2015
6,269
First thing I'd be looking at is why the switch burned up. Could be a bad motor - could just be years of use on that switch. I had an electric weed whacker. A friend borrowed it. He hardly used it when the switch went bad. He was concerned that he would have to replace it but I knew he didn't do anything to make it go bad. Sometimes switches just do that - go bad.

On another hand, if the motor is stuck and switching it draws tremendous current the switch might not like it. Since you said it's a snow blower I'm assuming you have a lot of snow to move. Could just be the machine was tired or maybe you worked it too hard. But basically a switch is just a switch. Turn it on - turn it off. But I'd be looking at the motor just to be sure there's no problems there before I spent money on a switch.
 

MaxHeadRoom

Joined Jul 18, 2013
23,639
Lawn mowers of mine have all had brush type motors. BUT an induction motor will brake hard if the line terminals are shorted while it is spinning. I made a 1750 RPM half-hose motor tumble over doing that one timme, just to prove it would. Of course, that was a no-load condition.
Induction motor requires some other means, it does not generate! , this is why DC injection braking or plugging is used.
Also a Universal motor is the same.
Max.
 

MisterBill2

Joined Jan 23, 2018
9,304
When an induction motor is running there is a lot og magnetic force lines passing through those coils and laminations. When the power is removed the magnetism does not instantly vanish. When the power is removed and the line terminals quickly shorted, current flows and force is generated. It would not be good enough for dynamic braking but it did work.
 

strantor

Joined Oct 3, 2010
5,646
Well I have something I want to try now: shorting the leads of an AC motor while it's spinning and see if it does anything to brake the motor. The concept seems to make sense; the residual magnetism in the stator induces a small current in the spinning rotor which induces a higher current back into the stator which induces a higher current into the rotor, all in the blink of an eye, which results in a screaming halt. I've never heard of it and certainly never seen it done, but it's the same or similar concept as how a generator works, so it seems plausible. But if it works, why is never done? And if it doesn't work then why does MrBill insist that it does? Life's mysteries. I won't be able to test it until next week so if anyone else wants to, I'll take a spoiler.
 

MisterBill2

Joined Jan 23, 2018
9,304
The exact way that I did that experiment was with a half HP 1750 RPM motor sitting on the floor of the shop with a cord with #12 wires and about 12 feet long connected quite adequately to the motor terminals. The other end had a normal AC plug. After the motor ran a bit I pulled the plug and immediately shorted the blades won a piece of round stock, so that the delay was about a second, certainly not more than two seconds.
The reason that this is not done in practice is probably because it is rather abusive to the motor, and there is the problem that a small timing error would put a direct short across the mains.
The reason that it is done on lawn mowers is that the makers see it as far more important to protect against fools who would stick their hands into a spinning blade than to deliver maximum motor life. That is also whyit would be included on snowblowers. Product liability trumps product life every time, thanks to a legal system that rewards stupidity very well.
 

MaxHeadRoom

Joined Jul 18, 2013
23,639
Well I have something I want to try now: shorting the leads of an AC motor while it's spinning and see if it does anything to brake the motor.
If it works then it would make the DC injection method that has been used for decades and the plugging method where the necessity for 'at zero speed' switches be both made redundant.! ;)
Max.
 

strantor

Joined Oct 3, 2010
5,646
Ok I couldn't wait until next week. I went ahead and did the experiment in my hotel room. I must have had the wrong kind of AC motor because I didn't experience any braking. Quite the contrary. It seems the residual magnetism generated a rotor current which was in phase with itself and rather than braking, the motor accelerated. And accelerated. And accelerated. It was some kind of positive feedback loop. It was like a field weakening free energy machine. It spun ever faster and faster and as I was thinking of how to harness the energy from it (couple a cordless drill to the shaft?) It began making a noise that prompted me to change my thought priority to finding shelter. I hid in my bathroom and observed a bright light from under the door. Then a loud bang. I came out to find the motor gone along with the TV that was beside it. There was sooty/charred spot on the wall near it but curiously, no shrapnel. Not a scrap of motor or TV. I think they might be in next week somewhere. Unfortunately I didn't check what kind of motor it was, didn't document any part numbers, and can't remember exactly how I wired it, so this experiment is totally unrepeatable, but at least now we all know that perpetual motion and time travel are possible.
 

MisterBill2

Joined Jan 23, 2018
9,304
Ok I couldn't wait until next week. I went ahead and did the experiment in my hotel room. I must have had the wrong kind of AC motor because I didn't experience any braking. Quite the contrary. It seems the residual magnetism generated a rotor current which was in phase with itself and rather than braking, the motor accelerated. And accelerated. And accelerated. It was some kind of positive feedback loop. It was like a field weakening free energy machine. It spun ever faster and faster and as I was thinking of how to harness the energy from it (couple a cordless drill to the shaft?) It began making a noise that prompted me to change my thought priority to finding shelter. I hid in my bathroom and observed a bright light from under the door. Then a loud bang. I came out to find the motor gone along with the TV that was beside it. There was sooty/charred spot on the wall near it but curiously, no shrapnel. Not a scrap of motor or TV. I think they might be in next week somewhere. Unfortunately I didn't check what kind of motor it was, didn't document any part numbers, and can't remember exactly how I wired it, so this experiment is totally unrepeatable, but at least now we all know that perpetual motion and time travel are possible.
OK, here is a report that describes the unique appearance of a miracle of energy generation. Suddenly out of nowhere energy sufficient to over-rev the motor appeared, and made the motor spin faster and faster. My guess is that this happened in "Hotel California."
 
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