Help me understand why this Shop Vac won't run

[rslaback]

I have a Shop Vac project I am working on that had been deemed scrap. It may or may not have been disassembled and diagnosed in order to make that determination.

I took a look at it today and the motor doesn't spin. I verified that the cord and DPST switch are good. The motor appears to be an Ametek based on the brush holder design. Where it gets weird is that I cannot find anything physically wrong with the motor. A visible check of the frame windings looks good. The rotor looks ok as well. I did check the rotor and none of the pads short to ground, the pads have consistent values from pad to pad and consistent values on opposite pad pairs. If I meter at the brush holders I get no reading between the holders themselves but each of them to ground gives 120vac. The system does not blow a breaker. I did see it flinch once.

What am I missing here? I was fully expecting to see a bad switch or a bad cord or consumed brushes or a burned winding and it doesn't appear to be any of those things.

 

[Old Man Roger]

Does it spin freely by hand? Did you make sure the brushes weren’t stuck?

 

[rslaback]

It does spin freely by hand. The brushes are not stuck and are making good contact with the commutator.

 

[Steve.S]

Check for an inline fuse in the wiring. Might be covered in shrink tube.

 

[BillK]

A visible check of the frame windings looks good.

Did you check them with an ohm meter ?

Did you try applying power directly to the brushes if possible ?

 

[Chuckster in NJ]

Bad cord, dirty commutator, no spring tension on brushes, high temp fuse, bad switch…….. "IF" not one of those things you may have an open (bad) winding.

Get an ohm meter and ring ever out starting with the cord.

TIP! Make sure you have voltage at the outlet.

 

[neblinc]

Probably a thermal fuse.

 

[alfredeneuman]

Get an ohm meter and ring ever out starting with the cord.

TIP! Make sure you have voltage at the outlet.

This should be fine tuned a bit. The ohm meter won't work if you have the shopvac plugged in.
Unplug it at the outlet first. then the ohm test, and finally make sure you have voltage at the outlet

 

[Chuckster in NJ]

This should be fine tuned a bit. The ohm meter won't work if you have the shopvac plugged in.
Unplug it at the outlet first. then the ohm test, and finally make sure you have voltage at the outlet

"IF" this needs to be explained to that detail maybe someone shouldn’t be working anything electrical related…… Maybe this will help “fine tune” my post #6.

 

[whateg01]

How can you have 120v to ground on opposing brushes?

 

[alfredeneuman]

"IF" this needs to be explained to that detail maybe someone shouldn’t be working anything electrical related…… Maybe this will help “fine tune” my post #6.

Many people here don't know what they're up against and haven't a clue to how to do it

 

[rslaback]

How can you have 120v to ground on opposing brushes?

each brush to ground gives me 120. I included that to explain why I am not getting a voltage across the brushes but I know that there is power to them. So in other words the power is going through the rotor. If it were not, the brush on the hot side would show power but the brush on the neutral side would not.

 

[dave*99]

each brush to ground gives me 120. I included that to explain why I am not getting a voltage across the brushes but I know that there is power to them. So in other words the power is going through the rotor. If it were not, the brush on the hot side would show power but the brush on the neutral side would not.

That would indicate an open in the neutral connection from the brush to the plug.

 

[TRWham]

I would be looking at impedance through the system rather than voltage. All of the coils in this type of motor are in series. As you read resistance from the line in through the various points in the motor moving toward the neutral connection, eventually you should find one measurement that goes to infinity (or at least very, very high), and that's your problem, right there.

 

[mm08822]

With the vac unplugged and the dpst closed (ON), measure the resistance across the plug hot and neutral blades. What do you get?

 

[rslaback]

Did some more work on this today. Used my Klein meter that does zero out fully.



This motor doesn't have any permanent magnets. The power comes in and goes through a housing coil and then to the brush holder.





Resistance through each of the housing coils is .2 ohms.






Resistance from brush to brush is consistent at 1.6 ohms as armature is spun.



The switch tests good and has been removed, powering each coil lead directly with no change.

Without knowing the housing coil specs, .2 ohms seems low. How likely is it that I have a shellac failure on both housing coils causing a short and low resistance?

 

[dave*99]

Did you confirm the power switch is working properly? Is there a thermal fuse hidden somewhere?
Looks like you checked that....



This link should work if you let it take you to Youtube.

 

[rslaback]

Did you confirm the power switch is working properly? Is there a thermal fuse hidden somewhere - in the tape?
Check for continuity across this device.



This link should work if you let it take you to Youtube.

I wired it directly to remove the switch from the possibilities. If there was a blown thermal fuse, wouldn't i get an overlimit instead of .2 ohms?

 

[dave*99]

I wired it directly to remove the switch from the possibilities. If there was a blown thermal fuse, wouldn't i get an overlimit instead of .2 ohms?

Yes I missed that in your photo. You would read overlimit across an open fuse.

 

[dave*99]

On second glance - that tape looks like it's only a splice. Is there an open fuse elsewhere?


It's worth putting a piece of plywood on your metal bench when working on electrical stuff....

 

[iron block]

Just to confirm -- when you test the motor, is it set up with the leads with the black terminals each connected to one of the brushes, and is AC power applied between the two leads with the bare metal terminals?

And what resistance do you read between those two leads with the bare metal terminals in this test configuration?

 

[mm08822]

Did you apply power directly to the motor leads? (Not using the original cord)

Did you do what post 15 suggested? Or maybe now, short the hot and neutral together after the switch and measure resistance at the plug blades?

 

[OccupantRJ]

 

[rslaback]

Score one for the good guys. It's running again. The motor from lead to lead measured in at 2.6 ohms. But at the plug end it was 8 megaohms. Cut off the plug and measured the resistance on the conductors and we are back to normal. Wired in a new plug.

I've always just verified a cord was good by measuring voltage at the ends. This cord had enough internal resistance in the plug that even though it was providing 122vac it didn't have enough amperage to magnetize the coils.

 

[alfadan]

There is a thermal fuse wedged into two fork like things near the windings. It looks like a piece of solder. I replaced one with...a piece of solder.

 

[dave*99]

Score one for the good guys. It's running again. The motor from lead to lead measured in at 2.6 ohms. But at the plug end it was 8 megaohms. Cut off the plug and measured the resistance on the conductors and we are back to normal. Wired in a new plug.

I've always just verified a cord was good by measuring voltage at the ends. This cord had enough internal resistance in the plug that even though it was providing 122vac it didn't have enough amperage to magnetize the coils.

Great. Keep in mind your high input impedance voltmeter will often be fooled measuring voltage on an unloaded cable. Look back to post 13. You found the open in the neutral right in the plug.