Electric Motor Fan for Vacuum Cleaners - Basic question

Hi All,

Our 2-year old vacuum cleaner (Kenmore Whispertone model - cannister type), seized up recently with a burner odor coming from the cannister compartment.

I pulled the unit apart and measured the voltage at the input terminals to this integrated motor fan unit and it was in fact 120V as it should be.

The on/off switch is on the handle of the vacuum cleaner and with the switch open, the motor armature still makes an intermittment revolutions while connected to grid power. But then eventually stops. Looking at the carbon brushes, they seem fine. There is a little bit of a build up around the brushes at the brush/slip ring interface. I ran it down to a local Vacuum repair shop and the fellow looked at it ever so briefly and said its a throw away and tried to sell me another vacuum. He indicated that the "field unit" was dead. Now from my understanding of electric motors (which is limited I admit), is that terminals connect to a series of field coil pairs with lots of copper windings which ultimately creates magnetic flux between these corresponding pole pairs causing the armature to rotate in the presence of the flux lines, so the principle concept here is electromagnetism as I understand it. Hence, I believe the primary components are magnetic pole pairs affixed to the stator, lots of copper windings, an armature connected to the fan, carbon brushes and slip rings. Now, I am assuming that this motor is constructed in a similar fashion.

Can anyone suggest some of the potential failure points in this motors? Are they typically field serviceable? How does a "field unit" (not really knowing what he meant and I asked for clarification but didn't get any from the fellow) die? Is it possibly a case where the windings from a given field coil are shorting to another set of windings? Can you rebuild these units easily? The replacement unit from Sears is $115 which accounts for

33% of the vacuum brand new. I have an electronics engineering background buts its been a while. I do have a fully equipped bench at home if necessary but I think this probably requires me to physically rewind the field coils more than anything else ... ? Am I on the right track? Or have i missed something?

Do these types of units typically have any capacitors in them which would potentially introduce a safety concern when opening it up while unplugged ... would need to discharge them accordingly? I wouldn't think so but not sure. Any / all info would be great. I would much rather fix it myself if possible.

What would account for the burning odor? copper melting? Are the windings typically enamaled or insulated? From what I could see, peering inside, it didn't look like it. It almost smelt like rubber???

Thanks in advance!

--Mike

Hi Richard, Thanks for responding. The belt(s) are located in the power head which is attached to the cannister via a standard vacuum hose so its very easy to isolate the two systems. I actually went as far as taking the motor fan unit out and putting it on my bench and applying a line voltage to it directly with the same behavior.

Unfortunately, very little came up on the Internet when searching for this model. In the interim I will try some more basic generic motor fan repair searches online but I am still interested in hearing back from anyone in this newsgroup that has been down this path before.

--Mike.

Hey Mike, In my experience vacuum cleaner failures are generally the result of moisture or dust causing: (1) the commutator of the motor (where the brushes contact the armature) to become resistive and the copper contacts burn up (fairly obvious from the scorch marks or lots of sparking when you try to run it). It sounds like the field coils are OK since these motors are usually series connected. Of course they could still be shorted. Possibly cleaning up the commutator as below will clear this up. (2) The bearings (usually sintered bronze) start binding. Does the armature turn freely? If it's binding you'll have to completely disassemble the motor. Usually two long bolts that run the length of the motor. Lift the brushes up when you do this. If they are removable do that keeping track of the orientation. If not I usually tie them up with a fine wire or surgical thread, then cut that when the motor is reassembled. Saves the brushes from getting banged up when you remove the armature or the brushes go flying and you're on your hands and knees on the floor. Usually cleaning up the armature shaft with 600 grit emery paper while spinning it in a drill and cleaning out the bearing itself clears this right up. If not I've gone so far as to turn them on my lathe. Resaturate the bearings in a good high speed oil before assembly. A lot of these fail because the filters protecting the motor have clogged/failed or too much plaster dust was picked up. They can get so bad they are not salvagable. That goes for commutators too. I have seldom seen the field coils burn up as these are thermal protected in UL rated motors but of course anything can happen and usually does! Richard (the other)

CMOS fabrication as well as some MEMS research in accelerometer applications. Now I am working in the industry but on the business side of things so tech skills are fading.

I have yet to pull it apart because there is a metal housing that covers the machine screws to release the motor assembly and it appears to be very firmly fitted into the base so not exactly sure how I will put it back together once I open it ... so wanted to see what the likelihood of repairing this unit was before hand. I suspect I will end up replacing the unit and pull this one apart to tinker with and then throw it away. Just hate to shell out 25% of the original purchase price on a 1.75yr old motor.

Are these shorts in the armature typically the result of overheating caused by extended periods of operation? Or is it more likely that this vacuum saw some high voltage transients from the AC mains?

Thanks for your input Rheilly.

Hmmm, what sort of "Electrical engineering" were you into?? It sounds like either the armature has " shorts" (the insulation between the wires in the coils has broken down and adjacent turns are touching) which is by far the most common failure in series motors or a similar fault in the field windings. Sure these things can be rewound but in the modern world much cheaper to replace the whole appliance. In particular rewinding the armature needs access to balancing equipment or the whole job is a waste of time. To the private person such as your self it would represent a lot of time with the great possibility of failure. Rewinding the fields would be a better bet but they are not the usual failure, have you measured the dc resistance of each coil ? they should be equal.

--
Regards ......... Rheilly Phoull

The most likely scenario is that somehow the airway got clogged - either in the suction end or by a full bag or clogged outlet filter. Since the cooling air for the motor in a canister is the same air that does the suction, when it gets clogged, the motor overheats and eventually burns out.

If the vac was over $300.00 only a couple of years ago, and the new motor is $115, then go ahead and have them replace the motor, and keep a stock of bags on hand, so that you can replace a full bag right away. Also, make sure the airway is clear all of the way through, and check/clean/replace the outlet filter regularly.

Good Luck! Rich

As to that housing around the impeller, I have taken one of these apart before and remember cursing the maker as the thing was glued together. I had to carefully pry the housing apart and reglue it back together. Some of the nuts holding the impeller to the motor shaft can be left handed but a careful inspection should tell you that. I used hard wood shims to freeze the armature so I could remove the nut, being careful of the copper windings. Richard

field unit sounds like the stator, stationary windings in your motor.

gets too hot, the conductors melt inside it causing an AC short-circuit

only on the rotor. but yes the windings do cross at the ends of the rotor

if you have plenty of time and patience.

sounds like a bargain.

I think so (right track).

Only small ones for noise suppression, nothing to be scared of.

The shellac enamel on the wires breaking down as the windings overheat. once it turns to carbon you've got problems.

Bye. Jasen

Some good information presented here. I would like to thank everyone who has taken the time to respond. Its much appreciated! Thank you.

They don't like getting too hot. running them for extended periods with clogged filters, blocked hoses, full waste bags etc will kill them as the air they pump is alo used to cool them, and if they can't pump air they don't get cooled.

>

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Bye.
   Jasen