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Electric Motor Fan for Vacuum Cleaners - Basic question

Discussion in 'Electronic Basics' started by Mike, Oct 11, 2005.

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  1. Mike

    Mike Guest

    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
     
  2. Richard

    Richard Guest

    Could the burning rubber smell have come from a beater-bar belt? That will
    seize up a vacuum pretty quickly. If so, it is also likely you will have tro
    remove the beater bar and the end bearing units to clean all the carpet
    fibers and pet hair (if you have pets).
    I would think shorted windings would pop the circuit breaker to the outlet
    it is plugged into.
    Starter cap could be a possibility.

    Did this happen suddenly, or did the performance slowlyu deteriorate?

    If all else fails, try an internet search for the make and model just search
    on Sears or Kenmore vacs.

    Good luck.

    Richard
     
  3. Mike

    Mike Guest

    Could the burning rubber smell have come from a beater-bar belt? That will
    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.
     
  4. 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.
     
  5. Mike

    Mike Guest

    Hmmm, what sort of "Electrical engineering" were you into??

    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.
     
  6. spudnuty

    spudnuty Guest

    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)
     
  7. Jasen Betts

    Jasen Betts Guest

    The on/off switch is on the handle of the vacuum cleaner and with the switch
    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
     
  8. Rich Grise

    Rich Grise Guest

    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
     
  9. Jasen Betts

    Jasen Betts Guest

    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.
     
  10. Mike

    Mike Guest

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

    spudnuty Guest

    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
     
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