Brush-type universal motor question
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  1. #1
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    Default Brush-type universal motor question

    I have a Master Appliance heat gun. Old, heavy duty, American-made. I love to fix and use such things.

    Won't run.

    Field windings show around 30 ohms each. Sounds like a probably okay range.

    Armature is OPEN between ANY combination of commutator bars. I assume continuity should exist between diametrically-opposite pairs of bars? The armature looks clean, no discoloration of (green) varnish. Where I delicately scraped varnish away from wires near bars, have continuity between bar and wire.

    Manufacturer has told me that a new armature for unit so old is not available, and the new model will not fit. They won't show me a drawing so I can see if I can make it fit, they say the new one is not electrically compatible, will not tell me details. Certainly a waste of their time as well as mine.

    Obviously I have already spent more time on this than the cost of a lovely new Chinese throwaway heat-gun. (New Master $108, new Chinese $20-$50)

    Just seems unlikely that EVERY coil is burned open, and none shorted, somewhere inside the armature, with no sign of distress at the surface.

    Am I missing something real obvious?

    Thanks to any who join me on this fool's errand, pleased to be of service to any who just want to laugh.

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    I believe you are correct about continuity between diametrally opposed commutator bars. It does seem unlikely that all the armature windings have "opens" in them, though. Scratching my memory, I believe it is time for an armature growler. This is a testing device used on DC and universal motor or generator armatures to test for bad windings. It was common in old time auto service shops. It is called a growler because you held a hacksaw blade against the laminations of an armature while the armature was bedded against two energized field coils. If the blade vibrated with a "growling" sound, you located the bad windings in the armature. A temporary fix was to cut the leads to the bad windings, disconnecting them from the commutator bars and possibly soldering the leads together. The result was the motor or generator would run, albeit with some reduction in capacity. The real fix was a rewinding job.

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    Quote Originally Posted by magneticanomaly View Post
    Am I missing something real obvious?
    Mebbe not obvious. Could simply be uncommon.

    You said SERIES wound motor. Not shunt wound, so:.

    - Is the Armature coil even the only item in the electrical path?

    - Is there a compensating coil ON the Armature - for regulation?

    - Is it multiple fixed-speed? If so, by what means?

    Not laffin' - puzzled meself.

    But series motors ain't really my bag, so...

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    Quote Originally Posted by Joe Michaels View Post
    I believe you are correct about continuity between diametrally opposed commutator bars. It does seem unlikely that all the armature windings have "opens" in them, though. Scratching my memory, I believe it is time for an armature growler. This is a testing device used on DC and universal motor or generator armatures to test for bad windings. It was common in old time auto service shops. It is called a growler because you held a hacksaw blade against the laminations of an armature while the armature was bedded against two energized field coils. If the blade vibrated with a "growling" sound, you located the bad windings in the armature. A temporary fix was to cut the leads to the bad windings, disconnecting them from the commutator bars and possibly soldering the leads together. The result was the motor or generator would run, albeit with some reduction in capacity. The real fix was a rewinding job.
    Good Old Skewl stuff, but that Armature ain't gonna whimper, let alone growl if every single turn is "open" - because.. they all ALSO go - in series - through some other SINGLE point of failure.

    Need a direct-hit lightning strike to wipe-out the lot, 100%, and those tend to leave some sore ugly evidence.


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    Growlers detect shorted windings. Even if there is an open winding, there should be continuity between some or most of the commutator bars. You can make a motor with a single winding between two opposite bars so you have a complete winding for each pare of bars, but manufacturers don't because it takes too much wire. Normally it is some variation of a coil between adjacent bars all the way around with two coils wound into adjacent slots, one for each bar on each side of a bar. That is a general statement. In use, there are all sorts of winding patterns, simplex, multiplex lap, etc. In any case, you normally expect to have continuity between all bars to all other ones. You would need to have all windings open to not show continuity somewhere.

    Where did he say series wound? With 30 ohm DC resistance fields, it has to be shunt wound. He can tell the difference by looking at the connections.

    I haven't had an armature rewound in a long time so I don't know who does it or pricing. It is not likely to be cheap.

    Bill

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    Quote Originally Posted by 9100 View Post
    Growlers detect shorted windings. Even if there is an open winding, there should be continuity between some or most of the commutator bars.
    But there is not....
    You can make a motor with a single winding between two opposite bars so you have a complete winding for each pare of bars, but manufacturers don't because it takes too much wire. Normally it is some variation of a coil between adjacent bars all the way around with two coils wound into adjacent slots, one for each bar on each side of a bar. That is a general statement. In use, there are all sorts of winding patterns, simplex, multiplex lap, etc. In any case, you normally expect to have continuity between all bars to all other ones. You would need to have all windings open to not show continuity somewhere.
    Something is still missing, here. Hope it isn't just his meter or leads!
    Where did he say series wound?
    Thread title. "Brush type universal"
    With 30 ohm DC resistance fields, it has to be shunt wound. He can tell the difference by looking at the connections.
    Thread title could be off a wrong assumption. Or my assumption could be wrong.
    Even so, there remains a mystery.
    I haven't had an armature rewound in a long time so I don't know who does it or pricing. It is not likely to be cheap.

    Bill
    Haven't seen a figure under $200 in ages for an actual "rewind", not just general cleanup and new end leads etc.

    Modern cost of fully-burdened skilled labour, most any skill other than burger-flipping, that could by now be double, and still be only the minimum. Wouldn't be surprised "small motors" aren't wanted, full stop.

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    Isn't there also a solid state device (Thermistor?) in there which prevents overheating? Once they fail the entire unit usually is non functional.

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    Thanks for all the kind and helpful suggestions.

    I may have misled with my assumption that it is a universal motor....Armature and windings appear to be in series.

    My meter and leads are okay, thanks Bill, always worth checking.

    Video from Groschopp reminded me that other times, checking other armatures, I have been puzzled that all bars seem more-or-less connected.

    Yes, the Growler! I even think I have one somewhere, probably acquired from a starter-generator repair place auction. A-C electromagnetic v-block, as I recall..

    But my poor little armature, without shaft and comm not much bigger than a walnut, is just OPEN. Really ought to just salvage the switch and junk it.

    Time probably better spent on the air compressor that won't turn all the way around before it goes "CLANK!" I can make a connecting-rod.

    Thanks again!

    Thanks again!

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    Now wait a minute. You got us all worked up looking for a solution and now you are just going to drop it? That's not fair. We need a solution!

    Tom

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    Quote Originally Posted by TDegenhart View Post
    Now wait a minute. You got us all worked up looking for a solution and now you are just going to drop it? That's not fair. We need a solution!

    Tom
    Wifey's world-mobile hair-dryer set to 240 but run off 120 does me OK. I think I HAVE a store-bought heat-shrnk gun... somewhere.

    Haven't used it since the FIRST time it got lost in the debris field of IJN Yamoto I seem to have managed to cram into me shop.

    "Drop it"? Why not. Scant chance it'd ever find its way to the deck, so no real harm...

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    Well, Tom, I have a last (maybe) e-mail in to the mfr asking about exactly why the new armature will not work in the old fields, but I think they already realize I am not a good sales prospect.

    'Tis a mystery how all connections are open with no visible damage, but I do not think i am crazy enough to try to unwind all that teeny wire and re-wind it.

    And an old hair-dryer does work okay, esp if you tape over most of the air inlet holes to raise the output temperature.

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    Quote Originally Posted by magneticanomaly View Post
    And an old hair-dryer does work okay, esp if you tape over most of the air inlet holes to raise the output temperature.
    Oh, well. I needed cooler, not hotter, but if it is "cheap hots" you need, instead?

    Just hang a length of perfed stainless tube for stand-off & blend onto the snout of a Bernz-o-Matic or Presto-lite clone - sized according to the need - preferably with pushbutton ignition - and you have it.

    Near-as- dammit instant ON/OFF vs 'lectric ones and potentially even "cordless" as well, free bonuses.

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