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AC motor question

Discussion in 'General Discussion' started by mage2, Sep 19, 2009.

  1. mage2

    mage2 Monkey+++

    So I acquired a OLD 1/4 hp 120v motor. I cleaned it up some and kicked it on. It spins but after a few min of running the motor was getting really warm. I notice that when i was cleaning it out some dirt came out of vent holes that are located below the shaft on the front and back of the motor.
    It doesnt make any noises (other than a hum when running) and the drive shaft seems to spin pretty freely.

    Is it normal for a very old motor to get really warm while running with no load?
  2. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    Yes, BUT (of course.) Assuming it is an induction motor, the most common: If it doesn't start humming louder, and doesn't seem to be getting hotter or slowing down with time, you are probably OK. However, look for bearing problems unless you take it apart and re-grease them. (I suspect that the grunge coming out is old grease.) Make sure you run it in an open area until you are sure it isn't overheating. If you can, find a rev counter and see if the speed is close to the nameplate data. If it is a 3600 rpm motor, it should run around 3570 or 3580 unloaded, and not much less than 3550 under full load. Any rated speed less than that should be around the same proportions.
  3. mage2

    mage2 Monkey+++

    well i left it running with a temp prob attached to the case and it hit 80 deg C (176F) before it shut down.
    I think im going to take it back to where i got it and see if they will trade it back in.
  4. Tango3

    Tango3 Aimless wanderer

    shutdown by itself???( temp overload tripped??)
  5. WestPointMAG

    WestPointMAG Monkey++

    What’s it smell like?
  6. Tango3

    Tango3 Aimless wanderer

    Good question westpoint mag, a burned motor has an obvious acrid bitter nasty smell to it..the smoke will make you wretch and you can pick it out across the factory floor....
    when they burn the insulation like that they're done.
  7. mage2

    mage2 Monkey+++

    i know well the smell of electronics components. (let me tell you sometime about my electronics instructor that worked for nasa full time and was a teacher part time)
    it seems like it was a thermal thing. the motor startted up again after it was let cool

    the previous owner of the motor has offered me 3 more motors and a arbor with pillow block bearings for my projects for a price of $40 total.

    I think ill get them all and then take this one apart and try to clean it out.
    There is a bearing store not too far from me.
    Do you think it could be the bearings are failing and thats causing it to overheat?
    Cause it doenst smell like buring electronics more like hot metal/oil.

    what do you think the chances are of me cleaning it/replacing the bearings and it working for me?

    at worst ill sell the copper. hah
  8. Tango3

    Tango3 Aimless wanderer

    A motor draws more current the more load is applied . some are even rated with "locked rotor current": as a stalled motor (0 rpm )will draw the maximum current available but also creates the maximum amount of torque before the winding goes up in smoke.Yours is puzzling it should not build up enough heat to trip a thermal overload running unloaded. there isn't much to an a/c induction motor, mark the end caps and windings( shell) with a sharpie,or better a punch or chisel ( punch marks can't be washed off in the solvent tank!) so every thing goes back together in the proper way, give it a good spin to feel what it feel;s like now the gingerly separate everything note the position of any wavy washers or shims on the shaft. Clean it and replace the bearings, aint much that can go wrong, spin the re assembled unit by hand anything bindinging (?) do it again...secure it to the bench; give it a noload test.

    just had a thought, before you take it apart just loosen the through bolts that hold the end caps on just enough to see if the rotor feels any looser,(perhaps the previous owner took it apart and installed an extra washer or something or the shaft is bent and binding; and the rotor is getting warm and expanding to the point of binding. If it feels better (but not so loose as to let the rotor come into contact with anything inside )... give that a short test.
  9. Tango3

    Tango3 Aimless wanderer

    Upon dissassembling you may find the front cover and bearing press fit to the rotor.Do not use a hammer around shafts and bearings, the proper tool is a mechanical arbor or hydraulic press ora threaded puller to slowly and evenly remove the bearing.Some auto parts places orauto machine shops maybeable to help, heat and cold help( heat expands, cold contracts.) i.e put the rotor in the freezer overnightwill help it gointo a tightly fitted bearing. if uyouusea metal hammer on the end of the shaft you create a mushroom then nothing goes back together right; and impact forces theball or roller bearings to leave marks in the races, guess how long it lasts then...
  10. mage2

    mage2 Monkey+++

    I have a press and promise not to use the 12lb sledge. unless i give up and im scrapping the copper out of it.


    when i spin it is seems to spin freely.. ill take some pics as soon as i get time to go out there and poke at it.
    i know when its running the tone of the hum seems to change, could that be it heating up enough to bind?
  11. Tango3

    Tango3 Aimless wanderer

    Just wanted to get you after saying:
    " oh yeah take it apart" its a snap!!!!"
    People's first inclination is to reach for the "hit with"...:) and beat the the rotor out of the bearing.You're "lucky" to have a press in yourshop.
  12. Tango3

    Tango3 Aimless wanderer

    Hokay this "no load heating" is a puzzler.

    Electrical motor controls" 3rd edition by Rockis and Mazur,1992,:
    They say :

    "Single phase motors are not self- starting. All have a means of developing starting torque.There are several types:"Splitphase, shaded pole, capacitor, universal, repulsion, and synchronous."

    common split-phase fractional hp A/c motors will have a "starting" winding(finer wire)and a "running" winding( heavier wire),When starting, both windings are in paralell at about 75% speed a centrifugal switch opens disconnecting the starting winding. Perhaps your centrifugal switch is stuck closed( gunked/rusted up??)???
    capacitor start motors adds a capacitor to the start winding circuit both disconnect at 75% like the plain split phase.[​IMG]

    there is also a permanent split-capacitor motor where the cap remains in the circuit. And even more complicated dual capacitor motors. Each has its own special torque/speed characteristics.
  13. tommy20/69

    tommy20/69 Monkey++

    check how many amps the motor is supose to draw and then check the amps the motor is drawing. i bet the motor is drawing way to many amps and thats what is causing it to overheat.it's just like a blower motor for an a/c unit it has to be restricted so it won't overheat by drawing to many amps.i get alot of them blowers out the dump and if i just plus it in without blocking some of the air opening they will overheat within minutes.now if i take and put a air restricter in front of the opening and only allow so much air to flow. the motor will sound like it speeds up but it's really slowing down. alot of people think you can just plug in a electric motor and thats it but their wrong you have to have some kinda restriction on it to keep the amps where they should be other wise you'll have a free spinning motor that will shut off every few minutes and restart when it cooled down but it will eventually burn up.
  14. Tango3

    Tango3 Aimless wanderer

    Won't/Can't argue with Tommys' personal experience (blowers) My experience is pretty much 3phase multi hp "prime movers":( conveyor and hyd pump installations) and its uncommon to run "unloaded" ( no reason to). Looking online There's a few questions and virtually no agreement as to what an unloaded singlephase will draw;Here's a troubleshooting page for single phasemotors, problematic fractional hp motors are generally thrown away, :
    same thing in pdf from the "fluke" site:

    Heres an interesting singlephase vibratory bowl motor problem discussed:

    They finally wittle it down to "power factor": a unloaded singlephase is a purely reactive load( "eli the iceman" ) a powerfactor of .5 . Where a loaded motor will run at .7: 1 is efficient (good.)
  15. mage2

    mage2 Monkey+++

    I will try to check the current this weekend,but its going to be a packed weekend so we will see.
    I took some pics of the motor to help you all visualize what im dealing with here.

    on a side note. I found someone that is giving away woodenboxes that are 5'x8'x7', here comes my storage and chicken coop and etc. :)

    anyway back to the pics.
    motor1. motor2. motor3.JPG motor4.JPG
  16. mage2

    mage2 Monkey+++

    so based on the pics it should be a
    ac motor type FJ
    thermoguard type A
    1/4 HP?
    1725 RPM 60HZ i dont know what the 1 is for
    115V 3.5 Amps

    i have no idea what 40 cont JC is
    and then some serial number i am guessing.
  17. tommy20/69

    tommy20/69 Monkey++

    ok here's what you do take the motor and get a bicyle and if it has a hand break just barely squeaze it just enough to get the brakes to touch the rim and then set the tire next to the motor when it's on that should put the motor in enough of a bind that it will not over heat. and let her run and see what happens.
  18. Tango3

    Tango3 Aimless wanderer

    Careful the capacitor may hold a charge.
    Capacitor start,115v 3.5Amps full load current,1725rpm (full load speed)takes into account a % of"slip".you can oil the bearings with a little 10w/30 motor oil in the little caps, I can see from the picsthe vents are dirty, obviously the neutral (white wire connection is bad... )
    and obviously its been hot. Make sure the cord is wired correctly: looking directly into the "spades" of the 3 prong plugthesaying is: "ground down" ( round pin held at the bottom down) "White right".( neutral wire to right prong looking into the plug. Left slot in the wall recepticle.)

    3.5Amps full load Amperage. (The current required bythe motor to produce the rated torque at the rated speed. From what I can find in my school texts tonight , noload current should be less than "FLA" because it takes less than max torque to just spin the rotor with no load. And I know a motor draws more current as the load is increased.
    "Locked rotor current" is not stamped in the i.d. plate. So 3.5 has to be "Fla" ( full load Amperage).
    Iwould redo the bluewire connection,
    (ground here) anyfresistance there will leave you with a voltage on the case in the event of a short. Ouch. Needs good cleaning and lube. Look for that centrigfugal switch (weighted mechanism) on the back of the rotor shaft.
  19. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    1 is single phase. Capacitor start (that is the can on top) can't tell if it is in or out of the circuit when running, but I think probably out after coming up to speed. (listen for a click during both starting and stopping, that is the centrifugal switch opening and closing.) Open construction means it is probably LOADED with guckus by the looks of things. Yes, 1/4 hp. 1725 is full load speed, unloaded it will be closer to 1775 or 1780. (This is the slip from synchronous speed of 1800 revs, otherwise there would be no way to start it without shunt windings or shaded poles. There is a bit more to that, but for now it will do.) 40 cont is the temperature rise above the surrounding ambient under continuous full load. That works out to be 112 F (more or less) in a "normal" atmosphere. The 170 deg F odd reported is WAY high, and evidently shutting off on high temp, and resetting the trip when it cools off. Can't see it in the pic, but the 3.5 is probably full load amps (look for an "FL" someplace close to the 3.5.) No idea what the JC means. Thermogard "A' is probably a code for the thermal trip, dunno for sure 'bout that.

    You have nothing to lose by taking it apart and cleaning it out, then trying again. Be careful with the centrifugal switch when disassembling, it might not be obvious how it comes apart. With nothing to lose, hose it out with water, then thoroughly drying. You can use a volatile solvent before the water (or after, then wash out again, depending on how it looks when you take it apart. Obviously do NOT use a flammable solvent.) If you've a notion, use the oven to dry both the stator and rotor for a couple hours at very low temp so long as it is above say 125 and less than say 150. Or let it in the sun for a day or three. You can also check the resistance in the stator and rotor with a VOM before washing it out and again when it's dry, might give a clue.

    It should not overheat under no load, but the experiment Tommy suggests might be useful as a test of the theory as well as your patience.
  20. mage2

    mage2 Monkey+++

    i plan on poking at it this weekend if i get the chance.
    It is full of gunk im sure.
    Thanks for the help and i will keep you all up with my progress.

    one more question. on the motor shaft. how much play should there be inline with the shaft. meaning if i set the motor down and pull on the shaft as if i was trying to pull it from the housing.
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