AC motors

Discussion in 'General Electronics Chat' started by ken roper, Mar 24, 2015.

  1. ken roper

    Thread Starter New Member

    Dec 6, 2014
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    Hi
    Don't know if this is the correct place to ask these questions but here goes.
    An induction motor runs on ac supply am I correct in assuming that if the motor was rated at 370w then from ohms law at 240v it would draw 1.5A, then from ohms law again drawing 1.5A at 240V the total resistance of the motor would be 160 ohms.
    The motor has 2 sets of coils in it (but joined together as one continious coil) one with 24swg wire and one with 28swg wire, the 28swg coil burnt out but there seems to be no damage to the 24swg coil, as I have all the time and patience in the world I decided to rewind the 28 swg coil, if I now check the resistance of the total winding it should read near to 160 ohms, all I have is a digital meter i do not have a megger to check the insulation.
    I can calculate the resistance of each part of the coil from the ohmic value of a meter length of each swg wire so then can take an ohmic reading of each part if below what it should be then can I assume that part of the coil is burnt out.
    I did manage to rewind the 28swg wire coil but when i switched it on it ran ok but after about 10 to 15 seconds it started to smoke so I switched it off do you think the other part of the coil the 24swg part is shorting where I cannot see it, it is not shorting to the casing.
    What I forgot to mention was when I removed the 28swg coil I noted the number of turns for each part that went into the slots, in total there were 8 sections but the number of turns for the first and last sections were not equal so i made them the same same number of turns as the 2nd and the 7th sections which were 40 turns as opposed to about 30, have I done something wrong or would it not make a great deal of difference
    PS I made a jig for winding the coil and took note of the direction of the winding
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
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    85
    Not true. This is an inductive device. Its DC resistance will be much lower, in the region of 10 ohms or so.
    The current it draws is usually shown on the makers plate. Because it is an inductive device it will have a low Power Factor. The current will be considerably higher than P/V.

    Induction motors can be re-wound but this is a skilled operation - usually carried out by specialist companies. Small motors are harder to do!
    You might have shorted turns. The professionals would impregnate the new windings in insulating varnish and bake hard.
     
  3. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Another thing to consider is the efficiency of the motor. As the motor get hot it is because you're converting power to heat, and that is not the purpose of the motor.
     
  4. ken roper

    Thread Starter New Member

    Dec 6, 2014
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    yes I understant it is an induction motor, but statically when not in use the coil must have an omic value and if the measured value is less than the calculated value then it must be short, on a rough estimate the length of wire in the 28swg part of the coil is about 100 mts so from tables 28swg wire has an ohmic value of 0.155 ohms per 1 metre length so by that the 28swg part should have a value of aprox 15 ohms the other part is 24swg which is thicker wire and has an ohmic value of 0.0703 ohms per metre if the two sections are the same length then the 24swg section should have an ohmic value of 7.03 ohms put them together the total ohmic value of the whole coil should be aprox 23 ohms that is what i get when measuring with my meter would as i said increasing the number of turns on the 2 outer most sections by about 10 turns each make any difference?
     
  5. JDT

    Well-Known Member

    Feb 12, 2009
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    85
    Probably not. Extra turns will increase the inductance and lower the working current slightly. If you do not have any shorts to ground (the casing of the motor) then I suspect shorted turns. This is often impossible to detect just by measuring the DC resistance.
     
  6. #12

    Expert

    Nov 30, 2010
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    I haven''t seen any professional motor designers here...yet.
    The obvious answer is, "Why do you think you can optimize a motor by guessing?"
    Just put it back the way you found it.
     
  7. ken roper

    Thread Starter New Member

    Dec 6, 2014
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    I know this might sound a bit OTT and a bit time consuming but would it be possible to rewind the separate sections (the 24swg and the 28swg) as they are only joined by a solder joint, manually but include 2 sleeves of heat shrink tubing on each part where the coil fits into the slots, omitting the varnish (even though I have the correct stuff)
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The coil slot is usually initially prepared with a Fish Paper tray for insulation, never wound directly into the slot, when the winding is complete then a fibreglass wedge is driven in at the top of the coil.
    Max.
     
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Wouldn't the 28GA coil be the start winding in the motor, if it is single phase? The last motor I stripped had plastic 'U' shaped sleeves in the slots to protect the wire bundles. The varnish is also to immobilize the windings, so vibration doesn't wear the insulation and cause shorts.
     
  10. ken roper

    Thread Starter New Member

    Dec 6, 2014
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    It's true what is commonly thought "you learn something new every day" apparently you cannot test a motors windings "loop to loop" with a megger, megger's test insulation from coil to coil or to ground.
    What is required to test loop to loop is an "impulse tester" and i don't have bottomless pockets.
    From what I can gather the 2 sets of coils in the motor are 1 for starting the motor and the other to run it at it's designed speed, now when it went belly up it was the thinner wire coil that burnt out, i might be wrong when I say this but the motor would still turn, but smoke and would be running at a slower speed with virtually no torque, from that I deduce , could be wrong, the thicker wire coil starts the motor and the thinner wire coil runs it at speed.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Either winding with a short will cause it to draw excessive current and lower rpm.
    If the start winding was open, it would run if you spin the shaft in either direction.
    The start winding is the higher resistance than the run.
    Max.
     
  12. ken roper

    Thread Starter New Member

    Dec 6, 2014
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    So by that reconning the 28swg winding which is the thinner wire has a resistance of 0.155 ohms per metre is the start winding and the 24swg winding (the thicker wire) which has a resistance of0.0703 ohms per metre is the running winding.
    Suddenly realised if the thicker winding has twice the length of wire the resistance could be virtually the same but on examination it does not look as though it has twice the length.
    If I am correct why is the start winding burning out when it is running at full speed?
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Does it have a centrifugal switch? Also you can tell the split phase/start winding by the connection of the external capacitor.
    Max.
     
  14. ken roper

    Thread Starter New Member

    Dec 6, 2014
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    Sacre Bleu I suddenly realised what i have done when trying to answer Maxheadrooms last reply,
    The motor cable has 3 wires one red, one black and one white, I labeled them when I took the 28swg winding out, one end of the 28swg winding was connected to the white wire the other end was connected to one end of the 24swg winding that in turn was connected to the red wire, the other end of the 24swg winding was connected to black.
    When connecting the motor wire to the power cord which was blue ,brown, and earth, the white wire was connected to the blue wire and nothing else, the red and black wire was connected to what looks to be a small coil then that was connected to the brown via the switch.
    What I have done, OH SILLY ME, was wire the motor cord thus 1 end of the 24swg coil to black (correct), the other end which was connected to 1 end of the 28swg winding to white (wrong) and the other end of the28swg winding to red(wrong) the red should be where the 2 coils join and the white wire should be on the other end of the 28swg coil.
    I checked the resistance of the 2 windings and they were very similar so will wire the motor up the correct way and report on the outcome, I could be lucky in the respect that when the motor started to smoke I immediately turned it off and prevented the loops from shorting, will also check to see if there is no earth fault before i wire it up and switch on.
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You may also have a outboard current switch to operate the start winding instead of internal centrifugal type (the small coil)?
    Max.
     
  16. ken roper

    Thread Starter New Member

    Dec 6, 2014
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    Thats probably it Max
     
  17. ken roper

    Thread Starter New Member

    Dec 6, 2014
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    Well I was correct it was wired up incorrectly, after correcting the fault it spun up and is working ok, that little coil is in fact a solenoid what it does is energise the starting coil a fraction of a second before it powers up the second winding and the 2 windings together keep the motor spinning.
    Well JDT i am not a skilled operative in rewinding motors but I managed this one and the rotor is only aprox 3in in dia, just shows what a bit of determination and planning can do.
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Its called a potential or "voltage' relay.
    [​IMG]

    Max.
     
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