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  1. #1
    Join Date
    Jan 2004
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    Mackay North Qld
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    6,446

    Default Electric Motor Capacitors

    Are there other members enduring a spate of capacitor failures ?.

    I have just had the capacitor replaced as the motor on my generic house brand 200mm 900w bench grinder had to be rolled over to kick start it. A nice new replacement of Italian manufacture was obtained and the grinder now operates as intended.
    A previous bench grinder was returned to the dealer as said capacitor expired within the warranty period and the above was the warranted replacement. Both motors are identical brand and model.

    I have been subject to a number of of electric motor capacitor failures over the last two years and it seems the more recent the electrical motors are ,the more likely they blow the capacitors over those capacitors in the older motors.

    For example I have a Taiwanese bench drill circa early 1990s that has not missed a beat.

    But motors in 3 bench grinders, a barrel grinder and a reciprocating saw have all spat the capacitor.The barrel grinder and recip saw were 5 years old but they were the good quality Bosch trade models not the DIY stuff.

    I realise that capacitors can blow from time to time but these failures of reasonable quality tools some just exceeding the warranty period in some cases seems unusual to me.

    I put it down to the source of the capacitors as I have done some internet reading where the circuit board caps caps from certain Asian sources were implicated due to the make up of the electrotye.

    Whether or not it applies to can types found in start & run caps I am not sure.

    What have your experiences been?



    Grahame

  2. #2
    BobL is offline Member: Blue and white apron brigade
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    Perth
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    Default

    I suspect some caps used in cheaper machines may be rated above their real rating both in terms of V and temperature.

    There may also caps made for the 220V market that may already be border line for use in a 220V machine hence more likely to fail at 240V.

    Most caps used in 240V machines are rated for V well above the mains in which they are actually used e.g. 400+V.

    Mostly I see 400V 65șC rated caps but I always try to get a 450 or 470V cap and 85șC rated especially on machines where the motor is under constant high load.

  3. #3
    Join Date
    Jul 2008
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    near Warragul, Victoria
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    3,718

    Default

    Just recently I had the capacitor fail in the ignition circuit of my old land Rover . The failed cap. was a brand new BOSCH cap. and it has lasted only 6 months before it failed. Took me a while to track it down because I've never had a automotive ignition cap. fail before. I replaced it with an old original cap. made in the 1950's and its been running fine !

  4. #4
    Join Date
    Nov 2017
    Location
    Sydney
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    Default

    If you are talking about motor start capacitors, I have found you need to check the centrifugal switch.
    The switch can take 25 to 30 amps on start up. If the points are bad or misaligned you will blow capacitors.


    On one machine I had trouble with, the points burnt out within a month.
    What I did was use a solid state 60 amp relay to carry the current. I energised it by the centrifugal switch. That was 2 years ago and still going strong. Arcing in the back of the motor was greatly reduced also.


    I see a lot of motors with a 115v capacitor and if I upgrade to 240v or higher they start cleaner.


    If it is under warranty - don't touch it.

  5. #5
    Join Date
    Jan 2004
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    Default

    The capacitor I purchased,I believe is a motor run capacitor 350V at 12.5uF to replace a 350Vat 13 uF.

    What I have read indicated the Chinese had a lot of problems with capacitor manufacturing a while back .

    I made a point of finding an alternate country of manufacture for my capacitor.
    The 2 bench grinder motors were of Chinese make.The second,a warranty replacement ,of the first, began to falter not too long after warranty expired.

    The first one just went bang and let the blue smoke out and the second one just got to a point where it would not run the motor unless it was rolled over. The replacement capacitor comes from Italy. To keep a record I have written the date on the can to give me an idea on its longevity.

    My understanding is that the motor start capacitors are above 100 uF rating.There is no centrifugal switch, nor can I find or any wiring for one.

    The bench grinder application is to power a wire brush and MultiTool Belt linisher. It is so nice to switch the darn thing on and not have to flick the belt to get it rolling.

    I am interested in improving my knowledge of capacitors as it begins to appear there are different levels of quality with them.
    Thanks for the tip of the parallel caps ,I will file that one.

    Grahame

  6. #6
    Join Date
    Oct 2007
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    Alexandra Vic
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    69
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    654

    Default

    Quote Originally Posted by Grahame Collins View Post

    My understanding is that the motor start capacitors are above 100 uF rating.There is no centrifugal switch, nor can I find or any wiring for one.

    Grahame
    Generally, double ended grinder motors are split phase cap run motors, fairly low start torque, slow starts, etc because they don't get that big kick in the pants from a start winding and start cap. Therefore I would not expect to find a centrifugal switch and start cap included. If the grinder head is a low quality design, (overspecced and underpowered) having a linisher unit hanging off it may just be too much for the particular unit, particularly if one unit died under warranty and it's replacement likes to eat run caps for lunch. Maybe an A&A or similar might be a better option, though I have replaced the run caps on two of the three I have dealt with in the past because they failed.

    Have said it before, but the best single phase grinder I ever came across was my dads home brew special, 2HP 2800 RPM old school cap start Crompton motor with custom made wheel mount adaptor and guard/rest. He built that in the 60's based on motor from the 50's, and it was still a beast when Dad departed 15 years ago. Not light, not compact, only single ended but definitely very reliable, and never ever bogged down.

    I doubt that there is a formal capacitance value demarcation between run caps and start caps. What is critical is the equivalent series resistance, the ability to carry the required AC current, and a value that produces the required phase shift when coupled with the associated winding.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  7. #7
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    Default

    I think you are right Mal.

    Doing any more to the unit is probably akin to putting lipstick on a pig.

    Mind you it is still ok at the moment as I had repaired the tracking cam which had worn over the years

    I using it mainly to clean up rust,dress metal ,radius the sharp corner off plates and sharpen drills and edged tools.

    I will look for a 2hp motor or better as I have another dedicated linisher frame ( sans motor ) which I want for knife work.

    Grahame

  8. #8
    Join Date
    Jul 2006
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    Athelstone, SA 5076
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    Default

    from my experience there are capacitors and then there are capacitors...
    I see a lot of failures associated with cheap products...ie my HM52 clone motor capacitor failed real quick...
    as has other cheap crap.

    An italian made capacitor is not a sign of quality...only make is....but they can fail at a moments notice

    however, a capacitor does like to be used..not sitting there idle for long periods as this does reduce the life.

  9. #9
    Join Date
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    Default

    Quote Originally Posted by eskimo View Post
    however, a capacitor does like to be used..not sitting there idle for long periods as this does reduce the life.
    The Multi tool is used daily so theres no danger of sitting around.

    Definitely the most used electric tool in my shed.

    What do you consider the top makes and where should I consider looking for the capacitors in the future?

    Thanks
    Grahame

  10. #10
    Join Date
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    Default

    Anything from a motor rewind centre would be okay..there are many good brands.

  11. #11
    Join Date
    Nov 2007
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    melbourne australia
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    Default

    Quote Originally Posted by malb View Post
    If the grinder head is a low quality design, (overspecced and underpowered) having a linisher unit hanging off it may just be too much for the particular unit,
    True.
    Chris

  12. #12
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    Default

    Quote Originally Posted by morrisman View Post
    The failed cap. was a brand new BOSCH cap. and it has lasted only 6 months before it failed.
    Was the cap definitely a genuine Bosch part? If purchased from a reputable retailer, then it most likely was. If purchased from eBay, then possibly (probably?) not.
    Chris

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