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  1. #1
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    Default Bench grinder capacitor replacement

    I need to replace a capacitor on a Chinese made 200mm bench grinder. I had the original bench grinder replaced under warranty as it let the blue smoke out of the capacitor.

    I can start this one by flicking the wheel over but it's not the right thing to do. Naturally its replacement now being being out of warranty the capacitor has pooped itself and the arc has taken out the uF marking.

    The electric motor sticker specs are 240V,750 watt 2850 RPM .The volt spec on the burnt capacitor is still readable and that is 350 volt.

    I have been online and checked sizing for motors and a table was given. From what I can garner its a run capacitor thats needed and at 350 volts it should be around 12uF.

    Can anybody confirm this or am I way out. I think the Chinese have a problem making decent capacitors so I am inclined to buy an European or US made capacitor or are they too ,no longer made made.

    Thanks

    Grahame
    Last edited by Grahame Collins; 23rd Nov 2017 at 11:14 PM.

  2. #2
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    Wander down to your local auto sparky. He may also be a motor re-winder. He will be able to assist with the correct size, or send you to where you can get one.

    Hooroo.

  3. #3
    BobL is offline Member: Blue and white apron brigade
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    Default

    Sounds about right. Chinese make caps as well as anyone as long as you are prepared to pay.

    I'd go for something like this
    https://www.jaycar.com.au/12uf-400va...citor/p/RU6604
    They are probably made in China

  4. #4
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    If you can start the grinder by flicking the wheel over then I would have thought it is more likely that the start capacitor has gone?

    Michael

  5. #5
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    Quote Originally Posted by Michael G View Post
    If you can start the grinder by flicking the wheel over then I would have thought it is more likely that the start capacitor has gone?

    Michael
    Say what? Wrong name?

    It is usually me saying stuff like that. Having a moment Michael?

    Dean

  6. #6
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    There are two types of common single phase motor using capacitors - one is a cap start motor where the capacitor provides a slight phase shift to get things moving and then relies on the load to provide a shift against the windings; the other is a cap start/ cap run motor where the run side of things is also shifted.
    In my experience (limited as it is), I have found that when a single phase motor will run after a flick of the wheel, it is usually the starting circuit that is not working, hence my comment that if it will run then it is more likely the start capacitor has gone.
    Without looking at the motor I would not know but as the two caps are differently sized, I thought I'd throw that in so that Grahame considers all options.

    Michael

  7. #7
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    See? I told ya so.

    Dean

  8. #8
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    Still not with you Dean -
    Quote Originally Posted by Grahame Collins View Post
    ...I can start this one by flicking the wheel over but it's not the right thing to do.
    Suggests a start cap to me

    Quote Originally Posted by Grahame Collins View Post
    ...From what I can garner its a run capacitor thats needed ...
    But in his original post Grahame is talking about a run capacitor.

    Perhaps I'm not the one confused

    Michael

  9. #9
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    I agree . If the motor runs after flicking the wheel around I would think it's the start capacitor that's crook .
    Had same issue on my mill and a new start cap fixed it . Go to an electric motor supplier and repairer the type that fix power tools etc. They will know which size capacitor you need . Voltage rating can be higher like 400 or 500 volts Uf ( microfarads ) has to be closer . Buy a good brand name not made in China .
    The volume of a pizza of thickness 'a' and radius 'z' is given by pi z z a.

  10. #10
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    All the bench grinders I own have only one capacitor. I had originally asummed they were a start cap but the table(see the link) designates low uF values as run caps ,so I went with that.

    I think the terms starter cap and run caps may have confused me as the site shows 2 tables, one for stater cap sizing and one for run cap sizing.

    Single phase capacitor sizing

    The starter cap table shows high uF values - in the hundreds and the run cap tables show uF values in the tens .

    The first bench grinder motor that exhibited the failed capacitor did have the uF values that were low - too far back to remember the exact value but they were low.

    To explain and make it confusing, the 12 uF value was calculated from the table stated as the RUN table. Looking at the 1 to 2 HP line it should be in the middle of the 1- to 20 uF rating and closer to 15uF, not 12uF I suppose.

    I have looked at another 8'' bench grinder and it has a 350volts x 8uF capacitor .

    Perhaps the naming for the tables is switched about???

    Grahame

  11. #11
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    Some grinder motors seem to avoid using a start cap and just have a permanently connected run cap so are effectively a split phase motor. A characteristic of these is that they have low torque and are slow to start when switched on, i.e they start to revolve slowly as soon as they switch on but take 3-5 seconds to get to speed and then slow significantly under load and recover slowly. I was working on an A&A about 10 yrs ago and it was configured this way. Most single phase grinder motors seem to be set up the same way. The only one that I can recall that had decent starting and torque was a 10" one my father made for himself based on an old school 11/2HP Crompton Parkinson cap start motor. That thing would be up to speed in less than a second and did not slow significantly under load, i.e it behaved fairly much like a 3phase grinder motor.

    If you could not hear the centrifugal switch cut out when starting or drop back in when slowing to a stop when the unit was in working order the machine most likely does not have a start capacitor, as there would be no way to isolate the start cap and winding.

    The thing with cap start motors is that they have two windings, a run winding rated for the motor's power and duty cycle, and a start winding with fewer turns of heavier wire that can handle a significant surge current for a couple of seconds maximum. The start winding is wired in series with the centrifugal switch and start capacitor, and relies on the centrifugal switch isolating the winding when the motor approaches operating speed. If the the switch fails to short circuit, or the motor is mechanically stalled, the start winding remains energised and burns out.
    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.

  12. #12
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    Quote Originally Posted by malb View Post
    Some grinder motors seem to avoid using a start cap and just have a permanently connected run cap so are effectively a split phase motor. A characteristic of these is that they have low torque and are slow to start when switched on, i.e they start to revolve slowly as soon as they switch on but take 3-5 seconds to get to speed and then slow significantly under load and recover slowly..
    Thanks Mal,
    You described the motor start up sequence to a t.

    Thank you everyone for your inputs.

  13. #13
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by Grahame Collins View Post
    Thanks Mal,
    You described the motor start up sequence to a t.
    Thank you everyone for your inputs.
    Some grinder instructions will even say - wait for 30 seconds before grinding - probably overkill but that's the instructions.

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