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  1. #1
    Join Date
    Aug 2008
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    Tennant Creek, Aust
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    Default Bench Grinder Capacitor

    The capacitor on my dadís old bench grinder has given up the ghost.



    On the capacitor it says 110 volts, can I replace it with a capacitor that is 240 volts?




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    Ratty 05/2004 -05/07/2010 COOPER 01/08/1998-31/01/2012

  2. #2
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    Sep 2008
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    Narrabeen, Sydney NSW Australia
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    Default

    Surprised it didn't go bang given that it's rated for ~50% of the actual line voltage, suppose it's a testimony to the quality of the original unit that it didn't, overbuilt with huge headroom and margin for safety - I would definitely be replacing it with the 240 volt equivalent.

  3. #3
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    Sep 2012
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    York, North Yorkshire UK
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    Default

    Hi Mark, Guys,

    With capacitors you can always use one of a higher voltage, never less. As far a capacitance is concerned, electrolytic capacitors have a quite wide tolerance value, usually as much as plus or minus 20%. So the actual value is not as critical as would be thought. AC rated capacitors can be connected either way round ! The only consideration is if it has a metal case that is connected to one of the terminals, in this case it has to be wired the right way round.
    Best Regards:
    Baron J.

  4. #4
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    Tennant Creek, Aust
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    Default

    The yellow cap covers the terminals.

    The grinder is probably older than me, it was in dadís shed as long as I can remember.
    What I donít understand is that Australia is 240volt, why would they put a 110 volt capacitor in an Australian built machine?


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    Ratty 05/2004 -05/07/2010 COOPER 01/08/1998-31/01/2012

  5. #5
    Join Date
    Dec 2013
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    Default

    Agree, I've seen higher rated capacitors like 280, etc, used in 240v, but are usually 240 or 250v.
    There are some good online tutorials to help out if needed.

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    Using Tapatalk

  6. #6
    Join Date
    May 2018
    Location
    Brisbane
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    14

    Default bench grinder gap.

    Quote Originally Posted by BaronJ View Post
    Hi Mark, Guys,

    With capacitors you can always use one of a higher voltage, never less. As far a capacitance is concerned, electrolytic capacitors have a quite wide tolerance value, usually as much as plus or minus 20%. So the actual value is not as critical as would be thought. AC rated capacitors can be connected either way round ! The only consideration is if it has a metal case that is connected to one of the terminals, in this case it has to be wired the right way round.
    The capacitor is in series with the start winding not across the mails. The voltage is split between the start winding and the capacitor so it is less than the supply volts Maybe half or more or less depending. But a 240 volt one would be a good choice if it fits

  7. #7
    Join Date
    Feb 2013
    Location
    Laidley, SE Qld
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    1,042

    Default

    If the grinder is 60 plus years old it could have had more than one start capacitor swap. In a moment of need the 110V unit from a piece of surplus US electrics was tried, it worked, and was left in place.

    Buy a 240V or more 150mfd Ī 20% cap with the correct terminals from your local electricial components supplier or online.

  8. #8
    Join Date
    Dec 2007
    Location
    Adelaide
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    589

    Default

    I've got the exact same 8inch grinder, purchased new in the early 1980's.

  9. #9
    Join Date
    Oct 2004
    Location
    Southern Highlands NSW
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    I've often seen motor capacitors with voltage ratings much higher than 240V.
    An explanation is that AC voltage is stated as RMS, whereas the peak voltage of its sine wave is higher.
    1.5x line voltage is recommended.

  10. #10
    Join Date
    Apr 2021
    Location
    Adelaide
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    231

    Default

    Quote Originally Posted by nadroj View Post
    I've often seen motor capacitors with voltage ratings much higher than 240V.
    An explanation is that AC voltage is stated as RMS, whereas the peak voltage of its sine wave is higher.
    1.5x line voltage is recommended.
    People making replacements might err on the side of caution but I think the manufacturer would stick to the rating to contain costs.

    AC capacitors are rated for AC voltage assuming a sinusoidal waveform. If you dig deep enough, there are limitations relating to harmonic currents as well.

    One manufacturer (Cornell Dubilier) writes:

    AC capacitors are designed and 100% tested to withstand a potential difference equal to 1.75 X rated AC voltage between terminals and 2 X rated AC voltage plus 1,000 volts for one second between terminals and case.

    How this motor operated for so long with a 110V capacitor I don't know.

    Jack

  11. #11
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    Aug 2021
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    Sydney
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    Default

    Latecomer here but sometimes there is a mechanism to switch out the start cap once the motor is running (centrifugal/spring or otherwise) so maybe this could be a factor in it living for some time. I've only ever come across start and run capacitors rated at 450V in my single phase 240V motor maintenance exploits - which hasn't been a lot but about half a dozen of them, maybe more. Thing about AC and LC circuits is that even if the capacitor is in series with the motor windings to start, I think you would get probably close to full voltage swing across the capacitor briefly for each cycle (If I recall correctly, Current leads voltage in a capacitor, whereas it lags in an inductor) But I am a bit rusty on the theory, not about to hit the books today

  12. #12
    Join Date
    Aug 2008
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    near Rockhampton
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    Default

    Quote Originally Posted by wm460 View Post
    The yellow cap covers the terminals.

    The grinder is probably older than me, it was in dadís shed as long as I can remember.
    I thought 70's 80's. Have an identical one here and it is an early 80's model. Taiwanese I suspect.
    Gold, the colour of choice for the discerning person.

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