motor start capacitor

[Scott]

Hi guys i need a new start capacitor for my lathe
it a 1hp motor 240v single phase 5.2 amp but i dont now what size to get as the old capacitor has no info on it worn off any ides guys ???

[Swarfing]

Scott

You can pick up start capacitors from Maplins, size is not a problem as long as it fits onto the motor and the ratings are correct. If they are open then pop along to any electrical wholesalers..

[irving2008]

I think when Scott said size he meant capacitance, which he can't read from the existing part!

Motor start capacitors are typically around 30uF per HP, my 1/2hp motor has a 12uF. The bigger the capacitor the larger the starting torque. For a lathe where there is little load at start, you might get away with a smaller one. Try 12uF and see... 440v rating.

[Scott]

thanks irving2008 your right i was after the correct capacitance / voltage i will try 12uf as there is very little starting torque required
new bearings smooth as a babys bum lol

thanks

[Swarfing]

LOL sorry guys i see the point. I had problems finding a cap to fit a shell on one of my motors until i tried a Maplins special. I followed the 12 - 15uf rule when i fitted mine as it was missing when i got the motor.

[Scott]

not to sound too stupid but whats the 12 - 15uf rule??

never had to mess with capacitors before first motor that i had to get one for lol

[irving2008]

I think whats being refered to is that most small fractional HP motors have a start cap of 12 - 15uF, so if in doubt try one... the worst that will happen is hasn't got enough start torque to run....This assumes we are talking about a start capacitor and not a start/run... can you confirm that your motor has a centrifugal switch that disconnects the start cap once its about 3/4 speed.

[Swarfing]

Scott Irving is right and not worth worrying to much about it. 30uf is roughly the norm for 1hp motors, it's more about what you can get as a lot of intermediate sizes are not available to keep manufacturing costs down. You can always go to the nearest highest value without much problem. A lot of small garage machinery uses between 1/2hp and 3/4hp motors (bench drills) so 12 -15 becomes a reasonable rule. If your motor requires a greater start torque to get going then a smaller cap will not start it and you would need the next one up. I have an old 3/4hp motor that runs quite happily on an 8uf cap (go figure that), it has very high spec bearings and not a lot of resistance to get started so only needs a small kick. This i suspect is the manufactures being a bit frugal with costs and opted for the lowest value they could get away with?

[Scott]

thanks for all the info guys got one off ebay no real shop round here lol

[AdieR]

Just one point worthy of note: single phase motors generally have one "start" capacitor to give it a kick on start up (where higher power demand is normal).

It won't generally be used to "run" the motor - motors which require capacitance on the "run" coil will often have a second, lower value capacitor for the "run" function, rather than use one capacitor for both functions.