motor start capacitor

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 ???

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..

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.

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

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.

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

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.

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?

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

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.