PDA

View Full Version : motor start capacitor



Scott
26-12-2010, 09:55 AM
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
26-12-2010, 10:38 AM
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
26-12-2010, 02:03 PM
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
26-12-2010, 04:04 PM
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
26-12-2010, 07:19 PM
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
26-12-2010, 09:49 PM
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
26-12-2010, 10:03 PM
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
27-12-2010, 01:09 PM
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
27-12-2010, 10:21 PM
thanks for all the info guys got one off ebay no real shop round here lol

AdieR
28-12-2010, 03:41 AM
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.

Swarfing
28-12-2010, 09:27 AM
Just for interest i came across this great site with a wealth of information all about wiring motors, encoders calculations and a lot more. Just make sure you click through the 'next' buttons to see it all.

http://www.patchn.com/index.php?option=com_content&task=view&id=49&Itemid=74

oadamo
31-12-2010, 12:18 PM
if i had seen this thread i few weeks ago, i put about 4 brand new start caps in the bin, i had them when i was making my 3 phase convertor. theres a motor rewind place by me that sold me them for about 4 quid each ill have to get there number and post it up.

Neil
11-02-2012, 03:56 PM
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.

hello, i purchase 4 GE motors 5KCP39HNB476S 1725 rpm 1/3 hp 115/230 volts to my surprise no mention of size start capacitor is require, i look all over can't find any info as to what size capacitor it needs.... also because I need it for each applications 115v= ? 230v=? or it doesn't matters?????? any help would be greatful.....