Discharging and testing 2 large capacitors  - Page 1
Forum section: electronics / beginners

author: Carsey
author_rank: Newbie
timestamp: September 26, 2018, 06:27:40 pm
content: HiI have 2 large capacitors which I need to remove and test from a wood working machine (Electra beckum hc260 motor) which is blowing plug fuses when we try to power it on.I have just bought a amecal st-9927t meter in hope of measuring the capacitance to hopefully identify a issue with the start and running motor capacitors.Does anyone have any advice how to safely remove and test the capacitors to avoid any risk of shock? They are pretty big in physical size and look like they could deliver a bit more than a tickle.I’ve attached 2 pics of them below with their specs on.Any advice would be appreciated and/or any clues why our plug fuses 13A keep popping when we try to run the machine motor.ThanksChris

author: SeanB
author_rank: Super Contributor
timestamp: September 26, 2018, 07:30:27 pm
content: Simplest check is to use the capacitance range on the meter and measure them. Short the 2 leads on the capacitor forst to remove any charge, but as you have them off anyway there probably is none. Reading should be over 45uF for them to be functional, and the simplest is to replace them, motor run capacitors of 50uF are a common value, and you get them with wire leads like that very easily from most large electrical wholesalers as well, or from any industrial motor supplier. Them going low value will result in the motor not starting, and blowing fuses, though first check, with the power disconnected, that the motor does turn freely. if the motor start capacitor is open the motor will try to start, and as it does not turn fast enough to disconnect the start switch it blows the fuse. Same if the start contacts are sticking for some reason as well.

author: Carsey
author_rank: Newbie
timestamp: September 26, 2018, 07:48:10 pm
content: Thanks for your reply.Is shorting the wires together on the end ok or best done with a insulated screwdriver?The motor does turn freely as this was one of the first things I checked when it failed to start. Can can actually hear it hum for a split second then the fuse goes.Is it just a case of checking both the capacitors once they are off the machine?I did read on another forum about there being contacts for the start switches but the motor seems a sealed unit with just a fan on the end so cant see anywhere can check any bushes that could be stuck or sticking.

author: SeanB
author_rank: Super Contributor
timestamp: September 26, 2018, 08:36:39 pm
content: Just short them together with power unplugged, it will be fine here. Hums then pops the fuse most likely the capacitors are either low or open circuit. Does the motor turn over at all in that brief period.

author: Carsey
author_rank: Newbie
timestamp: September 26, 2018, 08:52:56 pm
content: It turns over about a partial turn but no where near a full rotation. 1/32th of a turn at most!

author: Carsey
author_rank: Newbie
timestamp: September 27, 2018, 04:04:23 pm
content: Meter turned up today and got checking the 2 caps.The smaller 37.5uF one will read 37.2uF after a few seconds, which is well within the spec of +-5%.The larger 75uF one will not read anything then after about a minute, the meter will read OL. I checked with the ohms and its just sat at 6.9-7.2 ohms steady. Also tried the 9v battery trick across them both and the smaller one showed a voltage reading and the larger one just didnt store any energy.Therefore im thinking the larger one is dead.I believe the larger one is the actual motor run capacitor and the smaller the start capacitor.The faulty capacitor is the one pictured below.So had a look on ebay and found one in the exact same length and width, however it is 5uF higher capacity.https://www.ebay.co.uk/itm/CAPACITOR-START-RUN-MOTOR-BOSCH-NEFF-AEG-5UF-6UF-7UF-8UF-12UF-16UF-20UF-WIRED/273440654818?hash=item3faa5545e2:m:mStwqr3aV0EXE0g6Su1FeTANot sure if you can run a higher uF capacitor on a motor or not or whether it needs to be the exact same.Alternatively, there is this 75uF one:https://www.ebay.co.uk/itm/Electric-motor-Run-Capacitor-capacitors-2uf-to-100uf-mfd-400-450v-with-lead-new/202349738750?hash=item2f1cfbfafe:m:m-JBUxm2wyl1Ih_ldUFBu_Qbut the dimensions are a couple mm smaller than the one we have.

author: james_s
author_rank: Super Contributor
timestamp: September 27, 2018, 04:12:33 pm
content: You need to use the same value, although a higher voltage rating is ok if the capacitor will physically fit.

author: Carsey
author_rank: Newbie
timestamp: September 27, 2018, 04:22:25 pm
content: Thought so, so a 75uF rated one it is that will do 250-450vSecond option be ok then?

author: SeanB
author_rank: Super Contributor
timestamp: September 27, 2018, 04:31:02 pm
content: Larger one is the start capacitor, and yes a sightly larger value and voltage one will work. Just choose the 40uF 400V one and the 75uF 400V ones and they will work. Physical size is not a worry provided it will fit in the place on the machine, and these will fit.

author: Carsey
author_rank: Newbie
timestamp: September 27, 2018, 04:35:59 pm
content: Awesome, thanks.Is there any sort of brand that should be gone for or will the 75uF one in the second link be good enough?Machine/Motor is used very rarely.

author: Gyro
author_rank: Super Contributor
timestamp: September 27, 2018, 05:49:52 pm
content: Those two values are a little unusual 40, 70 and 80uF are more common.If you want branded ones, then CPC and Farnell sell Ducati, which is a decent brand. I checked RS but they have nothing cable terminated.As often happens, CPC are cheaper than Farnell, even though they are the same company...https://cpc.farnell.com/w/search?st=motor+capacitor&capacitance=40uf|70uf|80uf

author: Carsey
author_rank: Newbie
timestamp: September 27, 2018, 05:51:53 pm
content: a 70uF will work ok?The 70uf is a tiny bit smaller in diameter, but I should be able to tweak the bracket that holds it on no problem

author: Gyro
author_rank: Super Contributor
timestamp: September 27, 2018, 06:29:29 pm
content: Others will have a view, but the 80uF would give a little leeway for capacitance drop over time. I'm really not sure if a bit under or bit over is better in this case (start cap).40uF instead of 37.5uF for the run cap sounds less but of course is the same in proportion.Motor caps inevitably loose some capacitance over time due to self healing.

author: james_s
author_rank: Super Contributor
timestamp: September 28, 2018, 12:37:35 am
content: Anything within +/-20% really ought to be fine, these are not precision components. You just don't want something drastically different. Most of the motors ive dealt with have the capacitors inside camel hump covers on the motor but if they mount externally you have a lot more leeway.