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How do I check the capacitors on my Warco Mill?

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Bob Rodgerson19/10/2017 09:49:32
585 forum posts
166 photos

I have a Warco Super Major mill that I have had since new in 2008. \

It has been abused somewhat and over the last couple of years has seen little use because I tend to use the CNC mill for most things these days,.However, I like to keep it in reserve in case any urgent jobs turn up and the CNC mill has ongoing work or if my Son In Law needs to mill something while I continue with other stuff.

The last time I used it as soon as I switched on the power to the spine the fuse blew at the consumer box in the workshop. I tried twice and each time it blew a 16 amp fuse.

I suspect that the start capacitor may be the culprit but being a total Numpty when it comes to electrics How do I go safely about testing to see if this is the case?

I have a multi meter so perhaps this will come in handy.

I am a bit wary of capacitors because I do know you can get a nasty jolt off them if they happen to be charged when you touch the terminals.

Bob Rodgerson19/10/2017 09:50:57
585 forum posts
166 photos

Predictive text has got me again for spine read spindle.

Emgee19/10/2017 09:59:26
1401 forum posts
212 photos

Hi Bob

Sounds like a short circuit, as suspected could be a capacitor failing to earth.
You really need a 500v insulation tester (Megger) to check the motor circuit.


martin perman19/10/2017 10:15:17
1779 forum posts
75 photos


You can buy a multimeter which can test capacitors and its easy to use.

Martin P

SillyOldDuffer19/10/2017 10:44:01
5338 forum posts
1090 photos

Hi Bob,

The easiest thing to do is to simply replace the start capacitor - they aren't expensive.

However, with an analogue multimeter (not a digital) you can get an indication of capacitor health by:

  • Disconnect the machine and leave it overnight to reduce the risk of a jolt. (Even a good capacitor will discharge slowly)
  • Use the metal end of an insulated screwdriver to short out the capacitor terminals. If you get a spark (or bang!) the capacitor is probably OK.
  • Disconnect the capacitor from the circuit.
  • Set the multimeter to an ohms range and, whilst watching the needle apply the black lead to the positive terminal and the red to negative. You should see the needle kick and then return to zero, If the meter doesn't return to zero,  the capacitor's probably faulty. (A digital meter can be used to measure resistance. It won't show the kick, but the resistance should be close to zero.)

Some digital meters can measure capacitance directly. The capacitor must be disconnected.

A multimeter typically only uses 1.5V or 9V to test resistance. You can use them to detect many faults, but they won't see insulation failures that only appear at higher voltages. That's why the professionals use Meggers as recommended by Emgee. However, never apply a Megger to delicate electronics: disconnect any control boards / VFDs etc before using a Megger.



Edited By SillyOldDuffer on 19/10/2017 10:45:04

martin perman19/10/2017 10:56:45
1779 forum posts
75 photos

I personally would never short a capacitor to discharge it because it could damage it or cause it to dramatically fail.

Martin P

Bob Rodgerson19/10/2017 13:07:43
585 forum posts
166 photos

I think I will price up a new capacitor and see if replacement works.

Ian S C19/10/2017 13:30:07
7468 forum posts
230 photos

Just check that the centrifugal switch is working, it may not be closing.

Ian S C

Neil Wyatt19/10/2017 14:17:40
17344 forum posts
690 photos
77 articles

A start capacitor may not have enough capacity to kick the needle of a multimeter.

If you carefully short it through a large resistor that should do the job without causing any drama.


Bob Rodgerson19/10/2017 14:35:35
585 forum posts
166 photos

ok I've just had the cover off the Motor JB and there are two capacitors housed in it, presumably one is a start and the other a run capacitor. Am I right in assuming the higher impedance capacitor is the start capacitor?

Brian Sweeting19/10/2017 15:17:40
404 forum posts
1 photos
Posted by Neil Wyatt on 19/10/2017 14:17:40:

A start capacitor may not have enough capacity to kick the needle of a multimeter.

If you carefully short it through a large resistor that should do the job without causing any drama.



If you have a simple electric soldering iron then they are very good for discharging capacitors.

Bob Rodgerson23/10/2017 15:54:11
585 forum posts
166 photos

OK All, I bought two replacement capacitors the same rating as those fitted to my mill. I decided this afternoon to get the steps out and replace them. I climbed up and pulled both capacitors out of their housing to check the wiring and found that the 20 microfarad capacitor looks as though it could be the culprit for blowing fuses when attempting to start the mill. I noticed that the cap where the wires exit from it appeared to be not fully seated and there looked to be a pice missing around its edge. When I pulled at it a little the cap came away from the rest of the canister and it looks as though some of the contents may have melted and re-set at some time.

Now, before I go and risk another £250 worth of fuse I need to know how to wire in the new capacitor.

At present both capacitors have a singular wiring colour i.e. Black. but the new 20 Microfarad one has a Blue wire and a black one and the new150 microfarad one comes with a black wire and a Yellow wire.

Does it matter which wire goes where?

Any help would be greatly appreciated.

Currently it appears that the two capacitors have a common wire from the motor that they join to and this is coloured yellow so do I assume that the yellow wire on the 150uf joins the yellow wire from the motor? and which wire from the 20uf capacitor joins this common, blue or brown?

Bob Rodgerson23/10/2017 15:55:33
585 forum posts
166 photos

£250 for a fuse! read £2.50

SillyOldDuffer23/10/2017 16:16:09
5338 forum posts
1090 photos

Motor start capacitors should be non-polarised, i.e. doesn't matter which way round they're wired. Are there any other markings on the capacitor that might indicate otherwise?

The blown capacitor you found may have just worn out, or it could mean a switch stuck causing it to overheat. You might be wise to wait for a motor expert to comment - I'm just a dabbler!


Bob Rodgerson23/10/2017 17:03:37
585 forum posts
166 photos

Hi Dave,

thanks for responding. I thought that might be the case but wasn't altogether certain, however before I get out the soldering Iron I will wait and see what the general consensus is. If I don't get any further responses I will do the deed anyway and I will find out one way or the other.

John Haine23/10/2017 17:05:25
2898 forum posts
143 photos
Posted by SillyOldDuffer on 19/10/2017 10:44:01:

  • ...... (A digital meter can be used to measure resistance. It won't show the kick, but the resistance should be close to zero.)

More like infinity if the capacitor is OK! Also, not a good idea to apply a megger to a capacitor as the high test voltage may exceed the cap breakdown volts.

But it sounds like Bob has found the fault. Motor caps should not be polarised, they need to work on AC after all!

SillyOldDuffer23/10/2017 17:28:11
5338 forum posts
1090 photos
Posted by John Haine on 23/10/2017 17:05:25:
Posted by SillyOldDuffer on 19/10/2017 10:44:01:

  • ...... (A digital meter can be used to measure resistance. It won't show the kick, but the resistance should be close to zero.)

More like infinity if the capacitor is OK!


Doh! Got me again John.

On the bright side I expect to get First Prize in Neil's Most Wrong Post of 2017 competition. Surely inverting zero and infinity is the biggest mistake it's possible to make!



john fletcher 123/10/2017 18:09:36
572 forum posts

Sounds as if you have a capacitor start and run motor. if so, both capacitors are in circuit when the motor is starting up, two in parallel. Once the motor is up to speed the centrifugal switch opens (it is mounted on the rotor) disconnecting one of the capacitors from the mains. the motor continues to run one capacitor only. The run capacitor is a quality capacitor as it is in constant use and it doesn't matter which way it is connected polarity wise. The actual value can differ manufacture to manufacture. The start capacitor is a special AC electrolytic with a short time rating and it is this one which often fails when the centrifugal switch sticks closed, it is normally a high value 70/ 120 again depending on who made the motor, it doesn't matter which way round the leads go. As the capacitors are working on the mains they too should be tested to comply with the wiring regulations, all electric components and wiring should be tested at twice normal working voltage, so a 500 volt DC insulation tester ie a Megger is suitable.There are also other makes of insulation testers which can be used. A capacitance meter ONLY tells you its capacitance. Should you need further help send me a PM. John

Bob Rodgerson23/10/2017 18:51:16
585 forum posts
166 photos

Hi John,

thanks for the input. it sets somewhere on the motor J.B. that it is a Capacitor start run motor. Judging from the way the two capacitors share one of the wires from the motor would suggest that they are connected in Parallel.

The replacement capacitor looks to be of high quality so hopefully it will work when I replace it .


Bob Rodgerson23/10/2017 21:14:10
585 forum posts
166 photos

Well That didn't work. As soon as I switched on the power the fuse blew yet again

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