Table saw motor resistance?

[hunter27]

Several years ago I bought a used table saw and after using it for a month it started blowing 13a plug fuses a couple of seconds after switching on, If I spun the blade before switching on it worked fine. I replaced the only capacitor I can see on the motor with a 450v 30MF capacitor.
Initially this didn't seem to cure the problem but then it settled down and has worked faultlessly for 3 years or so.
Today it ran perfectly this morning then later in the day started tripping the ring main breaker (not the RCD) it's not blowing the 13A plug fuse at all and there is no delay between switching on and it tripping Using what little electrical knowledge I have I disconnected the motor and checked the motors resistance between L and N. I am only getting around 2ohm I think this is way to low , I disconnected the motors capacitor and still got the same 2ohm. Using an online calc I thing this motor at 2.2KW should be around 19ohm resistance, is that what you would expect? The motor is and has always been bone dry so it's not that. Is it worth trying a new capacitor or is that a waste of money at this stage?





Thanks Pete

 

[Red'n'Black]

Does the spin-then-switch-on trick still work?

A couple of ohms resistance for this size of motor is normal, it tells you very little about how much current it will draw once up to speed.

Here's a quick check for the capacitor, before you spend any money on a new one:
Remove the cap from the machine and connect a 100W tungsten light bulb in series with it, if you can still find one
Connect this combo to the mains.
If the bulb doesn't light at all, the cap has failed open circuit.
If it lights at about 2/3 normal brightness, the cap is probably ok.
If it lights at full brightness, the cap has failed short circuit.

 

[hunter27]

Does the spin-then-switch-on trick still work?

A couple of ohms resistance for this size of motor is normal, it tells you very little about how much current it will draw once up to speed.

Here's a quick check for the capacitor, before you spend any money on a new one:
Remove the cap from the machine and connect a 100W tungsten light bulb in series with it, if you can still find one
Connect this combo to the mains.
If the bulb doesn't light at all, the cap has failed open circuit.
If it lights at about 2/3 normal brightness, the cap is probably ok.
If it lights at full brightness, the cap has failed short circuit.

It doesn't spin at all now even though I can spin the motors shaft easily by hand. I will try the cap and bulb idea now and report back.
Thanks.

 

[Guineafowl]

^ +1 resistance is not the same as impedance, which is what AC ‘sees’.

Another trick, if your multimeter doesn’t do capacitance, is to take the cap out of circuit, discharge it by shorting the leads, then measure its resistance one way, then the other. You should see a low reading, climbing up to infinity in both directions. Set meter to highest resistance range.

Open up the motor terminal box and take a pic, so we can work out a way of measuring the start winding resistance. Should be higher, if not it’s probably shorted and may have taken out the cap too.

 

[hunter27]

Tried the bulb trick and it lights up but difficult to see if its dimmer with the cap in series.
Will try guineafowl suggestion with the cap and get a pic.
Thanks

 

[Red'n'Black]

Tried the bulb trick and it lights up but difficult to see if its dimmer with the cap in series.
Will try guineafowl suggestion with the cap and get a pic.
Thanks

Another quick test, if you have a meter that reads AC volts, is measure the voltage across the cap during the bulb test. I would expect somewhere between 30 and 80 V. If it measures zero, or only a few volts, it's failed short-circuit.

 

[hunter27]

I tried the cap with the meter both ways, it was OL and stayed at OL and didn't rise. The capacitance I read was 29.8uf which I think it means it's okay
Wires connected
Z2 and U2 to live on top.
Z1to cap in middle then cap to neutral.
U1 to neutral on bottom.

U1 to U2 = 2.2ohm
Z1 to Z2 ,= 5.6ohm
No connection between Z and U.
Hope this makes sense.
Thanks.

 

[Red'n'Black]

OK, another idea : What's the rating of the circuit breaker that's tripping? I have a 2.2kW motor on my compressor and that used to blow 13A fuses on startup until I upgraded it to its own 16A supply with a type C breaker.

Can you swap in a different MCB, to see if it's got over-sensitive with age?

 

[hunter27]

I have removed the motor and clamped it to the bench. Connected up with its existing lead directly to a socket and it is starting fine (without the 12" blade fitted) it's drawing just over 8amp? It works in its usual ring socket 32a breaker and on a 40a breaker spare. Whether the blade makes a difference I don't know. I will blow it out with the airline and put it back together testing it at every step.
Fingers crossed.

 

[Guineafowl]

Without further testing, looks like the motor windings are roughly as expected. Seems to spin freely so not a seizing/overgreased bearing.

Could be the MCB as mentioned above - the extra inertia of the blade might be tipping the balance.

32A type C might be the answer, but ideally you’d check the loop impedance is low enough to install one.

 

[hunter27]

Well it back up and running again. I have to admit on initial testing and getting a sub 3ohm resistance across the motor I mistakenly jumped to the conclusion 3ohm was way to low, almost a short circuit and the motor was the cause of the problem. With the motor running through a lead and plug it works fine, so I eventually checked over the other wiring on the saw and although can't find a fault that's started causing it to trip. A couple of years ago I asked for advice on here about fitting an electrical brake for this motor to stop the blade spinning for almost 2 minutes after use, it was suggested feeding DC current to the motor would work so I connected a 3 way switch up with position 1 running the blade and position 3 feeding a laptop charger outputting 18vDC to the motor. It has worked very well since then until now and that seems to be causing the trouble. I have now completely removed the DC supply and its working well without it. I think a 2 pole changeover switch may do the job though.
Thanks for all the advice I am a bit wiser re checking capacitors and would never have thought 3ohm resistance would work.
Thanks again.
Pete

 

[Guineafowl]

Ah.

The old ‘homemade DC injection brake’ chestnut. Should’ve spotted that…

Glad to hear you’re up and running