Chester 16V milling machine

[John MillisParticipant @johnmillis57685]

Thanks John I accept my limitations which I why I am using the forum to pick the knowledge of people more qualified than myself I will try reconnecting the potentiometer to the main board and see what happens when I have some daylight in the workshop as it takes out the lights as well. Is it possible to test the potentiometer before reconnecting. With the potentiometer out of the circuit does it takes the motor and speed sensor out as well? John

[John RuddParticipant @johnrudd16576]

I think it unlikely that the pot is causing the problem….Unless there is leakage current from the circuitry to earth via its body, which would need to be in contact with a metal panel that is earthed…which is why I suggested a replacement..

[Martin WParticipant @martinw]

Hi John M

I take it that when you applied power to the mill with the potentiometer disconnected the motor did not spin up. If the motor was stationary then this could mask a problem with the motor/motor feed or possibly even the drive circuitry. That said it is worth looking at the potentiometer its cabling and the area where it connects to the motor control board.

Without readily being able to conduct electrical tests then it will initially be visual only. Check the pot connections and as much of the interior of the pot as can be seen for any sign of contamination. Then examine the connecting cable for signs of damage and finally thoroughly check areas around the connections on the motor control board for any spurious material. The main culprit is likely to be very fine metal slivers or dust that is creating an intermittent low current short circuit, I take it the the main fuse doesn't blow when the RCD trips out.

If nothing is apparent then do as John R suggests reconnecting the pot and see if the fault is still there, should it run fine then there is a fair chance that you have disturbed some foreign matter. If the fault reoccurs it may well be worth while replacing the pot, how's your soldering?, as this will be the cheapest and easiest first point to start with.

The reason I asked about the motor running is that if the is some earth leakage on the motor or its cabling is that if it is not powered then there will be no leakage to earth and the fault will not show. A quick question, how far did you go when inspecting the motor for carbon brush dust?

Martin W

edit; PM sent

Edited By Martin W on 28/11/2018 19:16:43

[Martin WParticipant @martinw]

Hi John

I have a few more thoughts about the problem. The fact that you could control the brilliance of a light bulb connected to the output of the power board indicates that the pot is, in itself, working correctly. If there was a break in the track of the pot or problems with the slider then you would have seen the bulb either going from off to full brilliance at some point or if bulb brilliance changed erratically then this could be a dirty/worn track or poor contact with the wiper.

Another check that you could do is to take the pot off the front panel and with it plugged into the control board set the wiper to about a quarter speed setting. Then after ensuring that the pot is fully insulated/clear off any link to earth or other contact switch the mill on and see what happens if you run it. If the mill runs fine at about the right speed then there is a strong possibility that the pot has some form of leakage to earth via the front panel.

The last point to check is the speed sensor itself. This is located under the cover at the top of the spindle, the spindle has a segmented disc and the sensor is an optical, infra red I think, unit that senses the gaps and vanes on this disc. Again make sure that the board etc. is clean and that there is no swarf etc. that could cause problems. If possible it would be worth looking at the connecting cable, where you can get to it, from the speed sensor to the circuit board.

Cheers

Martin W

[John MillisParticipant @johnmillis57685]

JohnHi Martin. Just come in from the workshop I thought I had found a problem a cable joint going into the plug that connects the speed sensor to the main pcb looked like it was coming out so I reinserted into the plug put everything back together switched on the mill started, the pot controlled the the speed the Rev counter showed reasonable steady speeds, I thought I had cracked it switched off had a bit of a tidy up switch back and it tripped I’m at a loss the cables where I can see them look good. Why would it run for several minutes and then trip when switched back on?

[John RuddParticipant @johnrudd16576]

John,

I think you need to go back to the beginning..

Disconnect everthing that has AC mains applied to it…Then start with the speed controller/potentiometer and motor. With them all connected up, run the mill, turn it off, run it again…. Repeat and rinse until you are happy that it isnt tripping the rcd. Then, one by one reconnect the other mains powered devices. Do this one at a time, and repeat the test cycle again….Then reconnect any remaining items…..Make notes each time you reconnect something, then you can backtrack…if the trip re-occurs.

Might sound long winded, but it is a logical approach…

You didnt mention the condition of the brushes on the motor or any carbon build up …..

Edited By John Rudd on 29/11/2018 16:14:10

[John MillisParticipant @johnmillis57685]

Thanks John one of the first things I checked were the brushes which were in good condition and not worn I cleaned the com with a bit petrol damp rag on a lolly stick and rotated the shaft it was slightly discoloured when removed so I don’t think the brushes and com are the problem.

John

[Les Jones 1Participant @lesjones1]

Hi John (M),
Can you draw a diagram of how the mains wiring is routed showing things like NVR switches and connection blocks. Could you also post a picure of your V16 (I have looked on the web but there seem to be a number of variants.) The random readings suggest that it might be a bad connection arcing but only when a reasonable current was being drawn. (Like when the motor was running or the pamp connected as a load.) This is not something that you would normally expect to cause an RCD to trip as there is no leakage to earth but I have seen it happen with switch contacts not closing and opening cleanly.

Les

[Martin WParticipant @martinw]

The fact that it tripped when the bulb was used as a load tends to preclude a problem with the motor and associated wiring. As Les says if the contacts on the mains switch are not making near enough simultaneously then this can cause the RCD to trip, this will be exacerbated by having a mains filter in the circuit. Another possibility is that there is a capacitor intermittently breaking down or going open circuit within the filter unit. This will cause random failures as the earth currents will not be balanced out. This could be tested by taken the input filter out of circuit and running the mill.

As John R says it's a good idea start at the beginning and slowly connect the bits of the circuit in untilthe fault manifests itself.

Good luck

Martin W

[John HaineParticipant @johnhaine32865]

Since its the w/s rcd that's tripping and this started recently, was there anything else that was connected to mains in the shop just before the fault manifested? There could be a small earth leakage from the mill that takes the rcd over the edge when added to an extra leakage from something else.

[John MillisParticipant @johnmillis57685]

Hi John, I had considered that possibility and ran an extension lead from the house to the workshop with a plug-in RCD and it tripped that. I had the problem some 12 months ago I checked the brushes and com had a look in the control panel and the problem seemed to go away so saying the mill has not had a lot of use since and then only for short periods and not a lot of stop start cycles it’s only since the nights have started drawing in that I’ve got back in to the workshop and started using it again that the fault came back with a vengeance..

[John RuddParticipant @johnrudd16576]

Is your workshop dry? I'm thinking maybe a bit of damp has gotten into the motor….

Do you have any electrician friends with a megger? Or a local motor rewinders

Checking the IR of the armature would rule out a potential motor issue….

[Author]

Posts