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Wilf Baker's variable feed for mill
Arcing to the table !
| fred bateman | 12/10/2009 18:59:05 |
| 10 forum posts | Wilf's article has prompted me to drag the old windscreen wiper out from under the bench and proceed with it , I bought a 'Como' mini drill speed control from Maplins 'cos I couldn't make head or tail of the circuit diagram, sorry Wilf. At first I connected it with a 12v battery and it ran fine on the bench, just like the man said. Then I hooked it up to an old laptop supply unit, and it ran fine again. I knocked up the mounting bits for the mill, then the trouble started. I fitted it up, switched on the power, and instead of doing the chunter-chunter it had done on the bench, it just went tick-tick-tick and went round very slowly indeed and wouldn't adjust at all. I though perhaps it was jamming, so I tried woobling it about to see if there was a looser position - I can now hear arcing - on an electrical kind ? So I pulled it away and offered it up to the table and it is sparking. Now, obviously the mill itself is earthed (but not switched on at this stage) , the power supply to the speed control is earthed, but insulated from the mill. the wiper motor not earthed in the coventional sense, but is 'earthed' in an automobile sense, in that the case itself is alternativly positive or negative depending which way it's switched. What the hell is sparking ? Has anybody got any thoughts, before I screw up another Maplin speed control. The mill does have a Newton Tesla 3 phase motor and control but that wasn't even on - brilliant piece of kit I will say. I would have put this thread under 'Construction Articles 2009' but I can't find that one on the list. Fred Bateman |
| David Haythornthwaite | 12/10/2009 20:31:38 |
 45 forum posts 7 photos | Hi Fred, Just a thought - Are you sure that one side of the output from the mini speed controller is not grounded to the case and therefore earthed? if this is so then you may have a dead short causing the overload protection to activate. Edited By David Haythornthwaite on 12/10/2009 20:32:21 |
| fred bateman | 13/10/2009 12:01:15 |
| 10 forum posts | Hello David, thanks for the reply. I have mentioned above that, ,being an automobile part - the wiper motor - is indeed all 'earthed' ; in that as with most car hardware, the path back to the battery is via the casing. In fact the motor won't run without one lead going to the casing. I feel there must be something obvious I'm not seeing. |
| russell | 14/10/2009 00:23:49 |
| 142 forum posts | hi fred
it sounds to me like the one side of the output of the speed drill controller is in fact earthed, probably the negative side. How are you reversing the motor? (not having seen the original article). does it work in reverse?
you may need to either insulate the entire motor from everything, or maybe open it up and bring the 'frame' connection out to a separate terminal.
it might also be possible to modify the speed controller so the output is floating. - re-reading your post, are you still using the laptop power supply?
could you post a link to the controller you are using?
regards
russell |
| fred bateman | 14/10/2009 18:42:14 |
| 10 forum posts | Hi Russel, thanks for the reply. I can't get onto the Maplin website at the mo, but the controller is MFA/Como 919d2p, can't see that it's earthed - there are 2 wires in (power + and -) and 2 out (to motor). Although that pair of wires out are going direct to the casing of the motor alternately, depending on which way it's switched. I have had another poke around today and I have found that it is the power supply - tho old laptop unit - which I used 'cos I thought it would be well smoothed - which is causing the motor to just go tick-tick and also doing the sparking. I found the mains earth to this is going internally to the heat sink. Don't know what that implies. I'd have made up the power supply/control unit shown in Wilf's original piece, but I have no idea how to configure the Lego board or whatever it's called, to get the circuit to work. Looks like I must get another control panel and start again with a different power supply. |
| V8Eng | 14/10/2009 19:04:43 |
| 1826 forum posts 1 photos | Hello.
Please find a local Electrician to help with this, before you possibly do some real damage to yourself or the equipment.
|
| fred bateman | 14/10/2009 20:23:57 |
| 10 forum posts | Russell, the article is in MEW 152 page 24 I will peek inside the motor and see if it's possible to isolate the windings, sounds like a good idea anyway - the old Mig may only run on 12 volts but it makes a flippin' big spark ! |
| John Haine | 15/10/2009 09:16:25 |
| 5563 forum posts 322 photos | I have one of the speed controls awaiting inclusion in a powered
down-feed for my mill. Tracing the circuit, it works as follows. The
positive power input terminal goes through a diode (for
reverse-connection protection) and feeds one side of the motor (marked
"+" on the output terminals). The "-" motor connection goes direct to
the drain of a power MOSFET (effectively a switch). The source of the
power MOSFET (i.e. the other side of the switch) goes to the negative
power terminal. The MOSFET is gated by a little timer circuit that
switches it on and off at around 1200 Hz with variable pulse-width to
control the motor speed. Thus, the motor output is floating. Running
off a PC power supply, the negative side of the +12v output is almost
certainly earthed (i.e. connected to mains earth) as this is normal
practice. One side of the motor will also be earthed because it is
connected to the motor case and through that to the machine (assuming
that this is earthed for safety!). So connecting the motor to the
speed controller will either connect the motor winding to the + output
and the - output to ground; or short the + output direct to ground.
(That's assuming there is a reversing switch between the controller and
motor.) In the first case the PSU will drive
the motor through the diode; but the MOSFET is just shorted out (which
won't damage it). However the 12V ouput from a PC PSU doesn't have a
very high current capability so I wouldn't be surprised if the start
current of the motor just causes it to trip; then perhaps reset; then
trip again. Result would be intermittent and slow motor movement, and
the speed controller will have no effect since its controlling MOSFET
is shorted out. In the second case the PSU output will just be shorted
via the diode - depending on the current rating of the latter it may or
may not have blown. So all this may not have damaged the speed
controller if you're lucky. Making the
arrangement work needs a motor with a floating winding, which is true
of most small DC motors. You might be able to modify a wiper motor,
depending on how it's made. Make sure it's a permanent magnet type.
Alternatively there are loads of PM motors with gearboxes around on the
web which ought to have enough (geared) torque to drive a mill table
(though not necessarily very fast). Summary: speed control may be blown; motor with floating winding needed; PC PSU may not have enough current for wiper motor. Hope this is helpful, |
| John Haine | 15/10/2009 09:25:48 |
| 5563 forum posts 322 photos | Just to add a further comment - I've looked at Wilf's article and from the schematic it's clear that he expects a floating motor as well since the speed control method is identical to the MFA device - though with his circuit it could work with all the controller floating on the isolation of the mains transformer windings. |
| fred bateman | 15/10/2009 23:22:18 |
| 10 forum posts | Messy, Hi, You worry too much. If I could afford an electrician I could afford one of Warco's table drives ! At my age, 415 volts is not a bad way to go ! I can remember when the mains was DC. Thanks for your concern. |
| John Haine | 16/10/2009 09:41:50 |
| 5563 forum posts 322 photos | ...and after all, all this is working off 12 volts! |
| Ian S C | 16/10/2009 12:03:10 |
 7468 forum posts 230 photos | I had problems with my power feed on my mill,I use a old Lucas wiper motor,and a power supply with a transformer from a Zerox copier(its BIG),every thing was liveso I checked the transformer,found it was single wound(not isolated),there was room on top of the existing windings so I wound on wire to give about 18V,no more problem.No electronics used,3 speed via resistors,took about 1hr to build,been going over 10yrs. |
| fred bateman | 16/10/2009 18:27:42 |
| 10 forum posts | Thanks for that very helpful comment John, I am puzzled now because I posted a reply to you last night at about 10pm and it isn't here. The muse is gone and I can't remember what I said ! I thought I pushed the 'add posting' button. What would be really nice would be if Wilf could come up with a diagram including the method for dividing up that Vero board to enable one to make up his circuit. I was reading on the web the other day some guys were talking about a big diode across the output to stop 'spikes' from damaging the delicate components, is that what your diode is doing also ? Wilf makes no mention of stripping the motor to insulate it, perhaps his 'man wot did the electrics' realised that it needed to be floated and just did it ? fred |
| fred bateman | 16/10/2009 18:43:41 |
| 10 forum posts | Ian hi, your method is interesting, low tech so you can see what's happening. I'll make that my 'fall back' position I think. We used to have sewing machine motors many years ago that had a small winding attached to the main winding to bring out a 6volt feed for a light. Trouble was it did it by picking up the neutral going in and generating the 6v as a 'difference' between the neutral (zero) plus 6v from the winding. Trouble turned out to be, if the live and neutral going in to the motor were reversed the difference became between 240 and 246 volts. One day a young lady managed to touch one of these terminals in the light unit and got a very nasty surprise. I am pleased to say that Janet did survive |
| John Haine | 16/10/2009 19:42:28 |
| 5563 forum posts 322 photos | Fred, I know what you mean - couple of times I've thought I made a posting but found it vanished - frustrating. The big diode in the MFA circuit is to protect against connecting the supply up backwards as it is in series with the main supply. There is also another diode across the motor connections to avoid the spikes, no need for another one. You can cut Veroboard tracks using an ordinary drill, probably 5 or 6 mm should do the job, hold it and rotate in your fingers with the point located in the hole where you want the cut until all the track has gone around the hole - make sure it has all gone, sometimes there are very narrow bits at the side to get you very confused! (I'm betraying my misspent youth as an electronics student here.) Actually there is very little difference between the MFA controller and Wilf's circuit, except for the series diode which is the main thing limiting the MFA's current capability. You could take it out (it's the fat black cylinder with a silver band next to the speed control potentiometer, not to be confused with the thin black cylinder next door but at right angles to it) and replace it with a wire link with no problem as long as you were careful about connecting the supply up the right way. The current capability according to the specs of the MOSFET would then be 10 amps or more. However I'd try it under load first - if the diode goes you can always replace it with some wire. I don't really know why they used such a wimpy diode in the first place. If you don't want to fiddle round with the motor internal wiring the way to float the speed control circuit is as follows. First, you'll have to avoid using the PC power supply because the negative side is inevitably earthed and make your own. (Or use a car battery.) Next you'll need a mains transformer and rectifier as in Wilf's circuit, together with the 10000 uF capacitor, the 180 ohm 1W resistor and the 12V zener diode. Basically you wire up the circuit as shown in his diagram but everything to the left of the zener, and the BUZ11 and 10A diode, is replaced by the MFA board. Connect the reversing switch to the "-" output on the MFA board instead of pin 2 of the BUZ11. Now the important bit. Normally you would connect the negative side of the electronics - i.e. the "-" side of the bridge rectifier - to ground. However to avoid the problems that you have had you SHOULD NOT do this but leave it floating. Connect the outputs of the reversing switch to the motor - i.e. one to the wire from the motor and the other to the motor case (and hence to earth). What will now happen is that the secondary winding of the transformer will be floating at around plus or minus 12 volts relative to earth depending on the reversing switch setting, but that shouldn't matter because the winding is insulated from the transformer core and the mains. It isn't wonderful electrical engineering but it'll work! You will have to be careful not to accidentally earth the negative side of the circuit when you assemble everything into the box. I hope these thoughts are helpful. Cheers, John. |
| V8Eng | 18/10/2009 17:00:46 |
| 1826 forum posts 1 photos | Hi Fred & John.
My comment about finding an Electrician was prompted by Fred's first post making me think he did not have enough basic knowledeg to takle this.
Burnt out electronic components can cost quite a bit if mistakes keep getting made as well.
As to it only being twelve volts, that does not automatically make everything safe, even lower voltages can cause fire or damage etc.
Sparks can set fire to solvents or gases.
I too was born when DC was supplied from local power stations in London. People used to do things like charging batteries indoors with just a lamp in series from the mains, that was pretty dangerous with 200+ volts at the wire ends if the battery got disconnected, to say nothing of the explosive potential of a gassing battery!
Anyway thats enough of this, I hope you get the variable drive working. Edited By MessyEng on 18/10/2009 17:01:42 |
| V8Eng | 18/10/2009 20:09:20 |
| 1826 forum posts 1 photos | OOPS.
Just noticed a number of errors in my post, and now the edit feature seems to have vanished! |
| Ian S C | 19/10/2009 07:16:17 |
7468 forum posts 230 photos | Suppose Janet didn't need a perm for a while,246v enough to curl anyones hair.I find the simple way is ok for my use,others may like something a bit more sophisticated,that gives better,more accurate control.IAN S C |
| fred bateman | 21/10/2009 12:00:35 |
| 10 forum posts | I have now been inside the motor and removed the connection of the negative wire from the case and it now has no apparent interface with the outside world. I guess it is now properly connected to 'earth' when attached to the table. V9Eng ? I thought you were 'Messy?' Is that your alta ego ? Thank you, I appreciate your concern. You're right about basic knowledge, it's the basic knowledge that causes most of the trouble sure enough. |
| V8Eng | 09/11/2009 20:44:10 |
| 1826 forum posts 1 photos | Hi Fred
Sorry not to have replied to your post earlier, but I have only jut picked up on this thread again.
Yes I did initially use Messy but did not like it very much, as I'd only done a couple of posts thought best to change quickly, I chose V8 as I like the sound of those engines.
My post about electrics was not intended to be some sort of 'Elf n Safety' thing, just a general concern for your good self.
Edited By V8Eng on 09/11/2009 20:44:59 |
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All Forum Topics > Model Engineers' Workshop. > Wilf Baker's variable feed for mill