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Old 26th Nov 2013, 2:18 pm   #1
sentinel040
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Default Tonum regulator

This may be a bit off the beaten track, so to speak, but a friend had just rebuilt his standby generator, which had suffered an engine failure (Lister JP4 with coolant in the sump). Now it has been discovered that the control panel, which worked last time I saw it in action (about 10 years ago) has a fault in Auto. In Hand the output can be varied but there was no movement on the carbon pile regulator. Now this beast is allegedly a former WW2 searchlight set, but being fitted with a 3 phase AC Vickers machine it seems more likely to have been a general purpose 27kVA set from this era.

Anyway, I have pulled the panel off, I have some experience of carbon pile regulator in the shape of the AVR's on some loco's, but obviously this beast is different in detail. The sampled AC comes onto the slate panel and one side goes to one leg of an ancient contact cooled bridge, the other side goes to a wirewound pot that sets output volts and thence to two large circular objects wired in parallel before going to the other AC terminal of the bridge. The bridge DC output goes to the electromagnet that decompresses the pile to reduce excitation. These mystery objects, which one would expect to be resistances of some form read open circuit and explains the lack of volts on the bridge and hence to the regulator, but they are devoid of markings. At this point the obvious comment is HELP!

Regards

Ian
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Old 28th Nov 2013, 8:29 pm   #2
Lucien Nunes
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Default Re: Tonum regulator

I may be able to help, as I have a similar or identical regulator plus some bumf on adjusting it. I have had a preliminary look but it does not seem to state the resistance of the precision resistors that are open circuit in your unit. It gives the regulator type as VS/75/18M and the coil current as 0.265A.

Clearly the genset is not a searchlight set, they were all DC. There were half a dozen models of Lister JP4 powered AC sets but if your friend's plant has a Metvick alternator I suspect it might be an Air Ministry 20kVA Mark II, as most Lister sets had Mawdsley's alts due to the connections between the two firms, as well as Isenthal regulators. The 20kVA had the Tonum regulator and a rather complex electrical system, due to the spec of the electrics for its particular application.

It's worth noting that I have had trouble with the carbon piles in these before and never got really satisfactory results from my reg. It is theoretically on the shelf for a rainy day, along with another that I am trying to acquire for parts. If my resistors are still OK, I can measure them, although I wonder whether they were matched to the torque-motor coils.

First, though, are your plant and electrics anything like this (less the trailer etc):
http://electrokinetica.org/d3/4/1.php
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Old 30th Nov 2013, 10:10 pm   #3
sentinel040
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Default Re: Tonum regulator

Lucien;

Thank you!

I am away from home at present and will come back again when I am back. The regulator is the same one and the panel looks as if it ,may once have been the same. Perhaps its best to add that the panel has a lot of carefully welded up holes in it, mainly in the lower part of the panel, yet the wiring seems to he unmolested, Blackly tape under zinc buckles for cable lacing and all very tidy, perhaps a re-work or built in a modified form.

This one seemed to control in the usual laconic fashion, but experience with the ones in our class 08 shunters taught me to treat with respect, we sometimes got Dowdings to re-work them, until they decided they could not anymore, then I used to have a go myself with varying results!!

Regards

Ian

So I suppose the big question is how to find out the resistance values?
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Old 30th Nov 2013, 10:25 pm   #4
Lucien Nunes
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Default Re: Tonum regulator

I'll measure the resistors in my plant, won't be for a couple of weeks though. I don't know whether they were matched to the torque motor or what tempco they had; if they simply swamp the coil resistance rather than compensating it then presumably any ordinary modern resistor will do instead.

Lucien
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Old 3rd Dec 2013, 9:52 pm   #5
sentinel040
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Default Re: Tonum regulator

Many thanks for the kind offer Lucien.

In the meantime I have as carefully as possible decided to carry out an investigation of the resistances. Prompted by the fact that that there appeared to be just steel discs separated by thinner discs I pulled one apart and indeed there are three solid discs separated by "Metrosil" discs, bearing the part number 912/306. A pic is included.

By the way looking at the photograph on the website there seem to be some minor engine layout differences, in that the air cleaner on this one is adjacent to the cooler group (radiator) and the fuel tank sits above the main alternator.

Regards

Ian
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Old 4th Dec 2013, 4:17 pm   #6
Lucien Nunes
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Default Re: Tonum regulator

Metrosil products are normally varistors, I wonder whether they are intended to have a tempco that compensates the coil resistance? Will investigate when I get a moment. There's no mention of this in the description of the Tonum regulator in the AP, perhaps we will have to reverse-engineer a solution if your discs are not salvageable. Of course one could sidestep it entirely and make the alternator self-exciting using a modern electronic AVR.

BTW the layout of my set was altered from the original AM spec when converted for use by the NHS for the mobile X-Ray outfits. Various things were removed and relocated, e.g. my fuel tank is under the floor. The autotransformer was only used to provide a low current 400V 3-phase supply, from the 230V delta alternator. For single-phase use it's not needed and mine has also been removed.
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Old 10th Dec 2013, 12:36 pm   #7
sentinel040
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Default Re: Tonum regulator

Lucien;

I have resurrected this unit. having dismantled the metrosil units and cleaned the rusty surfaces on the steel discs they now seem to function again. Testing by means of a variac fed into the input. I assume the non-linear characteristics of the metrosil discs was used to get the voltage trim to function sensibly...

Regards

Ian
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Old 11th Dec 2013, 9:39 pm   #8
Lucien Nunes
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Default Re: Tonum regulator

That's good to hear! I wonder whether there would be an advantage in replacing the plates with a non-ferrous material to avoid corrosion in the future, or at least replace them with new ground flat ones to ensure best possible contact, once the unit has been proven to operate correctly?

It wouild be interesting to plot voltage/current curves for your repaired devices, to find out how much effect they have on the regulation. There's no need to precisely linearise the transfer function of the complete unit, so my hunch is that it provides compensation for the coils, independent of the external multi-position temperature switch that accounts for the winding resistance.

I'll mail you my scans of the adjustment instructions as they're too big to post here.
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Old 11th Dec 2013, 11:52 pm   #9
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Default Re: Tonum regulator

Hi Ian (and Lucien), looks like a nice set.
Well done on the repair, it's been a few years since I last played with a carbon pile regulator.
I have several ex-equipment electronic AVR's if you want one to try out.

Cheers, Rob.
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Old 18th Jan 2014, 4:18 pm   #10
sentinel040
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Default Re: Tonum regulator

First of all sorry for taking my eye off the ball on this but other issues have taken fore stage in life I am afraid.

Rob; sorry for not replying, I made the error of saying to myself "lets see how we go, I will know in a few days"; then life sort of fell apart again!

Lucien; I have plotted the voltage across the regulator's coil with respect to panel input voltage (from a variac - before I refitted it) to see if my hunch that the Metrosil units provided an offset so the coil was only energise over the upper part of the voltage range. this appears to be the case. I also had a look at the resisitance of the pile over the controlling range as well. Here goes:-

(machine o/p volts - coil volts and where given pile resistance)

70v - 1v
100v - 7v
125v - 11v
150v - 14v
175v - 21v
200v - 26v
210v - 27v
215v - 29v (coil starts to buzz)
220v - 33v - 9.1ohms
230v - 35v - 10.3 ohms
240v - 40v - 11.6 ohms
250v - 42v - 18 ohms (but wanders a bit)

I noted that someone has scrawled on the exciter terminal panel "13.5vDC for 250v".

Regards Ian
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