switch rating

Hi all
I'm constructing a petrol-electric loco for 5" gauge. B&S engine turning a car alternator and also a 12V DC motor (ex power shower) as a generator to supply the alternator field current through a high power rated rheostat. The alternator output runs a pair of 24V, 12A DC motors to drive the wheels.
Reversing the connection between the motors and alternator will obviously reverse the loco. I have quite a number of double pole, double throw switches rated at 10A 250V AC which can perform this connection reversing. Its pretty essential that the current is at zero before this reversing occurs and this is easy enough - just turn the speed control down to zero.
The question is - how many of the AC rated switches should I be using in parallel to safely carry the DC load assuming the load is zero when switching occurs? How are switch ratings assigned? Is it based on switching full load current and issues of arcing or on current capacity to avoid overheating?
cheers for your thoughts,
Toby, Fareham

Toby wrote:
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If there's no current passing through the switch when the contacts open, you'll likely get away with using one switch per traction motor. You might even get away with using one switch for both motors, but that's pushing your luck. Of course, the switch manufacturer won't officially approve of this, but they might not approve of you using the switch for DC at all (DC creates more persistent arcs). But it'll probably work fine until you flip the switch with the motor running.
Arcing is what destroys switch contacts. There will be little heating of the contacts when they are closed, probably less than a watt of power loss.
If the switches use a common type of mounting, like the 15/32" round hole, use one switch and see what happens. If it doesn't work you can always buy a more suitable switch to fit the same hole.
Best wishes,
Chris

Toby wrote:
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It's essential, yes!!!
In practice I think you'll have to consider the cases when the motor is switched off when running at full whack, when the switch is switched on when the voltage is at full, and even the case when it is switched on in reverse when going full speed forward.
You can't really rely on it never, ever, happening.
If you don't design for the ignorant they'll always get you - and you thinking they are stupid, which they probably aren't anyway, won't change anything. Besides, you may fall and accidentally operate the switch yourself?
A switch rated at 10A AC will probably not be able to reliably switch more than 2A DC.
When a switch opens an arc is formed, and this can quickly destroy the switch, maybe leaving it permanently on with the contacts welded together, or maybe permanently off, melting, catching fire, or even blowing up!
In 50 Hz AC the voltage is zero 100 times a second, and this quenches the arc quite quickly. In DC however, this doesn't happen, so switch ratings are much lower for DC than for AC.
When a switch closes, it bounces open for a short time - and again an arc is formed. This isn't usually as bad as an opening arc, but it can still weld the switch closed, again leaving it permanently on.
If a permanently on situation is dangerous - and it sounds it - then just for that reason, never mind idiots or accidental operation, you'll pretty much have to use a much beefier switch.
For reliability people don't usually use electrical or electronic components at their full ratings. Manufacturers tend to like to give the biggest numbers they can, and that's maybe under specific conditions etc - it is usual practice to derate to no more than 60%, and even less for semiconductors. This doesn't mean they won't run at full rating, but reliability tends to suck if you do.
Your switch will probably not be able to carry much more than 10A (AC or DC) continuously before the tiny copper bits inside start to melt. The manufacturer's rating is really about switching off, and to a lesser extent about switching on, but they don't usually make the tiny copper bits any thicker than they have to.
Unfortunately, switches don't really parallel very well - more current tends to go through one switch than the other. Bipolar transistors are notoriously bad for this, but it happens in any kind of switch, even MOSFETs (though MOSFETS tend to cancel this out a bit). The higher current in one switch will most likely kill reliability.
Also, if one switch welds on, any paralleled switches won't make any difference - unless of course someone switches one of them to the other direction, when you'll get a huge short circuit!
It would be be best, by *far*, to use a single switch for the 12 A each motor takes. Even a single 10 A unsuitably-rated switch would be better than two paralleled switches in your case - one switch switched the wrong way, and BANG!. Wires melting, fires everywhere. molten and vaporised copper spraying everywhere ... DO *NOT* PARALLEL SWITCHES HERE!
It may be better to use just one switch for the whole 24A the two motors take, but I don't insist on that. :) Are you ever going to want only one motor to operate?
Lastly, the loads will be inductive. A lower voltage usually means a better switch-off capability, but an inductive load will cause the voltage to rise when it's switched off, which will probably more than overcome that advantage.
I don't know offhand where you'll find a suitable switch for 24 A inductive loads @ 12V DC, but I'd start by looking at car/lorry parts.
Hope I've already answered the rest of your questions, as below. It seems I've written quite a lot about one switch - and could go on a lot longer!
-- Peter Fairbrother
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Peter Fairbrother wrote:
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Memories of Moorgate......

Hi Peter, Peter and Chris
thanks for all your thoughts, especially the possibility of someone (even me) selecting reverse when the loco is at full speed. Thinking about this is a bit scary - the diode pack in the alternator would conduct if the motor switch(es) were reversed, producing some (maybe lots of) braking. Not a pleasant thought.
How about ensuring that the switch(es) cannot be changed over unless the mechanical brake that I'm also fitting is engaged and the speed control is set to zero? Some form of rugged mechanical interlock should make things somewhat safer.
If I can ensure zero load at switch over by such interlocking, do you still object to parallelling switches to obtain higher capacity? My plan was to couple as many as required in such a way that a single control lever moves them all so there's no possibility of me forgetting to move one.
I cannot think of any need to have separate control over the two motors, so one switch or group could serve both for simplicity. I'm not keen on relays as they need their own control supply.
thanks again for your insight and thoughts,
Toby

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You can get 4-way power relays, we had one of the Matsushita J series knocking around, I'll see if that is still in the workshop 'spares' bin.
Albright do DC motor reversing contactors, and some of their product can be got from fork truck dealers.
You could rig up a simple speed detector on the wheels or axles to prevent change of direction until the loco was at rest.
Peter -- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk http://www.oldengine.org/members/diesel http://www.stationary-engine.co.uk http://www.oldengine.co.uk

Toby wrote:
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I'm against it, as no matter how good your control lever is they will not all switch at the same time, which would cause a momentary short-circuit. Bad news if the alternator can supply any significant current.
If they are interlocked so they can't be operated when any current is flowing then you don't really need to parallel them anyway - but try and get some switches rated to least 15A for each engine, and preferably higher. One switch for each engine doesn't have as much potential for disaster as paralleled switches.
Plus, as I said, switches don't parallel well anyway. So basically, forget all about paralleling switches, it's a bad idea.
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I must admit I'm a bit surprised you aren't using a battery, I thought that was the point of fuel/electric hybrids, to boost acceleration when starting so you need a smaller fuel engine, and maybe to recover energy when braking.
However you already *have* a suitable control supply - the output from the small genny before it feeds the rheostat and alternator feed coils.
Relays have several other advantages beyond availability, current capacity and cheapness. First, the switch on the "dashboard" doesn't have to carry the full current, and can be almost anything you like.
Second, the main current-carrying wiring does not have to be rerouted via the dashboard, as the relays can be inserted almost anywhere, and thus the main wiring can be shorter.
Those are the main reasons why cars and lorries use relays. If you have trouble finding suitable relays, one relay for each motor is also okay and might be useful for diagnostics, a low power running mode, etc.
If you want a neutral as well as forward and reverse positions you could go for two normally open relays, one for forward and one for reverse, rather than a single changeover relay, with a low-current direction switch on the dashboard with a center-off position.
You could even use a keyswitch on the dashboard, deterring unauthorised operation at fairs etc. while leaving the motor running.
With a center normally off position on the dashboard switch you can be pretty sure one relay has opened before the other closes. You could also disable the operating supply from one relay when the other relay is on, and do some slightly finicky stuff so that the relays simply wouldn't switch on if the engine was already travelling, but it's too early in the morning to describe it..
-- Peter Fairbrother
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Hi all again,
Thanks once again for your thoughts and insight.
How about a pair of these http://www.maplin.co.uk/Module.aspx?ModuleNod36
They are simple change-over relays rated at 40A 28V DC with 12V coils. I could use the normally closed contacts for forward running and the normally open contacts for occasional reversing. Movement of a switch to energise the reversing coils could be interlocked with the brakes and speed control.
Toby

<>
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I always recall a maxim hammered into me many years ago: 'For every idiot-proof solution there is inevitably an idiot greater than the proof'
Richard

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Most toggle switches are inherently fast-break by their design, but there's nothing stopping you using a pair of DPDT relays to do the job, you can get 30A rated ones with Lucar terminals.
Make sure you have a fuse in the circuit, as if the relays are not mechanically linked, there could be a small problem if one fails to operate for any reason.....
Peter -- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk http://www.oldengine.org/members/diesel http://www.stationary-engine.co.uk http://www.oldengine.co.uk

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Got some 1/4"x1/8" brass bar and a few bits of piano wire to make contact clamping springs, a bit of Delrin and a bit of Tufnol sheet???
Make a lever change over switch with a little delrin handle and a Tufnol base.
If you're worried about limited contact life due to arcing, fit spring loaded auxiliary blades.
It ain't high tech, this is railway engineering :-)
Mark Rand RTFM

Toby pretended :
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I would rate an AC switch as able to carry, not switch, the same DC current plus a bit. So 10amp AC rated switch should easily be able to to carry 12amps DC continuously.
However, were you to make or break the DC load with the switch it is liable to arc and destroy the switch, so you do need some form of interlock to prevent this. Perhaps a cover over the switch and as you lift the cover it breaks the supply.

On Wed, 13 May 2009 23:02:03 +0100, Harry Bloomfield
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Contact arcing is no problem because you have a low voltage known polarity DC source driving a motor load.
Arc generation can be eliminated by a reverse polarity connected diode across each pair of switch contacts. Forward conduction of the diodes short cicuits the inductive overswing and 24v is not enough initiate and arc.
It's natural to think of four separate diodes but a neater solution is possible with a standard full wave bridge rectifier - the internal connections happen to be correct.
Your existing connections - the DC supply in to the two changeover wipers and the fixed contacts to the motor. Connect rectifier AC terminals to motor, rectifier + and - terminals to supply DC+ and supply DC-.
No continuous current flows through the rectifier so the rectifier can be pretty small - 5A is ample.
Jim

snipped-for-privacy@yahoo.com wrote:
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Also I wrote:
> Lastly, the loads will be inductive. A lower voltage usually means a > better switch-off capability, but an inductive load will cause the > voltage to rise when it's switched off, which will probably more than > overcome that advantage.
I worded my thought entirely wrongly, and ended up not saying what I meant. Sorry, I'm a bit 'lergy just now, and not thinking too clearly.
The problem is not (just) the back emf from the motor, it's that the alternator voltage will spike if the load is removed, and as there is no voltage regulator and the field is manually controlled that could come to quite a high voltage, especially during a disconnection under high field currents!
Reverse-connected diodes can't protect against this sort of spiking, as the spike voltage is in the forward direction.
Also spiking might well blow the rectifier diodes in the alternator, they are/were* very sensitive to overvoltage.
* (it's a while since I did this sort of stuff, and things may have changed - but in those days anything over 60V would kill them)
It's just possible that even a residual field might cause a damagingly high voltage if the alternator speed is high and there is nothing to soak up the voltage, but eg a 10W 470ohm resistor or 48V bulb should protect against this while taking minimal current under normal operating conditions, assuming the field current is zero when switching occurs and we are only talking about residual fields.
BTW, 24V most certainly can arc! Arc welders often work at less than that voltage.
BTW2, the Maplin relays look okay, but a bit expensive - try a scrappy, or ebay if you want new. It might be an idea to use a pair for each motor, as they are designed for 40A at 12V, and 24A at 24V may be a bit too much.
I couldn't find any 40A DPCO ones either!
-- Peter Fairbrother

On Thu, 14 May 2009 19:47:09 +0100, Peter Fairbrother
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It must be a very long time. I've yet to see automobile diodes rated for less than 200v PIV and most test out at more than 400v PIV
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Both stick and MIG welders initiate an arc by virtue of the built in self inductance and the absence of any reverse diode to absorb the inductive overswing. In MIG welders the inductance is a separately added component. In stick welders it is the deliberately high leakage inductance of the power transformer. Without this inductance a low voltage arc is self extinguishing.
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Jim

snipped-for-privacy@yahoo.com wrote:
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Yep, mid 70's. New cars had alternators by then, but many old ones still had dynamos.
I've yet to see automobile diodes
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That's good to know, in the old days the diodes were the weakest link. I blew several up!
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Sure, but that's for arcs which are deliberately stabilised over wide operating parameters.
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Not necessarily. It depends on the arc length (and other things). Besides which, an arc doesn't have to last long to weld a switch shut.
A mate of mine regularly stick welds using two 12V car batteries, and sometimes only uses one. I can't do it myself, I tend to stick the electrode to the work, but he does - and there is no inductor involved.
Get a 6V motorcycle battery, and touch a wire to both terminals - you'll see sparks fly. That's caused by an arc, and it's very similar to the action of a switch.
-- Peter Fairbrother

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Dangerous advice.
Both ends could weld to the battery, and if wire is of sufficient gauge (eg a spanner) then explosion + fountain of hot acid results

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Heavy duty switches similar to auto starting solenoids are commonly used to control 12VDC vehicle winches in both directions. They tend to be a bit pricey. Electric lift trucks also use similar switches.
Don Young (USA)

Toby wrote:
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Maybe a suppresion circuit would reduce the back EMF from the motor and protect your switch. Try this site as a start http://www.reed-sensor.com/Notes/Protection_Circuits.htm
Don

Toby wrote:
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Hadn't read that bit carefully - I presume it's an ordinary 12V nominal output car alternator?
If so, why are you using 24V motors?
Running them on 12V, you'll probably only get about 24W [1] delivered power from each?
-- Peter Fairbrother
[1] at 4A and 50% efficiency - depends on type, gearing etc, but that's fairly typical