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Post by andrew7931 on Feb 28, 2015 11:19:48 GMT
Hi guys
I have almost finished by 5" ruston type loco and I am thinking of what I could use in line with the motor to stop the electric being generated from free wheeling going back into the speed controller I was thinking of a diode but carnt seam to get one to take the amperage any ideas?
Thanks
Andrew
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Post by alanstepney on Feb 28, 2015 12:36:01 GMT
Let me know what ratings you require as I probably have something that will do the job.
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Post by Roger on Feb 28, 2015 13:08:57 GMT
Are you sure you need anything at all? Does the specification of the speed controller state that it's required. I was just thinking that the DC motor will generate a voltage in proportion to the speed, so when it's coasting, the voltage it produces will be within the normal voltage rating. If you ran down a hill to twice the maximum speed, it would be twice the voltage. You may well find that it doesn't matter because the output devices can cope with over voltage since the voltages we're talking about are very low. It's worth contacting the controller supplier to see what they say.
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dfh
Hi-poster
Posts: 197
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Post by dfh on Feb 28, 2015 13:21:26 GMT
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Post by Deleted on Feb 28, 2015 14:32:10 GMT
That was a good link there David, and well worth scrolling up + down to see the whole thing as well........
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Post by andrew7931 on Mar 1, 2015 22:29:38 GMT
Hi guys
Thanks for the comments I got my speed controller from china 24v 60A 3000w my motor is reted at 13.5 A at 250w 24v so not too sure what I would need if you could help Alan that would be great as this is the last part of the build
Thanks
Andrew
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Post by Deleted on Mar 2, 2015 0:52:28 GMT
Are you sure you need anything at all? Does the specification of the speed controller state that it's required. I was just thinking that the DC motor will generate a voltage in proportion to the speed, so when it's coasting, the voltage it produces will be within the normal voltage rating. If you ran down a hill to twice the maximum speed, it would be twice the voltage. You may well find that it doesn't matter because the output devices can cope with over voltage since the voltages we're talking about are very low. It's worth contacting the controller supplier to see what they say. -------- unless you incorporate a free-wheel device ??.......No "Back-loading" involved but then your brakes must be up to standard, just like an automatic car for instance.... |
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marcf
Hi-poster
Posts: 167
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Post by marcf on Mar 2, 2015 6:48:56 GMT
I use the same controllers as there cheap and work extremely well.
I've stuck a diode across the motor to take any back feed.
I'm running Sinclair motors in mine and there rated 12v 29a 250w
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Post by Roger on Mar 2, 2015 7:32:26 GMT
I use the same controllers as there cheap and work extremely well. I've stuck a diode across the motor to take any back feed. I'm running Sinclair motors in mine and there rated 12v 29a 250w I think I must be missing something here. I read that as being a diode across the terminals of the motor which means you can't run it in reverse? Perhaps you mean a pair of Zener Diodes back to back across the terminals to absorb the over-voltage? |
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marcf
Hi-poster
Posts: 167
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Post by marcf on Mar 2, 2015 7:36:21 GMT
Sorry mine is across the outputs of the circuit board so not an issue when reversing as have a 3 way switch after the speed controller
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Post by Roger on Mar 2, 2015 8:43:25 GMT
Sorry mine is across the outputs of the circuit board so not an issue when reversing as have a 3 way switch after the speed controller Ah, that makes more sense. Even so, I'm not sure why you've put it there? If it's across the output, ie from the output to 0V then it can only stop the output from going -ve. If it was the other way round, it would short out the motor output. If the intention was to prevent the output voltage from climbing above the battery voltage when the motor is acting as a generator then it isn't doing that. Maybe the diode is a Zener Diode, in which case it can do what you intended. Do you have the number of the device? Internally, there will be a standard diode across the output anyway to handle the back EMF of the motor. When current is passing through an inductor, in this case a motor, the current takes time to build up but when it's flowing, it wants to keep doing that. When you try to interrupt it, the magnetic field collapses and huge voltages are generated in an attempt to keep the current flowing in the same direction. This drives the disconnected terminal -ve and without a diode to provide a path for the current to flow through in that direction, the power transistor would be destroyed. My guess is that you don't need any of these external circuits. I doubt if any motor is driven by the load fast enough to generate enough voltage to destroy the output transistor. If it was a problem, a normal diode won't help, you'd need a Zener diode to clip the voltage and it would need to be able to absorb the energy so would probably need to be on a heatsink. It would be a good idea to google Zener Diodes and back EMF to get a better understanding of what these things are. |
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steam4ian
Elder Statesman
One good turn deserves another
Posts: 2,069
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Post by steam4ian on Mar 3, 2015 12:08:52 GMT
What Marc is referring to is a "free wheeling diode" it deals with the currents induced in the circuit by the motor and lead inductance which would otherwise make it difficult for the output semi conductors in the drive to switch off.
Andrew, I would not worry about any additional device, diode or otherwise, in series with the motor. The only time you might have a problem is if the loco ran away down a hill and gained a speed well in excess of the speed at full speed. If this is really possible then a diode between the output of the controller and the battery active, connected in reverse to the current flow in the controller, would shunt the motor energy in over-speed back to the battery.
BTW, I prefer to drive electric locos without regenerative braking for which you need a 4 quadrant controller.
Ian
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marcf
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Posts: 167
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Post by marcf on Mar 4, 2015 6:56:44 GMT
I hate the regenerative braking as well. The speed controller I use doesn't have this and benefits from costing just under £10! It's by far one of the best controllers I have come across.
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Post by andrew7931 on Mar 4, 2015 10:10:33 GMT
Hi guys
Thanks for the information I think I will not bother with any thing and see how I go. I built this loco as cheep as possible just to see how much I could save by not buying components from the "special railway suppliers with there special prices" and I have manage to build it for the sum of £160 so I am happy with that I will have to see how it runs on the track now
Thanks
Andrew
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Post by Deleted on Mar 4, 2015 20:20:38 GMT
Any photos available ??
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