Cheap DC
Cheap DC
(OP)
Another rail problem...
A lot of old rail cars ran on wheel driven generators that kept gigantic battery banks charged. Wheel driven generators are verboten now. This leaves cars with 2HP 32VDC 40A motors in a bind. The first idea is always, "lets just swap in a 240V 3ph motor."
The problem with that suggestion is that the DC motors are bazaar. They aren't NEMA anything. They have looooong shafts an asymmetrical shafts. Different diameter shafts on opposite ends, etc., etc., ad nauseam.
Most cars have 240VAC 3ph.
There is often the suggestion: "Get a 36V battery charger." That doesn't work because they put out more like 40V and often look for a battery which is not going to be present.
Of course half these cases are for rail museums and no one has any money so solutions need to be low budget.
My current thought is this:
32 x 40 = 1280W
3pole 240VAC breaker.
Three 32V 500VA buck/boost dry transformers.
Six 40A, 100V stud mount diodes.
Some aluminum flat stock for mounting the diodes.
Various isolating mounting hardware bits and pieces.
String it all together for a 3 phase rectifier and directly feed the motor.
This would come to $100 per transformer.
$8 per diode = $48.
$15 for the breaker.
$100 for details.
Total: $463
Wad'da'yah think?



A lot of old rail cars ran on wheel driven generators that kept gigantic battery banks charged. Wheel driven generators are verboten now. This leaves cars with 2HP 32VDC 40A motors in a bind. The first idea is always, "lets just swap in a 240V 3ph motor."
The problem with that suggestion is that the DC motors are bazaar. They aren't NEMA anything. They have looooong shafts an asymmetrical shafts. Different diameter shafts on opposite ends, etc., etc., ad nauseam.
Most cars have 240VAC 3ph.
There is often the suggestion: "Get a 36V battery charger." That doesn't work because they put out more like 40V and often look for a battery which is not going to be present.
Of course half these cases are for rail museums and no one has any money so solutions need to be low budget.
My current thought is this:
32 x 40 = 1280W
3pole 240VAC breaker.
Three 32V 500VA buck/boost dry transformers.
Six 40A, 100V stud mount diodes.
Some aluminum flat stock for mounting the diodes.
Various isolating mounting hardware bits and pieces.
String it all together for a 3 phase rectifier and directly feed the motor.
This would come to $100 per transformer.
$8 per diode = $48.
$15 for the breaker.
$100 for details.
Total: $463
Wad'da'yah think?



Keith Cress
kcress - http://www.flaminsystems.com





RE: Cheap DC
You might need a cooling fan to minimize the smoke.
Mike Halloran
Pembroke Pines, FL, USA
RE: Cheap DC
Muthu
www.edison.co.in
RE: Cheap DC
Muthu; You completely lost me.. Probably because I lost you first.
240VAC 1ph and 3ph is now present. That is ALL that is present. There is no 24V? ??
BTW: These are hulking DC brush motors.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Cheap DC
Muthu
www.edison.co.in
RE: Cheap DC
Love your animation!!
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cheap DC
Nope they just dumped them across a few thousand amp-hours of batteries originally.
They're running squirrel cages so their isn't much starting effort required.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Cheap DC
Don't forget that to get 32VDC from a rectifier your AC buck transformer secondary will have to be 45V, not 32V.
You are going to have a lot of ripple. What's the load on the motors?
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Cheap DC
I thought 32V / 1.41 = 22.7V
22.7 + 1V + 1V = 24.7V (diodes)
So a 24V transformer is closer to what I want. No?
Motors are just air handler blowers.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Cheap DC
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cheap DC
RE: Cheap DC
Compositepro: Ah...
But, with no caps yes! Good point. Back to the 32V transformers. Ah wait a minute. That would be true with single phase but what about 3 phase?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Cheap DC
D'oh! I'm so used to explaining to people why DC bus voltages are higher than AC RMS that I confused myself... it's too easy to do that sometimes. I did 32/.707 instead of 32*.707
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Cheap DC
rasevskii
RE: Cheap DC
RE: Cheap DC
RE: Cheap DC
rasevskii; Thanks for that 1.35 multiplier. That's what I was looking for.
If I use the following circuit I can ground one of the rectified outputs can't I?
[img ttp://www.box.net/shared/static/gfiglqr7n4.png]
sreid; I don't see how I can use them in autoX mode.
100A.. um.. Now we're talking $25 times 6!!
85A is still only $13 a piece.
Compositepro; Thanks for that info.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Cheap DC
Lots of ideas here. Why make your own diode trios? Or maybe make a modification to one of those outboard rectifier units.
RE: Cheap DC
RE: Cheap DC
David; Thanks for the confirmation.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Cheap DC
BTW you will also need a heat sink for the diodes. More than one.
There is such a thing as a "Rectifier Handbook" around to design these things. Maybe on the net..
rasevskii
RE: Cheap DC
Why ground one side of the DC ?
Muthu
www.edison.co.in
RE: Cheap DC
At a certain museum I was once involved in, a colleague built a silicon rectifier into an open oil drum to rectify 440VAC into about 600VDC for overhead trolley supply. Negative grounded to the railcar frame. 440V from a rental genset.
rasevskii
RE: Cheap DC
http://www.mcmaster.com/#70525kac/=8zgjxx
2) Go with the 85 Amp Rectifiers [Price].
3) There are no Regeneration concerns. The motor freewheels on decel [no source for negative current]. But maybe you should have a reversing switch.
RE: Cheap DC
A couple of comments;
I would suggest going with the transformer with the highest impedance if you have a choice. The transformer impedance may be the main limiting factor for the starting current. This may contra-indicate an auto transformer as well as grounding issues. I like auto transformers but this is one case where I may not use one.
In regards to the use of automotive rectifiers. I was under the impression that automotive diodes were rated around 100 Volts so as to withstand load dumps.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cheap DC
RE: Cheap DC