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Cheap DC

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?

 

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

RE: Cheap DC

You sure come up with interesting problems.

You might need a cooling fan to minimize the smoke.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Cheap DC

Why 240 V, ac machines ? Why not directly use 24 V, ac machine and use the 3-phase bridge rectifer to get the 32 V DC output ?

Muthu
www.edison.co.in

RE: Cheap DC

(OP)
Mike that's a good point.

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

Keith - Forget it. I read it as train lighting generators. Too early in the morning.

Muthu
www.edison.co.in

RE: Cheap DC

Do the motors have resistance starters? DC motors are very close to a dead short when stationary. Initial starting current may be estimated as being limited by the resistance of the starting resistors, disregarding the armature resistance.
Love your animation!!

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Cheap DC

(OP)
:)

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

Keith,
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  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Cheap DC

(OP)
Jeff; 45V?

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

I would design for a short circuit, tapering down to running current as the motor comes up to speed.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Cheap DC

Wounldn't the rms a.c. voltage be equivalent to the full wave rectified rms d.c. voltage?

RE: Cheap DC

(OP)
Thanks Bill.  I'll keep that in mind.  The transfomers should be good for 10 seconds of abuse and rectifiers take hellish abuse for a short time.  I'll bump up on the current ratings.


Compositepro: Ah...   Normally all my DC supplies have capacitors in them which essentially means no.  The diodes and the caps result in a dc voltage near the peak voltage - far higher than the RMS equivalent.

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

Keith,
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

For a 3-phase supply the DC voltage output is 1.35 the rms AC voltage and the AC current input on each phase is 0.82 of the DC load current in amps. Without caps involved...That is a 6 diode rectifier. The AC neutral, if any, is left disconnected. There is a galvanic connection between the DC and AC sides and the DC output side may not be earthed if the AC input side is earthed in any way. Otherwise an isolating transformer has to be used for the AC input.

rasevskii

 

RE: Cheap DC

The ratings for most Buck-Boost transformers is when they are used as a conventional primary secondary transformers [low voltage lighting].  When you are stepping down from 240 VAC to 24 VAC connected as an autotransformer, the rating is much higher.  Other might find the formulas for you but 3 500W buck-boost transformers connected as auto transformers will laugh at 1200 Watts.  Including the inrush.  I'd use maybe 100 Amp rectifiers to deal with the motor starting current.

RE: Cheap DC

I have observed that DC motors designed to run on batteries often do not run well on unfiltered rectified AC. Three phase would work much better than single phase, of course. Many DC motor are designed particularly for use with SCR controllers.

RE: Cheap DC

(OP)
Ah there you go Jeff!


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

With the circuit you showed, you can ground one side of the DC if you don't ground the wye point of the transformer.

RE: Cheap DC

(OP)
Nice idea wayne!   Not sure I save much.  And I'm a little worried that 12V diode stacks would deal with the 32V they'd be subject to.



David; Thanks for the confirmation.

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

RE: Cheap DC

Yes, as davidbeach said, the DC can be earthed if the AC neutral is left open. On the choice of diodes they have to withstand the peak inverse voltage at non conduction, which is something like the DC output voltage plus the peak AC inverse voltage. There is a way of calculating this but I don't know it off the top of my head... See if you can find the spec sheet of the diodes you have where this value is  stated. I doubt 12V diodes would be suitable. In other words they would fail immediately... due the non-withstand of the peak inverse...Add some fuses...

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

Don't you need a surge diverter too in parallel with the dc output ?

Why ground one side of the DC ?

Muthu
www.edison.co.in

RE: Cheap DC

Oh yes, you will also need a fan to cool the heat sinks...or you can build the whole thing into an oil drum or large paint bucket and immerse it in trafo oil. A fire hazard, then has to be outdoors. Or convection cool it by building it into a large drum, open top and bottom to get a chimney effect.

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

1) Three Phase Buck Boost;

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

Hi Keith;
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

I like the idea of using an old automobile alternator.  Not only do you get the diodes, you get the Heatsink! The price might be right too [0?]!

RE: Cheap DC

A pair of rectifiers for a Delco 33SI or 34SI alternator (100A 24V) will probably be up to the task at hand. I have used a similar 12V 100A alternator as an expedient welder and source of 110VDC; it lived to produce 12VDC with no ill effects. I agree that you may not save much money, 2 rectifiers at 29.95 = about $10 each for diodes. The major advantage is that they are more generally available, and prepackaged.

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