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Author Topic: Why Does A Tall Power Thing Hum ?  (Read 15486 times)

Offline Geezer

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Why Does A Tall Power Thing Hum ?
« Reply #25 on: 06/09/2009 23:51:44 »
You could also get about 50 car batteries (old submarine batteries might work too) connect them in series and also include a current limiting resistor, attach the whole thing to a couple of long cables with big croc clips on the ends, and nip out after dark and hook up for a quick recharge. Then, attach the batteries to a large inverter that produces 240 V, 50 Hz. to power your home. What could be simpler?

Be sure to connect the red wire to the positive rail and not the other way around, although, in the US that might be the black wire. Anyway, I'm sure you'll figure it out. (Is the third rail positive or negative? I've no idea.)
 

Offline syhprum

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« Reply #26 on: 07/09/2009 07:47:09 »
I would expect it to be positive as a negative rail would be more prone to corrosion.

PS Lead acid batteries work best when charged from a constant voltage source of 2.35 volts per cell so use the appropriate number.
« Last Edit: 07/09/2009 07:50:21 by syhprum »
 

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Why Does A Tall Power Thing Hum ?
« Reply #27 on: 07/09/2009 08:57:34 »
50 car batteries would cost you quite a lot. Still, you could always sell them to pay your fine when you're caught!
I can't help feeling that track maintainance would spot your cable or other evidence of your connection.
 

Offline Geezer

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« Reply #28 on: 07/09/2009 19:11:24 »
SC - I think you missed my point about the big croc clips. You only sneak out at night and hook them up for a couple of hours so the track crew never see them. Mind you, that does increase the risk of being flattened by a train somewhat and humans do seem to have a very hard time judging the speed of oncoming trains, but nothing ventured.

Syphrum - Re. charging voltage, I don't think you can count on the track voltage being very constant. I would imagine there are some fairly significant voltage drops in the rails when trains are accelerating, and the voltage may exceed 750 V at certain times, so if you match the number of cells to 750 V exactly, you may get no charge at all, or far too much, which can cause very nasty problems. Other factors can affect the cell voltage too, so you really need some sort of regulator to accommodate these variables. A resistor is all it would take. Something a little more sophisticated might take a little more time to design, but while you are "in the nick" you will have plenty of time to work it out for the MK2 version  ;D
 

Offline LeeE

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« Reply #29 on: 07/09/2009 21:53:18 »
Quote
A resistor is all it would take

A bit of an understatement methinks: it would have to be a mucking hefty resitstor.  ;D
 

Offline Geezer

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« Reply #30 on: 07/09/2009 22:51:20 »
Quote
A resistor is all it would take

A bit of an understatement methinks: it would have to be a mucking hefty resitstor.  ;D

Not all that hefty really  :)

With 50 car batteries, a 3Ω resistor would limit the current to about 42A and it would "only" dissipate 5.2kW. It ought to be possible to rig up something appropriate with a network of 1kW radiant heater elements. And, the heat produced would be quite nice while hanging around outside in the middle of the night while the batteries were charging.
 

Offline LeeE

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« Reply #31 on: 07/09/2009 23:06:16 »
Hmm... I believe that the typical max continuous current draw for railway traction motors is around 1800A, and this figure may be exceeded by quite a large margin (as much as 2x) for short periods, but even if you do only draw 42A, and only have to dissipate 5.2kW, that's still quite a mucking hefty resistor  ;)
 

Offline Geezer

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« Reply #32 on: 07/09/2009 23:18:50 »
We could do all kinds of fancy stuff to eliminate the resistor, and probably increase the number of batteries also. For example, we might use a switching regulator to limit the current instead. That would reduce the amount of power dissipated considerably. But, as it's not our power, why would we care?

Thanks for the data on the current drawn by the traction motors. At those levels they are hardly likely to notice a mere 40A "leakage". (Crap, I hope nobody is taking any of this seriously.)
 

Offline syhprum

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« Reply #33 on: 08/09/2009 12:00:56 »
Lead acid batteries do not like being charged from a constant current source i.e from an over voltage source via a resistor.
They bubble off Hydrogen gas which depletes the acid solution and can explode, batteries last much longer on modern cars because they are charged from a well controlled constant voltage source (2.35v per cell)
 

Offline Geezer

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« Reply #34 on: 08/09/2009 17:51:42 »
Lead acid batteries do not like being charged from a constant current source i.e from an over voltage source via a resistor.
They bubble off Hydrogen gas which depletes the acid solution and can explode, batteries last much longer on modern cars because they are charged from a well controlled constant voltage source (2.35v per cell)
Agreed. With a simple resistor, we would have to disconnect the croc clips when the batteries were sufficiently charged to prevent gassing.

Supppose we connect a partially discharged 12V battery which is at, say 12.6V, to a charger at 14.1V which has almost unlimited current capacity (as the third rail supply does). The charging current will only be limited by the internal Resistance of the cells. I suspect we will easily exceed the maximum charge rate and cause the battery to vent, or possibly explode.

To prevent this, we still need something to limit the current. A current limiting circuit possibly, or perhaps a resistor. If we are using a very low resistance current source, the charger should not operate on voltage alone. Battery chargers and car alternators have some internal resistance which limits the current they can supply. The third rail system would be able to supply far more current than the batteries could safely consume.
 

Offline syhprum

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« Reply #35 on: 09/09/2009 13:12:55 »
Car generators have well designed electronic current limitation, if you wanted your installation to last it would be just as well to use a proper charging regular and not just a simple resistor.
The 723 chip schematic would be a good place to look for inspiration.
« Last Edit: 11/09/2009 20:36:26 by syhprum »
 

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« Reply #36 on: 09/09/2009 15:58:38 »
Would it work at 750 volts?
 

Offline syhprum

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« Reply #37 on: 09/09/2009 17:45:03 »
There are of course more complex chips that work in a chopped mode fashion and are suitable for a 750V system but I showed the 723 as an example of a regulator that has voltage and current sensing.
 

Offline Geezer

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« Reply #38 on: 09/09/2009 18:37:01 »
BTW, the 723 regulator requires resistor R5 in series with the load to create a voltage drop so that the 723 can measure the current supplied to the load. Resistors are hard to avoid  ;D
 

Offline Bored chemist

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« Reply #39 on: 09/09/2009 19:20:23 »
"Resistors are hard to avoid"
Ask the Borg or the Vogon guard.
 

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« Reply #40 on: 09/09/2009 19:30:41 »
Rats! We are discovered. Quick! Hit the probability drive.
 

Offline Waldo Pepper

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Why Does A Tall Power Thing Hum ?
« Reply #41 on: 16/10/2009 19:05:46 »
Lead acid batteries do not like being charged from a constant current source i.e from an over voltage source via a resistor.
They bubble off Hydrogen gas which depletes the acid solution and can explode, batteries last much longer on modern cars because they are charged from a well controlled constant voltage source (2.35v per cell)

Lead acid cells are fine being charged from a current limited supply as long as the voltage behind the limited current is set at 2.35v/cell. That is how domestic chargers work, albeit not a very well regulated end voltage.

The current a 200Ah+ truck battery would take when flatish being charged at constant voltage would blow your house electrics.

Exactly the same with Li-Ion cells. Charge at constant (or limited) current to the datasheet recommended rate (normally C/5) then let them take what they need until fully charged from a constant voltage 4.2V supply.

Just came across this site. Awesome! So much for me wanting to cut down on Internet usage and do something else :)
 

Offline Waldo Pepper

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« Reply #42 on: 16/10/2009 19:10:57 »
Would it work at 750 volts?

Yes if you dropped the supply to the device and used some high voltage transistors. BU208 ones used to be used in the EHT circuits in CRT Tellies (now old technology). High voltage regulator designs are complex and require a good knowledge base and some scary procedures unless you want to end up dead.
 

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Why Does A Tall Power Thing Hum ?
« Reply #42 on: 16/10/2009 19:10:57 »

 

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