High voltage DC question

[bhm1712]

Hi All,

I am trying to help my dad mend his Historic Tram. The tram has a dewirement indicator consisting of a 600 volt DC supply and a rather ancient coil and relay contacts. Anyhow, the coil has burned out and you cannot get them anymore (1948 British Thomson Houston). I am trying to figure out what voltage the coil would have been.

You have the Positive 600 volts coming in through a 5 amp fuse and then through a 9K ohm resistor, and then onto the positive side of the coil.
On the negative side, you have the O volts also going through a 5 amp fuse, and then through another 9K ohm resistor onto the negative side of the coil.

My question is, How can i work out what the coil voltage originally was? I am getting ridiculous figures and values when doing ohms law and voltage drop calcs - a 540 coil surely is not possible?????? can anyone help me out?

 

[bernardgreen]

There are some tram way experts at the Tramwau Museum at Crich wjo rebuild trams and very likely can provide you with information about the relay and might even have spares

http://www.tramway.co.uk/

 

[bhm1712]

My dad is a member........they are scratching their heads!

No spares available.

I just want to know what calculations to use to work out the voltage after the resistor.

My calcs are showing that it will be 300 volts - 2 equal value resistors - potential divider.

 

[Paul_C]

How can i work out what the coil voltage originally was? I am getting ridiculous figures and values when doing ohms law and voltage drop calcs

Unless you know something else about the coil (i.e. its operating current or resistance) you really can't calculate it.

You can determine from the 18 kilohms series resistance than the operating current could have been no more than about 33mA (600V / 18K), but that's about it.

As Bernard suggests, I think you need somebody who is familiar with the specific equipment and would know the type of coil used. There would have been a fairly wide margin for supply variation designed into the circuit anyway, because as I'm sure you're aware the 600V supplies for things like tramways were subject to considerable variation from the nominal voltage in actual use.

 

[Paul_C]

My calcs are showing that it will be 300 volts - 2 equal value resistors - potential divider.

If you just joined the two resistors together (bypassing the relay coil) then yes, you would end up with 300V at the midpoint. But when you insert the relay coil between them, the voltage which appears across the coil will be dependent upon the total current flowing and the resistance of the coil. With 9K on each side of the coil, the average of the voltage appearing at the two ends of the coil will still be 300V, but the actual voltage across the coil will be something different.

For example, with a 2000-ohm coil, you'd have a total series resistance of 20K, giving a current of 30mA. The voltage appearing across the coil would then be 60V (with one end of the coil at 270V and the other at 330V with respect to earth).

For a 1000-ohm coil, the current would rise to approx. 31.6mA and the voltage across the coil would, correspondingly, be about 31.6V.

 

[bhm1712]

ok, but what would the voltage have been after the 9k ohm resistor on the positive side? 300vdc?


It wired like this:



+600v------Fuse----9K resistor----Relay Coil-----9k Resistor---fuse-----0v

Sorry Paul C i posted this before i saw your other post.

If i cant measure the resistance of the coil, am i Knackered???? all the spare ones available, all have broken coils!! no wiring diagrams available either!

 

[bernardgreen]

The circuit will be three resistors in series 9K0 + Rcoil + 9K0 If R coil was 9K0 then each would have the same voltage, namely 200 ( one third of 600 )
'
Current in the coil and resitors would be 22mA, power in the coil and each of resistors 4.44 watts ( 13.33 watts in total ) That is possible if it is a large relay but still quite lot.

A higher resistance in the coil would reduce the current but the voltage drops in the resistors would be lower so the relay coil would have a voltage higher than 200 volts.

A lower coil resistance would result in a higher current but lower voltage.

Can you post a picture of the relay and the resistors and any lettering on them. ? Is it possible to investigate the remains of the coil to measure the diameter of the wire and an estimate of how many turns. Re-winding he coil is not impossible and if it its a high current low voltage coil the number of turns is small.

The fuses being 5amps is a bit strange, The highest current possible is about 35 mA ( zero Ohm coil )

 

[Paul_C]

ok, but what would the voltage have been after the 9k ohm resistor on the positive side? 300vdc?

It's impossible to say for sure without knowing (a) the current, or (b) the resistance of the relay coil, or (c) the operating voltage of the relay coil.

The voltage at that point would be somewhat above 300V, since that 9K part of the potential divider formed will be lower than the other 9K resistance plus whatever resistance the relay coil happened to be. How much above 300V depends upon the coil.

With a 2000-ohm coil as mentioned above, the voltage at that point would be about 330V. With the 1000-ohm coil, it would be around 316V. Put a 4000-ohm relay coil in the circuit, and it would rise to about 355V.

Edit: Looks as though Bernard & I were typing at the same time.

 

[bhm1712]

i will take some pics in the morning as im now on a night shift,

The coil and relay is quite large, not by any means a small one. The coil is around an inch and a quarter tall and 25mm diameter. I possibly could investigate the coil deeper, but id have to be careful as its sitting on a sheet of asbestos board.

I was thinking about replacing the coil and relay contacts with a modern one, hence me trying to work out the coil voltage!

The only lettering is on the 2 green resistors : AW113, 9Kohm, 5% mJ

 

[efiste2]

this may sound a silly idea but would it be worth contacting the people who run and maintain the tramways at Blackpool....

 

[Igorian]

Not sure if this will be of any use, but it might be worth a look.

 

[bhm1712]

Igorian - thanks this may help.

The wire gauge is actually AW3113 not AW113.

I will tell my dad to contact them at blackpool if we cant find a solution.

 

[ban-all-sheds]

bhm - Google Ohm's Law.

You, or somebody associated with the rebuild of this tram, is going to have to get their heads round it.

 

[big-all]

london underground liverpool 3rd rail and southern/south eastern /south west trains are 3rd rail are all 600-750v dc so try contacting one off the workshops or perhaps the southern electric group
http://www.southernelectric.org.uk/

 

[bhm1712]

ban all sheds - your answer is useless. We can all google ohms law all day long.

The point here is trying to find a replacement relay with the correct voltage coil from modern equipment, not 1940's technology. The tram is one of a kind and there are no like for like equivalents available for comparison. Ohms law can tell me what resistance i need to get the required volts drop, but finding a relay with the right voltage coil and with the right resistance is not that easy.

Rs does not list the resistances of the coils on all of its relays and its not easily found in technical documentation either.

The tram is already fully restored, but this part has failed and means the tram cannot run.