12v 725w generator, Electrical question?

[cowpat]

Having used my diesel gen set for 26 years now. The wire wound vairable resistance/rehostat that controlls the field coil current and generator output rate is now worn and very tired. I find it difficult with a multimerer to measure the low resistance of it so that I can order a suitable replacement. Has anyone had experience of measureing low resistances that could help? I am guessing it is between 0 to 2 ohms at 15Amps ?

 

[ianj99]

Does it look anything like this 25watt 4.7ohm type?

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0170433

It cannot pass 15amps, because even at 1 ohm the voltage drop would be 15volts and the power dissipated would be 225watts (I^2 * R) so it would be a beast!

If its in the field circuit, the current is probably less than 5 amps, meaning the resistance could vary from zero to 2ohms to vary the voltage drop across it from 0 to 10v. Even so a 50watt rheostat would be needed.

To measure the resistance, set the multimeter to its 10amp current range and put it in series with a 10watt bulb (wattage not important - its just there to limit the current) and the rheostat and connect to a 12v battery.
Now measure the current I, and then reconnect with the meter removed and connected to measure the voltage across the rheostat. Its resistance R is simply V x I.

I

 

[cowpat]

12v gen. rehostat.

Thanks, janj99,
YES it does look remarkably like your link photo.
My initial guesswork on values was not such a good idea.
Thanks for the advice on measurement I shall use the multimeter as directed in a day or two,when on the boat again, because I haven't brought it off the boat... I take it that the second stage of measurement is with all in the same circuit but set on voltage and not amps? chris.

 

[ianj99]

Thanks, janj99,
YES it does look remarkably like your link photo.
My initial guesswork on values was not such a good idea.
Thanks for the advice on measurement I shall use the multimeter as directed in a day or two,when on the boat again, because I haven't brought it off the boat... I take it that the second stage of measurement is with all in the same circuit but set on voltage and not amps? chris.

Once you have measured the current, remove the meter so the bulb is still in series with the rheostat.
Then connect it, set to volts, across the rheostat to measure the volt drop across it, when set on max resistance (min current).

If current was 1amp and voltage was 2v, then rheostat has a 2ohm resistance (assuming it was on max)

Ian

 

[cowpat]

12v ge. rheostat.

Got that.I will check this as soon as i can.
Thanks for your kind help and link. Chris.

 

[John100156]

......Now measure the current I, and then reconnect with the meter removed and connected to measure the voltage across the rheostat. Its resistance R is simply V x I.....
I

Surely resistance R is V/I not V x I?

 

[ianj99]

Surely resistance R is V/I not V x I?

Yes of course, slip of the pen!

I

 

[John100156]

I do it all the time! Good to have some experts on here!

Liked your reply to the OP, very practical/informative.

Power=I^2 x R.... its been many years since I did my elec theory but you just never forget P=I^2xR or V^2/R or IxV in a DC circuit......

 

[ianj99]

I do it all the time! Good to have some experts on here!

Liked your reply to the OP, very practical/informative.

Power=I^2 x R.... its been many years since I did my elec theory but you just never forget P=I^2xR or V^2/R or IxV in a DC circuit......

Anything much more complicated like algebra or calculus and I'm totally lost though! Took me 3 attempts to get 'O' level maths so there's wasn't much point in doing physics beyond 'A' level...

These days, you'd probably be made a professor if you passed a 1970s ' A' level!