Re: (ET) field weakening potentiometer with a Curtis 1204

["David Roden" <etpost drmm net>]

On 21 Jun 2017 at 11:32, Robert Laird wrote:

>  If 1/30th of the resistor is 7.9 watts, then the entire resistor has
> to be around 237 watts. 

I don't understand this at all.  Where on earth are you getting 237 watts? 
 
Nothing like that is being dissipated anywhere in the example field 
circuit. 
No current is passing through the other 29/30 of the resistor.  It might 
as 
well not be there.

Even with no series resistance, the motor field itself was dissipating 
only 
72 watts.  And as you add resistance, the total power dissipated in the 
circuit will fall.  So there's no way you can get to 237 watts.  
(Actually, 
there IS a way to get 237 watts on the field: raise the battery voltage to 
65 volts and connect it right to the field. I don't recommend trying that, 
however.)

> But when you first put the rheostat into the circuit, (the first
> click, or first turn on winding), it pretty much has to carry nearly
> the entire current of the field, since the resistance is still quite
> low at that point. 
  
No.  First, that one turn is NOT carrying the full field current.  That's 
the whole purpose of the series resistance, to REDUCE the current in the 
field circuit.  As I said above, by Ohm's law, if the total circuit 
resistance increases, the circuit current falls.  And remember, in a 
series 
circuit, the current is the same everywhere.

More importantly, on that "first click," the added series resistance is a 
small fraction of the total circuit resistance.  Therefore the voltage 
drop 
across that resistance is a small fraction of the system voltage.  Since P 
= 
E * I (power = voltage times current), power dissipation in the added 
series 
resistance is a fraction of the full-field power.  

Maybe you're still thinking of the series resistance as if it were the 
entire load across the power supply (battery).  It's not.  It and the 
motor 
field IN SERIES form the load.  That changes everything.

Look at the calculations in my previous post again.  Run them for 
yourself.  
I think you'll see that there's no need to panic.  :-)

And if you don't see that, fine.  For your own ET, just use whatever power 
rating your intuition suggests is necessary for the field rheostat.  You 
can't harm anything except your wallet by absurdly oversizing the field 
control.  


David Roden - Akron, Ohio, USA

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