What are the benefits or the drawbacks of connecting the return (“ground”) wires of two transformers? As I understand it this can be done provided that the transformers are in phase, but why?
Pete
First of all, there is no requirement that the transformers be in phase. Not only can they be out of phase, but also at different frequencies. This is of course unlikely except for one special case: A zero-frequency (DC) supply can be very useful for implementing capacitive-discharge turnout-throwing and diode-logic schemes. I’ve got them all over my layout.
(When the frequencies are different, the overall RMS return current is the square-root of the sum of the squares of the individual return currents. This is generally very close to the larger individual current, except when the individual currents are nearly equal and 40 percent greater when they are exactly equal.)
One benefit of a common return is that you can use control rails to operate turnouts and accessories from supplies separate from the track supply. In this particular case, the return current from an accessory transformer that is out of phase with the track transformer actually cancels some of the train current and can somewhat reduce the voltage drop in the outside rails.
A block-control system, in which the blocks can be assigned by (elsctrical) switches to more than one transformer, would require switching both rails among the transformers if there were not a common among them.
Those who unwisely run their trains between blocks powered by separate transformers, or worse, by separate outputs from the same transformer, need to have their transformers in phase to reduce the fault current that flows when they change blocks. (They also need a precise match of voltage and waveform.) But, even with a proper block system, it is a good precaution to have the track-supply transformers in phase, to minimize the damage for the occasional accidental crossing between separately-powered blocks.
Having all the outside rails (except the control rails) connected together can
Thanks, Bob, for the in depth response. Sometimes you lapse into “engineer speak” and lose me, but this time except for the “the sum of the squares…” part I was able to follow you all the way. [:)]
Pete
I put it in parentheses, just so you could ignore it!..
What are you referring to when you say ‘control rail’? thanks
A control rail is an outside rail that is insulated from the other outside rail on a track section, and from the outside rails on the adjacent track sections. It is used on the common/return connection to an accessory, like a crossing gate or a signal, so that when a train enters the control section its wheels complete the circuit to the accessory via the uninsulated outside rail. The principle is the same as the non-derailing feature found on many remote control turnouts.
Pete
Gotcha. I believe that is how my fastrack road crossing section with semaphores is initiated.