RailPro > RailPro Specific Help & Discussion
Large Scale Power
Alan:
Yes, measure the resistance between drivers. Would be better to measure right at the motor terminals to eliminate any wheel wiper influence. You aren't directly measuring wattage although Ohm's Law would allow you to calculate it from the measurements. Wattage is a power measurement - amps x volts. Knowing the wattage is not necessary for your purposes. You are trying to determine only one half of that equation - amperage.
When a DC motor is stalled it effectively becomes nothing more than a coil of wire that has resistance (the winding + brush contact resistance). The resistance of the winding and the voltage applied determines how much current will flow. By measuring the resistance of the winding with an ohmmeter while the motor is at rest you are measuring the winding and brush contact resistance the same as if the motor were stalled under power. Ohm's Law then tells you the amperage at any specified voltage. At this point you could then calculate wattage if you are curious however there is no need to do so. Your power supply choice will be based on the desired voltage and the required amperage. The power supply will have a wattage specification but that is used more to indicate the load it places on the mains (wall plug).
Take several measurements slightly turning the motor shaft between measurements. Make sure the shaft is still when measuring. Average the readings. Plug the averaged resistance measurement into Ohm's Law along with whatever voltage you have selected for your trains. The result will be the stall current of the locomotive. Be aware, if the shaft is turned while taking a measurement you will get an incorrect reading because the motor acts as a generator when the shaft is turned. Sensitive digital multi-meters will detect the small generated voltage and give an erroneous resistance reading.
There is no need to be ultra-precise with this measurement process. Close is certainly good enough. Notice in the earlier post it was mentioned to double the needed amperage when selecting the power supply. There are several reasons for that. First, power supplies wear out quickly when continuously operated at or near 100% rating. Heat makes components fail sooner. Second, your track, layout wiring, and the motors connected to it will have natural capacitance and inductance. The power supply voltage regulation response time will be affected by such especially so when a power supply is operating near its rated output. Lastly, you do not want normal operation of the trains to use enough current to be anywhere close to the power supply internal circuit breaker setting. Nuisance trips may result. In summary, ample power supply headroom is a good thing. It is also why precise loco measurements aren't required. Close is good enough.
atsfguy:
Thank you for the explanation, Alan. It makes more sense than my first reading with my limited electronics knowledge.
The forum is extremely fortunate to have you and several other electronics experts on board.
Cecil
atsfguy
Dean:
In RailPro the 'stall current' is when the wheels are spinning and the train is not moving. This current reading will be lower than the calculated value. Actually, I think it is almost impossible to get a model train to stall with the wheels not turning.
Because I run HO, these power supplies for large-scale give me the willies. My 225 Amp Lincoln arc welder has an output of 12V to 50V DC and a current output from 40A DC to 130A DC.
Like Alan mentioned, be sure to have circuit breakers or fuses in your locomotives and protection for the layout. Be sure the fuses and circuit breakers you choose are rated for DC.
Alan:
--- Quote from: Dean on June 30, 2018, 09:19:56 AM ---In RailPro the 'stall current' is when the wheels are spinning and the train is not moving. This current reading will be lower than the calculated value. Actually, I think it is almost impossible to get a model train to stall with the wheels not turning.
Because I run HO, these power supplies for large-scale give me the willies. My 225 Amp Lincoln arc welder has an output of 12V to 50V DC and a current output from 40A DC to 130A DC.
Like Alan mentioned, be sure to have circuit breakers or fuses in your locomotives and protection for the layout. Be sure the fuses and circuit breakers you choose are rated for DC.
--- End quote ---
Exactly right. The reason for the manual measurement is to help select a suitable power supply, not to set the RP stall current setting. To the best of my knowledge there is no means of manually entering the RP stall current.
Dean:
You can manually set the stall current for a locomotive. Without the hand controller in front of me, I couldn't tell you how. But I have done it while experimenting with all the capabilities of the RailPro system.
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