For those of you that are "challenged" backing up

2oldman wrote:

Wes Tausend wrote:
I will guess that each motor draws as much as 1500 amps,

at what voltage?

2oldman,

Voltage gets a little tricky. Underway, everything jumps around. I interpolated* the 1500 amps from a DC unit to relate to a more powerful AC locomotive which doesn't have a read out in amps. Unlike the older DC units, the AC locomotive computer screens do read out in main generator (MG) voltage, maybe 2kv at times. If I remember, the KW of instantaneous power reads out in either per axle, or per truck (bogie) of three axles each. The digital readout jumps around a lot when under extreme pull because of constant wheel-slip correction, whereas the old amp guages were slow to respond and therefore gave a very handy average.

I believe the old DC locomotives would hit around 700 volts, but there was no analog volt meter (like the valuable analog ammeter) that would show voltage in real time. When the amps went up on the old DC units, because of a hard start or slowing on hills, I believe the volt output went way down, maybe to 200 VDC.

I kind of get what you are getting at; it seems if we knew both the amps and rpm, and there was no slippage, we could theoretically calculate the wattage and convert it back and forth to HP, ftlbs of torque, voltage etc:
1 hp(I) = 745.699872 W {P(hp) = P(W) / 745.699872}

T = HP * 5252 / RPM
HP = T * RPM / 5252
RPM = HP * 5252 / T

Ignoring slip, if we knew the exact wheel rpms, we could eventually derive voltage. The speedo's are quite accurate on a properly calibrated locomotive. They are adjustable to match wheel size as the steel wheel "tires" wear down. Knowing the exact wheel size, one can calculate fairly precise axle rpm from the indicated speed.

There might be another way to calculate voltage by realising the diesel rpm in "8 Throttle" (about 900 rpm) and supposing all 3000 HP of an old SD40 is "pegged" and being delivered as torque. What again thwarts an exact conclusion is at max torque the traction control is generally periodically cutting power to one or more axles and causing terrible voltage spikes. The max torque is always traction limited so voltage and amps quickly swap back and forth, so more powerful units did this even worse. At slow speeds, a 3000 HP SD40 unit essentially pulled no better than a SD60 3800 HP unit, or a 4000 HP MAC70, if they weighed the same. At high speed against the wind, the 4000 HP wins because it doesn't run out of legs.

The torque of a locomotive is simply directly related to the delivered traction at any given time. The voltage goes up as the amps and torque go down.

* My reasoning was that magnetism, therefore torque, is always directly related to circulating amperage on a similar diameter armature, whether DC or equivalent AC. The amperage doesn't care if it is driven by 1 volt or 20,000. Of course the wiring size, and subsequent number of windings varies by the chosen amps vs available voltage.

And, in keeping with this thread, they'll do it all in reverse too. :)

Wes
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