April 26, 2024

# Locomotive auxiliary power FAQ

## Q: Why is 72 volts used in locomotives for lights?

A: The auxiliary battery on a locomotive consists of 8 x 8 volt batteries. So the nominal voltage is 64 volts, the open circuit voltage is 68.8 volts, and the float charging voltage is 72 volts. So 72V is about as high as the locomotive's power bus can get. It is really a 64V battery. Sometimes the "boilerplate" will list 74V, but it is still a 64V battery.

## Q: What voltage are the train's lights?

A: The train's lights are rated at 75 volts, which is equivalent to 14V for a 12 volt battery.

## Q: Can an engineer use the 72VDC to charge his cell phone?

A: Yes, if you find the right charger. Some of the AC cell phone chargers are very flexible on their input voltage. Switchmode power supplies immediately rectify the AC, so they will run on higher voltage DC. The problem is that 72V is pretty low, since even rectified 90VAC is 127VDC. But some of these chargers will still put out a regulated 5V to charge the cell phone when powered with 72V. But of course not all of them will. There are DC converters that are rated for 72V input and work very well for powering 12V and 24V equipment in a locomotive.

## Q: Train light bulbs take the same socket as household bulbs. Will they work at home?

A: They use the same socket, but their filaments will vanish in a puff of smoke when used at 110VAC.

## Q: Why 72V?

A: The higher the voltage, the lower the resistive losses. Using the same wire you can transmit 4 times as much power with only twice the voltage. But as we all know, when the voltage is high enough it can give you a nasty shock. 72V is at the ragged edge of being in shock territory. But because it won't kill you it is safer to run around the cab, and doesn't require the safety codes of higher voltage.

## Q: How much power does it take to start the locomotive's diesel engine

A: The engine can take as much as 25 horsepower of electrical starting power. This is about 19000 watts. (If it used 12V like cars it would take 1600 amps).

## Q: Do diesel-electric locomotives use 72V?

A: The diesel-electric system runs on 600V. But the auxiliary power is run off of 72V alternators. Other drive voltages have been used, 750V is common, and 3000V has been used. But these high voltages are limited strictly to the engine-motor compartment.

## Q: Has the locomotive voltage always been 72V?

A: No, it seems that many steam engines had 32 volt dynamos. In that era passenger cars were also 32V and often had their own batteries and generators. Some of these generators were axle-driven. Cars with AC tended to be 110V because of the convenience of using 110VAC air conditioners.

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