Western Railroad Discussion > 72 volt question on locomotives

Hello

Someone asked me tonight at work couple question that I couldn't ansewer, I was wondering if anyone could explain?

How come locomotives have 72 volt lites not 110 volt like houses?

Is there any history behind the choosing of 72 volts?

Didn't passager car run on 72 volts?


Chris

the 72 volts, I have also seen a badge plate for 74 volts, is generated by the auxiliary generator that runs off of the diesel engine ..that generates the electricity for the lights, refrigerator and other uses.

The batteries are 8 volts each, a set of 8 on a locomotive for a total of 64volts DC. Aux. generator puts out 72VDC for charging, running lights and control circuits. Most bulbs today are 75V, although on older locos, headlights were 30V and aux. bulbs were 75V.

Since we are talking about electric things on the locomotive, then I have a question.
I have seen a lot of engine crews using the outlet that is available on some locomotives and charging there cell phones.
Will this fry your phone or do any damage in the long term? The people I have seen doing this say nothing bad happens to their phones.

I've seen a lot of them use the charger for cell phones, and it seems to work fine. I've also seen custom made pigtails that tap into the 110v outlets for the refrigerator. I remember back when they had number board doors that you could open from inside the cab. We woule unscrew one of the bulbs, and screw in one of those two prong outlets in it's place. We all carried a plug in water heater, and would set a bottle of water up there by the lights, and heat it up for instant coffee,tea, bullion cube or instant soup.

I'm not quite sure why the 64 volt system but many marine applications are 32 volt as were passenger car systems. GE put 32 volt systems in their industrial locomotives meant for military and industrial applications where they used 64 volt systems if they were meant for railroad aplications. An example would be a 25 or 45-ton unit which is an industrial model would be 32 volt...where a 44-tonner or 70-tonner built at the same time would have a 64-volt system. Most likely 64 volts is just doubling the quantity of 8-volt batteries to arrive at the higher voltage system.

The underlying reason for the higher voltages is you get twice the horsepower out of 64 volts vs 32 volts for the same gauge of wire; thus, larger displacement engines that draw more current when being cranked can get by with more smaller batteries to get the required cranking horsepower. A typical EMD switcher's starting circuit is fused at 400 amps and the 400 amp fuse usually is good fdor normal starts but sometimes will blow under extremely cold or prolonged starts. Assuming the starting system draws 350 amps at 64 volts you have 22,400 watts or slightly over 30 horsepower (22,400 / 746 = 30.026 hp) cranking the engine (theoretical). If the locomotive had a 12-volt system, such as in your automobile) would draw 1,867 amps to do the same job.

The 74Vdc standard has been around since the days of steam. One thing it does is to reduce the theft of railroad light bulbs for use around the house. While the RR light bulbs will indeed screw into the household lighting fixtures, one flip of the switch and POOF! the 'free' light bulb is gone.

A lot of newer locomotives have 110 volt outlets in cab
The 74 volt was chosen because of electrical code governing power over a certain tresshold to be labeled as high voltage.
the control voltage going from locomotive to locomotive is never higher than battery power.
the actual power is 64 volt, the charging power of the aux generator is 74 volt.

Cjcheely Wrote:
-------------------------------------------------------

> Didn't passager car run on 72 volts?

In the pre-HEP "battery box" era, the passenger cars voltages were either 32, 64 or 110 V, depending on individual railroad preference.

Bob

I bought a locoverter that coverts the 72/74 voltage. It's nice when your sitting in a siding and I'm able to use my laptop without worring about my battery running out.

Doug Wooten
UP Engineer
Kingwood, TX MP 21.5 UP's Lufkin Sub.

If I remember my high school electronics class correctly (from 100 years ago with Mr. Marconi), 72 volts DC is the root mean square of 110 volts AC. What that means is that if you rectify 110 volts AC to DC, and add the upper and lower portion of the sine wave together (the positive and negative portion of the alternating current), it's average is 72 volts DC.

How things have changed. Back in the steam days the steam dyno was 32 volts. Passenger cars were 32 volt and had batteries and generators that were hooked to the trucks. The big steam engines had two or three to run all the electric stuff. Example, 4449 was built with three. One for lights, one for the Mars light and one for the electric braking they came out with.

Then came the diesels with 600 Volt DC generators. They needed low voltage for lights and had 74 Volt. Most of the relays were air operated back then. Then when they started using electric relays they were all 74 volt. As time went on, they installed alternators for the cooling fan motors (AC) but still kept the 74 Volt DC for everything else. The SP engines had a 74 volt plug in the breaker case and a lot of guys used that to heat coffee, etc. Some of the coils didn't last long but they were cheap. They stayed with that voltage right up to the new ACs.

Every engine still has 74 volt because all the MUs are that voltage. Someday that may all be gone but not in the near future.



Nudge

mojaveflyer Wrote:
-------------------------------------------------------
> If I remember my high school electronics class
> correctly (from 100 years ago with Mr. Marconi),
> 72 volts DC is the root mean square of 110 volts
> AC. What that means is that if you rectify 110
> volts AC to DC, and add the upper and lower
> portion of the sine wave together (the positive
> and negative portion of the alternating current),
> it's average is 72 volts DC.

Sorry, but not even close. Time to go back to class. :-) The DC voltage in a locomotive bears no mathematical relationship to the AC voltage used in buildings.

If you were to rectify and filter 115 volts AC, you'd wind up with about 81 Volts DC.

As an electronics engineer, I find the explanation of using eight, 8 Volt starting batteries, and charging them to 74 Volts makes total sense.

The reason for 72 v (nominal) is that it's the highest voltage which won't cause shocks (unless you're that rare individual who is hypersentive to electricity). This has been ensconsed in the NEMA code for a long time, although probably not exactly in this language, which code BTW does /not/ apply to the railroads. As mentioned earlier, passenger cars of the steam heat/axle generator era were 32, 64, or 110 v DC depending on the owning railroad. Lightweight cars with electrco-mechanical air conditioning, like the CZ, tended to be 110 v because this cut down on the amount of copper needed to run the AC compressors.

>Every engine still has 74 volt because all the MUs are that voltage. Someday that may all be gone but not in the near future.

They can't even decide on how to change the 27 point MU cable, although they've been trying for several decades.

>The batteries are 8 volts each, a set of 8 on a locomotive for a total of 64volts DC.
>As an electronics engineer, I find the explanation of using eight, 8 Volt starting batteries, and charging them to 74 Volts makes total sense.

A fully chaged lead acid cell puts out about 2.1 volts. In your 12 v car battery, there are 6 of these cells making for 12.6 volts. Since charging a cell requires more than 2.15 v, the alternator in your car is designed to put out voltages as high as 14.4, particularly at high charging rates. And yes, your headlights, radio, etc all run at 14.4 volts when the charging rate is this high, hence the term "12 v nominal".

In a locomotive battery rated at 72 (or 74) v (nominal), there will be 36 cells total, wired in series. This means two "batteries" of cells of 18 cells each, wired in series. A charging system is designed for the total battery voltage, not any other grouping of cells, so at high charging rates, the voltage reaches the 82-85 v range.

Common usage of "battery" is that it's an electrical device, but it actually means "a grouping together", as in a grouping together of a number of lead acid cells or a grouping of guns, a "shore battery", for which Battery St in both NYC and SF are named. Decades ago when electrical generating stations and substations used batteries for peaking (the high loads of the rush hour--the NWP electrification had these), they were large agglomerations of glass jar cells in huge rooms, all wired together, but the whole assemblage was "a battery".

The shop staff at Cranbrook BC took a bunch of single element hot plates, and removed half the wire in the elements.

They worked for many years for heating tea and coffee, and were stored on shelves at the stations where coal train crews went to work. The crews supplied the coffee pots, and the shop staff provided the hot plates. The hot plates outlasted the M630 diesels originally used on the coal trains.

The yard engine at Nelson had a much more elaborate arrangement, the same idea with an electric teapot with half the wiring removed. The old MLW S-series switchers did not have plug ins, so an extension cord was plugged into a cab light.

The teapot and cups were stored in a special wooden cabinet. At tea time, the engineer would join the rest of the crew in the switchmans shanty.

It all seemed very civilised, sipping freshly brewed tea in a small trackside shed next to Kootenay Lake.

By a bizarre coincidence, the day yard engineer was named Earl Grey (name of a very popular tea blend)

Not all passenger cars had 32 volt DC systems. Some cars were built with 64 DC volt systems, and some with 110 volt DC systems.

In 1959, CN built 12 new business cars but they used some second hand electrical equipment from retired coaches. The car I am familar with, TIOGA PASS, was built with an axle generator (spicer drive) that generated 72 volts DC and fed it into a 64-volt battery pack. Everything on the car was DC powered, even to the point the CN purchased and converted toasters to run on DC instead of AC. Then, they decided to add a diesel generator to these cars which meant that the output of the gen set had to run thru a rectifier to make it DC and feed the batteries. Later, a side by side refrigerator was added in the galley (it is not known how they got it into the galley as it won't fit down the hallway) so an inverter was added to power the refrigerator and the dishwasher. Then they added AC lighting in the lounge area and dining room, which required even more AC.

The car ended up equipped with multiple invertors and rectifiers, all manipulating electrical controls straight from the 1940's. The electrical sytem finally gave up the ghost in 2005 and we rewired the whole car to run on 110 volt AC with a 60 KW diesel generator, or on Amtrak 480V current.

Other cars of the 12 built in 1959 ended up with 110V DC systems, but the car assigned to the CN's chairman was all 110 volt AC. In short, there were no standards - each location seems to have done what they thought was best.

Norm

Radar,
Things have changed some since I was in school. House voltage is 120 Volts, and is expressed as an RMS (root mean squared) value. That being the case, the Peak voltage is 170 volts peak, or 340 volts peak-to-peak. For purposes of discussion, to determine the unfiltered DC voltage resulting from a full-way rectifier, the formula is = 0.6366 x vp, where vp is the peak voltage, or equal to the average voltage when calculated from the peak voltage. Thus the unfiltered DC voltage resulting from a full wave-rectifier would be 0.6366 x 170 or 108 volts. This assumes a relatively pure sine wave, and no filtering, or a battery that would act like a filter. To answer the original question, the 72 volt choice is all about reducing IxR voltage drop in copper conductors, thus allow smaller conductors with higher voltage to achieve the required power for the starter motor. Years ago automobiles went from 6 volts to 12 volts for the same reason. I suppose we could add to the confusion by discussing the 600 volts used for the DC traction motors, or the AC voltage used for the AC units.

>I suppose we could add to the confusion by discussing the 600 volts used for the DC traction motors, or the AC voltage used for the AC units.

OK, in 1880 Sprague selected 550 volts (DC) because higher voltages tended to cause "serious discomfort". This was raised by 1900 to 600 v. By 1910, came 1200 v which eventually got raised to 1500, 2400, 3000, and 3300 v DC. Today's "light rail" is 750 v, except for the older operations like the SF Muni, MBTA, and SEPTA, and some of these may even be upgrading to the new voltage. The reason for the AC-DC drive in diesels is that the DC generators were getting physically too big for the carbody, and in order to raise electrical output, "they" decided to do it by raising voltage, which is a snap with AC, so to speak. What made it all possible was the solid state diodes (the E-33s and early E-44s has ignitrons, complete with water cooling; I believe all were retrofitted). AC-AC drive has been made possible by mircoprocessors which are used to change the propulsion current frequency. The electricity is created as AC and rectified to DC, which is then chopped up into 3 phases and varying frequencies and fed through some smoothing reactors.

I used to work in telecommunications, where the standard battery voltage is 48 VDC, here again, not high enough to cause a dangerous shock to personnel. Like the abovementioned peaking battery at the NWP, they're usually made up of glass-jar cells, although some of the newer installations are horizontally mounted gel cells. They come in many sizes, ranging from about 100 amp hours to 2400 AH. The engineers specify a battery with enough capacity to run a comm site or central office for at least 10 and sometimes as much as 48 hours depending on the remoteness and importance of the site.
Getting back to locomotives, some industrial switchers use 24 VDC electrical systems, usually if they have truck-type engines. At OERM we have two GE center-cab switchers; one is 72 volts with two Cat diesels and the other is 24 volts with two Cummins. Between my pickup and another member's sedan, we were able to "jump start" the Cummins unit when its battery was depleted.
The old way of starting a diesel is by applying the 72 volts to the generator, which acts as a motor and spins the crankshaft. When locomotive designers went to alternators with rectifiers in the 1960's, this wouldn't work anymore. I remember the first time I was in an F-45, noting that it had two very large starter motors, and a warning that if the "mill" hadn't fired up after 20 seconds, stop and let the motors cool for 2 to 5 minutes. I suspect the motors were 24 volt and hitting two in series with 64 to 70 volts would be rather "stressful".
One of the challenges of dealing with 70 volt electric systems was for the radio designers to develop a power supply for a two-way radio that would work on this oddball (to the rest of the world) voltage and, of course, survive in the hostile railroad environment. I was even more fun back in the days of vacuum tubes, but they did get the job done.