Head End Power

[Anonymous]

Is the electricity used in the passenger cars AC or DC? I have read several articles stating the same thing that HEPs generate AC then rectified to 480 VDC 60 Hz before being distributed to passenger cars.

Karn[:)]

[Anonymous]

Thank you M.W. Hemphill!!

Also why do passenger trains need to connect electrical system to yard power unit when the trains are at the yard? Are they required to have electricity while they are not in service??

Karn[:)]

[M636C]

Karn,

Your description of "480 Volts DC 60Hz" is by definition incorrect, since direct current has no frequency measured in Hertz.

Locomotives with solid state HEP systems take power generated in the locomotive traction alternator, with variable voltage and frequency, dependent on the engine speed and power output, rectify it to DC, generally about 1500 volts DC. It is then fed through an inverter, using Gate Turn Off thyristors (GTO), or Insulated Gate Bipolar Transistors
(IGBT) to provide 480 volts, 60 Hz. The conversion to DC is required to provide a fixed frequency for the train power system. Similar systems are used to provide variable freqency for AC traction motors, also requiring conversion to DC before inversion at variablr frequency. The Alaska railroad SD70MAC locomotives are understood to be able to operate either as six motor locomotives, or as four motor locomotives with the traction inverters for the other two motors diverted to provide HEP (at 480 v 60Hz). The inverters can produce either variable frequency and voltage, or fixed frequency and voltage as required.

British High Speed Trains use the variable frequency and voltage from a secondary winding on the traction alternator of one of the two class 43 power cars to power the train lights and air conditioning. The particular (1500 rpm) high speed diesel engines in these cars Paxman "Valenta" and VP185 engines, provide a frequency high enough that British (50Hz) equipment can be used (some with small modifications).

The Joint UP/SP/Milw "City" trains of 1937 were the first conventional trains to use HEP, provided by GM diesels in a separate power car. EMD builders photos often show this car coupled to the E-2 locomotives. The 1940 trains also used the same HEP arrangements.

Peter

[AntonioFP45]

Peter,

It's been a while! As always a pleasure reading and being educated by your posts. HEP always seemed a bit confusing.

I remember back as Amtrak gradually switched the long distance trains to HEP in the late 70s early 80s, those F40s really took a pounding as their 3000 horsepower, 645 diesels were constantly idling in Run 8 to keep the HEP going. Were they noisy! Engineers were not too fond of this, needless to say.

Cheers!

[M636C]

Yes,

I've been to the other side of the country and back chasing trains where applicable!

The F40 had to run at 900 rpm (notch 8) to produce 60 Hz power. The more recent locomotives with separate diesel generator units were quieter and much cheaper to run and to maintain.

HEP can't usually be run in "Multiple" unless the generators are precisely aligned to run at exactly the same frequency. So with two units on the train, one F40 would supply the HEP power and the other would have all its power available to haul the train. With dedicated generators not also providing locomotive power, controls to keep the generators in phase can be applied.

It is possible to 'Split" the train, providing part of the load down the cables on one side and the remainder down the cables on the other side, using two generator sets not in phase with eachother, (such as two F40s).

Peter

[Randy Stahl]

As I recall from my daze working in the Milwaukee depot "splitting"the train is exactly what we did. The difficultys of parellelling alternators was apparently too technical. although on the South shore we parellelled alternators all the time with light bulbs connected between the machines. I wasn't old enough or experieced enough to have a respected opinion but it always seemed to me that once the HEP voltage dropped down to a certain level, frequency was a moot point any how, so parellelling 2 machines would have worked on a long train.
It was funny to confuse new electricians... 3 phase D.C. we used that one alot, after all every rotating machine can only create A.C. We use a commutator to pick off the parts of the cycle we want on a D.C. generator. A good way to look at it is that if a car has flourecent lighting , it must have an A.C. power supply, direct current will not work.
On the F-40 PH I am familier with , stand by power was obtained from the AR-10.
Randy

[Overmod]

Randy's right: if HEP voltage is sagging, almost certainly either the frequency will be off (e.g. the engine is lugging down) or the AC waveform will be distorted.

Fluorescent lights, BTW work much more effectively at AC frequencies substantially higher than 60Hz. The 400Hz many aircraft systems provide, for example, give substantially less perceived flicker.

Amtrak's HEP is an almost classical example of the problems inherent in 'cutting corners' (although evolving technology is largely responsible for making the 'corner cutting' more obvious in hindsight than it was at the time!)

Attempting to provide precise "consumer-grade" AC frequency control by direct engine speed control -- assuming 900rpm as the engine-crankshaft speed! -- makes much better 'sense' when using dedicated generators than it does for locomotives. It's much cheaper to do that, though, than to go through the conversion to DC and then arrange for the necessary energy storage in the inverter components to get a clean AC sine wave (or even halfway-decent synthesized pseudosine wave) when the chief purpose (at the time) was to use lighting and AC components originally sourced to use 60Hz commercial AC. Motors and lighting ballasts are not particularly sensitive to the normal kinds of 'unclean' power that a traction alternator producing nominal 60Hz AC would provide, I think!

Obviously, the AC-to-DC-to-synthesized-AC approach is much better, and essentially allows the traction alternator to turn at any efficient speed. Note that tying this to DPU engine-control electronics would make still more sense than cobbling up something that uses the modal Run settings on the locomotive throttle (or as provided at the engine governor) to set the engine output speed independent of load.

More, Randy! More!

[Anonymous]

I remember when I was in the navy, the distribution cabinets for ship's power had light bulbs in bridge circuits that would give a quick indication if two generators were paralleled or not by how the bulbs were lit.

[Anonymous]

Oh, synchronus generators control their exitation by an outside reference to parallel their output. Engines in this application use an isochronus governor without "droop".

[Paul Milenkovic]

Ah, the head-end power question!

Back in the Diesel streamliner era (i.e. Heritage Fleet originating days), locomotives had "steam generators" (train heat boilers) and steam was trainlined for train heat. How they did the airconditioning I am not sure. There was something called steam-ejector airconditioning. How did that work? Lights were done with passenger car axle generators and with lots of storage batteries. Doesn't sound too cost effective. Was electric AC ever done off generators and batteries?

These systems had a reputation for not being very reliable. On the AC side, I can use my imagination as to the problems. On the heat side, my workplace is heated with steam heat piped from a central plant, but I guess there were issues with steam heat on trains, especially if the train was long and the winter was very cold, and then there was the possibility of frozen pipes.

For a while a lot of things were tried. The Santa Fe Highlevels each had a small Diesel in them to run the AC, but were probably steam heat. I rode a Highlevel Chicago to LA back in the early SDP40F Amtrak days, and I was wondering where that bus engine sound was coming from when on one station stop I got out and noticed a slender exhaust pipe going up the side of the car and louder bus engine sounds coming from a louvered area above one of the trucks.

Now you might think it kind of expensive to have a little Diesel in every last passenger car just to run the AC, but if you think of it, every freight reefer (and every truck reefer) has one of those little Diesels in it too. Think of it, you are not going to get head-end power on a freight! (Maybe that could be a cost advantage for Amtrak to hook reefers up to their trains to run off head-end power -- look, no reefer Diesel.)

Anyway, it was the commuter trains such as the C&NW bilevels that pioneered the head-end electric power from a separate Diesel generator in the space the steam generator used to be and to run all-electric cars -- both AC and heat were electric. I rode those bilevels, first to school and during summers to work, and there were never any problems with heat or AC at a time when Amtrak was having chronic heat and AC problems, hence Amtrak jumping on the HEP bandwagon.

I also remember the F40C on what was the Milwaukee Road commuter line (us railfan Northwestern commuters would sometimes walk over to Union Station to see the other trains). That was the first I saw (and heard!) of the constant speed main Diesel to run both the traction alternator and the HEP, a system adopted by Amtrak for the F40PHs.

My understanding is that the F40PHs were fuel guzzlers compared to the Genesis. Was the constant speed Diesel the culprit (the FDL 4-stroke is only a little bit more efficient than the EMD 567-610-710 2-strokes)? Don't suppose a constant speed Diesel is as bad as a Big Blow turbine, but running an engine at full revs may not be the most efficient way for part load.

I also read that the P32AC Genesis uses electronic HEP, but what do they do on the P40/P42 Genesis? The non-AC Genesis does not make the mile-a-minute sound of an F40PH in the station, and it makes that GE "chug-chug-chug" sound when it revs up for the train to leave the station, and I didn't hear the sounds of a separate motor generator set. What does the non-AC Genesis do?

Do you suppose trainlined steam heat may make a comeback? HEP heat is, of course, electric heat, and no one uses electric heat in their house these days if they can help it. If you have a 3000 HP locomotive and are tapping off 600 HP for the HEP to supply electric heat (my quick back-of-the-envelope suggests that the HEP is sized for the electric heat rather than the AC demand), that suggests that the HEP is a non-trivial addition to the fuel bill, and we all know how much gas, Diesel, and #2 heating oil is costing these days. Or do you suppose they will run the AC units as heat pumps?

[Randy Stahl]

There was a day when things were simple..... I love the older passenger cars, they operated almost entirely from batterys. They had spicer driven generators to charge the batterys and to provide for all the cars needs while the train was moving. Nothing like the smell of battery gases in the evening. I made the mistake of under estimating the old CN battery cars, I got 110 volts right from the battery... It really hurt !
Randy

[jchnhtfd]

Part of the reason for the F40's running at constant speed for HEP was that when they were being built, the AC/DC/AC conversion process was... um... er... pretty durn unreliable, to put it mildly. In the last few years -- well, decade or two (I'm getting old) -- the solid state devices used for the conversion have become sufficiently reliable and robust that the process described by Peter and others above is extremely reliable and does generate consumer grade electricity (and it's amazing what consumers will try to plug in...). It has also made possible AC traction, which requires -- as noted -- the same AC/DC/AC conversion sequence to work properly.

The steam heat/battery combination was very reliable, too -- and dead simple. But it does require a boiler. Which in the days of steam engines was right there, handy by. The train heating boiler on both diesels and electrics was a first class pain.

[oltmannd]

Do the ARR SD70MACs have 3 inverters? I thought EMD's AC design was two inverters - one per truck - but I've been out of that loop for the past 10 years....

[Randy Stahl]

As far as I know EMDs have 2 phase modules. I am waiting for some one to elaborate on that.
Randy

[daveklepper]

You are all wrong about "firsts." Erie 4-6-2's and other steam commuter power had oversize dc generators to power the lights in commuter trains as far back as I can remember, possibly introduced in the 20's. The usual axle-driven generators could not keep the batteries charged with stop and go commuter trains, so the oversized generators and jumper cables were installed. I think, if I remember correctly, this even applied to open-platform wood coaches and 1905-era 2-6-0's on the Boston and Maine! In use as late as 1953! Of course there was no air conditioning, and heating was steam from the locomotive boiler, but isn't even that a form of "head end power?"
Am I correct in saying that I rode C&NW steam commuter trains with the same arrangement in the summer of 1952? I'm pretty shure the power was dc but I don't know the voltage.

[M636C]

OK guys,

You've found the major flaw in my oversimplified explanation of the Alaska SD70 MACs. I wrote down what I'd been told, but after a while, I recalled that the EMD locomotives normally had two inverters, each feeding three motors.

It is possible that Siemens provide different inverters, one capable of powering four motors and one for two motors (or HEP) as required.

Alternatively, it might be possible to power four motors, at reduced power but adequate for passenger service, from a single "three motor" inverter, leaving the other unit to provide only HEP.

Without any data to support either option, the second sounds more likely to me given the desire of locomotive builders to minimise changes in design.

I believe in India they have an EMD model GT46 MAC, basically an SD70 MAC cut down in height and modified for 5'6" gauge. There is also a passenger version, GT46 PAC, which is basically the same but has only four motors being a 1Bo' Bo1'. I hadn't thought about why it only had four motors until I heard about the Alaska SD70 MACs, but clearly, the other inverter is being used for Head End Power.

This doesn't help explain whether standard inverters are used, or whether EMD provided a "4+2" inverter arrangement for Alaska (and India). However, If you were providing special inverters, why not put in an extra one for HEP, leaving two standard units for traction power. So the more I think about it, the more I think that standard inverters are used in the Alaska units.

Remember that the same inverters and motors are used (as far as I know) in the 6000 HP SD90MAC, so there may be a reserve of power in the inverter to power four motors on a 4000HP unit.

This would be relatively ineffecient use of a big traction inverter for HEP, but it will certainly work!

I believe the P32AC DM has five identical inverters, one for HEP. In case of failure, any inverter can be used for HEP with that motor being unpowered.

I don't know about the P40 and P42, but a single inverter of the P32 type would be able to do the job, and the two types were designed at the same time. But I don't know - I never thought about it!

In Australia, we had a number of air conditioned cars that relied on axle driven generators for power. Unlike the old leather belt and small generator used for 32 volt DC lighting, this had six heavy duty vee belts driving from drive wheels mounted on the axle to a stub axle with a bevel gear that drove a DC generator that charged batteries. These then ran a DC to AC motor generator set that ran the air conditioning and the fluorescent lights. This equipment came from Stone of Deptford, England, and was also used in Malaya. The Australian cars were eventually converted to HEP. We had HEP trains in Australia from 1949 (the Stones system dated from 1936), and the HEP was powered by two Detroit 6/71 75kW generators mounted in an end car. One generator could usually carry the load. The locomotives were 1943 steam locomotives (4-6-2s), so there wasn't any significant electric power available there.

Sorry if I oversimplified my initial explanation!

Peter

[Randy Stahl]

I recall seeing photo's of Milw steam locomotives with oversized dynamo's. I also remember reading that many baggage cars came with dynamo's for train lighting. I don't know much about these cars. I wonder if any have been preserved? I think this was the first step in electric lighting for passenger trains. I think electric headlights were even a new thing back then.
Randy

[M636C]

There was a very short branch line North of Canberra from the main line to the town of Yass in NSW. They had a single passenger car powered from the locomotive lighting generator, because the car didn't run far enough to charge the batteries.

Peter

[Eddystone]

From reading these posts it seems that the output of the HEP unit and the electric load of the passenger car limits train size unlike the old steam heated equipment. What would be an average train for 1 F40?

[M636C]

There are at least two types of HEP alternator on F40s, at least on Amtrak units. There may be other variations on Commuter units, apart from the later MBTA units (and others) with separate HEP diesel alternator sets.

The first Amtrak units (200 to 229) were built with a 500 kW HEP alternator, and were intended for smaller short distance services with Amfleet cars. From somewhere I recall that this was adequate for about 12 coach cars. The later units, which were intended for long distance service, had 800 kW HEP alternators and larger fuel tanks. The position of the tanks changed to assist longitudinal balance (along with the heavier HEP alternator up front). Diners, Superliner lounges with their dome-like glass roofs and sleepers with showers would require more power than Amfleet coaches, so I can't comment generally on how many cars an F40 can supply.

I had to check these figures (in the "Field Guide"), so I looked up the P40/P42 as well and noted that a GTA 33A1 HEP alternator was indicated. From comments in earlier posts to this thread, it is possible that this is allowed to run at variable speed and feeds through a static frequency converter, a simpler form of inverter that only provides 60 Hz power at 480 volts, three phase. GE certainly can make such equipment, the Boeing F/A-18 Navy fighter being fitted with two very compact alternators of this type (producing 400 Hz power) driven by the GE F404 gas turbine engines, which have worked well for more than twenty years.

Peter

[DRBusse]

While "head end power" is the defacto railroad definition of "HEP", the more correct engineering definition is "hotel electric power" which more accurately describes what HEP really is...electricity in passenger-carrying vehicles required for non-propulsion uses.

You'll see "HEP" used in engineering descriptions of modern cruise ship power plants, because most cruise ships have computer-controlled systems that allocate certain amounts of power for propulsion and certain amounts for "HEP" -- keeping the lights and a/c on and margarita blenders running. Last cruise ship I was on had something like 8 Sulzers...how many of 'em were actually running depended on whether ship was underway, docking (where use of many bow and side-thrusters creats more demand for juice), etc. Italian ship's engineer diagrammed it all for me in pidgeon English with liberal use of "HEP" term after I asked why several engines could be heard in startup mode just before docking (in St. Lucia, if I recall), yet previous night had heard engine in "shutdown" while underway (at less than 5kts in calm seas for a short trip).

The term was explained in an issue of Trains (1980s) involving story of Superliner design. In the world of Amfleet and Superliners, railfans and railroaders assumed HEP was a railroad-specific term involving generators on head-end of train...but it ain't.

[M636C]

I've been thinking about the earlier references to F40PH units as gas guzzlers, and my mind made a number of connections to running engines at high speed and low power. It occurs to me that although the engine is running at 900 rpm, the low power involved would mean that the EMD turbocharger would not be getting enough exhaust pressure to "lift off" and would be taking quite a large proportion of power from the crankshaft, so the specific fuel consumption would be as bad as it gets, since the blower would be providing more air than needed anyway. This also reminded me of one of the bad features of the M636, that the engine had to run in notch 8 and low power when the units were in dynamic brake, because the dynamic brake resistors were cooled by the central blower rather than by an electric fan in series with the resistors, as is EMD and current GE practice. The crews used to tell me that the (Alco) engines were "shaking themselves apart" in this setting. This might relate to the relatively large number of stored Amtrak F40PHs that were unserviceable, more than would be expected for the usual EMD unit which can put up with a lot of use and poor maintenance (as in Egypt, for example).

And in a not immediately related thought, I did an analysis of the relative fuel consumption penalty of running GE Marine LM2500 gas turbines at synchronous speed and low power as part of a study of the possible costs of running turbine-electric warships. There was a serious increase in fuel consumption involved. I put this data to Rolls Royce, who were suggesting electric drive in conjunction with their recuperative WR21 turbine. They never did give me low power synchronous fuel consumption figures for the WR21, but the problem, running the power turbine at high speed and low power, still requires more fuel than runnng it at low speed and low power.

I hope that's not too confusing!

Peter