I have wondered about this for a long time. Why do they use 74 VDC for the low voltage in locomotives? It’s such an oddball voltage. And why DC? AC would seem much more practical, especially with all the various electric / electronic devices.
It is probably so the lightbulbs cannot be taken home and used by the crews there.
[;)][:-,][zzz]
Sam
Just a wild guess, here…
If I remember correctly, most passenger car electrical systems were (before HEP) powered by batteries that were recharged by car mounted generators when the car was moving. I believe these batteries were 12v DC, so the electrical systems were in multiples of that. I would guess that most on-board locomotive electrical systems that were shared with other equipment (lightbulbs, maybe some small motors) were originally developed to use the same current, rather than introduce another parts inventory. Once steam engines (with onboard turbos) and steam heated equipment (with on board generators) went away, the diesels were already set up for DC, so it just hung around. (I know 74 is not an even multiple of 12 but my guess is that it’s like household appliances that can be rated in a range from 110-120v AC).
So, did I get any of that right? Do I qualify as an ametuer industrial archealogist, a conspiricy theorist or just plain delusional?
Actually, I think car batteries and similar types run alittle over 12 volts- 12.4 to be exact. Therefore, if you had a half dozen connected in series, you’d get 12.4 x 6 = 74.4 volts.
The locomotive system is (8) 8 volt batteries for a total of 64 volts, the charge voltage is 74 volts. The 10 volts difference in battery voltage/charge voltage is the same ratio as 12 volts/13.8 volts in a car. We have always used higher voltages in my industry to reduce wire size, reduce voltage drop and provide starting/crank power to heavy loads, I suspect that is part of the reason here.
I believe you’re right. The control wiring is 14 gage as it is.
Early diesels’ starting motors were the main generator. A 600 volt machine won’t turn very fast on 12 or 24 VDC, but it does OK on 64 VDC.
'Aight, engineering students turn at explaining it…
Nothin more oddball than the 12v of your car, or the 6v of old cars, or the 24v of military trucks. You just happen to be more familiar with 12v All are multiples of the 2v units of wet cell batteries.
Why would you want a certain voltage, (and usually high)? Any certain device requires a certain and exact amount of power to drive it. This is measured in watts and the basic formula is Power (watts) = Voltage (volts) x Current (amps). Since the wattage remains fixeds for any given load, and you always want current to be as low as possible, voltage must go up.
The same device will use half as much current at 24v than at 12v. Locomotive elecrical systems would use MASSIVE currents at 12v for example. Solution is to up the voltage and the current naturally drops. Advantages include smaller diameter wire which saves money ('specially big honkin copper ones).
DC systems have their advantages over AC for certain requirements and are used how they are to be most cost effective.
Adrianspeeder
Look forward to 48 volts as the new automotive standard. All the electronics are increasing current draw such that wiring needs to be increased. Wire increases cost and weight. 48 volts and a digital control bus will eliminate about 70% of the wiring in current designs.
dd
Wait till the bailing wire mechanics try to jump start one of those.[(-D]
As has been said, nothing really special about 74 volts, except that it is a convenient voltage (8 8 volt batteries or, to put it another way, 32 lead acid cells in series). It was also used in some telegraphy and early telephone (modern is about 90 volts, still DC. Not much current available, but it is possible to get a jolt off a telephone wire), according to one of my old texts. You don’t want to go much lower in a diesel – it takes a lot of amps, even at 74, to crank a big diesel, and lots of amps means big wires, and big wires are a pain.
Why DC? Because that’s what batteries store and produce. And that’s what you’re after here.
Try the new hybrids - it’s more than 74 volts, and lots of amps. We in the fire service are casting a wary eye at them, especially since they’ll get up and go without the engine running.
Actually telephone is -48volts with 110volt AC ring signal.
BBBZZZZZ WRONG! [:D]
POTS (Plain Old Telephone Service) (usually analog) is -48VDC when referenced to ground. Ring voltage is 90VAC (when referenced to ground) and this is applied only to the POTS circuit during ringing. Repeaters used high voltages to drive the signals over longer cabling, but the voltage deliver at the telephone would still be -48VDC/90VAC. This by no means is modern technology.
Today’s modern technology (Ethernet, DSL, T-Carrier, etc) uses different voltages and a digital carrier methodology.
I wrote this answering a question on another boar but though I would throw it out here to expand on the digital carrier crazydiamond mentioned…
Railroad radios are 15Khz wide single sideband Frequency Modulation.
FM=Frequency Modulation
FM means that the carrier of a certain center frequency and a fixed amplitude is “Modulated” by the “carried” signal (baseband audio in this case) by shifting the phase (frequency) of the carrier. So with FM signal the amplitude is constant but the frequency varies.
AM=Amplitude Modulation
AM means that the carrier frequency is fixed and the “carried” signal modulates the carrier amplitude (Level). So with AM the frequency is constant but the amplitude varies.
FM
2-way radios
Analog cell phones
FM radio (obviously)
Television audio carriers
Television color carriers
Long wave radios
Short wave radios
AM
AM radio
CB radios
Long wave radios
Shortwave radios
Television video carriers
Digital uses a combination of AM and FM called QAM, Quadrature Amplitude Modulation. Digital is a series of ones and zeros. Instead of useing a simple signal to convey the 1s and 0s they use a complex signal with a “constelation” of points that indicate a group of bits, multiple 1s and 0s. A basic 4 point QAM can indicate 4 possible combinations, 00, 01, 10 or 11. Every time you add another amplitude or frequency element you double the points in the constalation and get one more bit of data out. In reality these “constalations” are much bigger. A 64-QAM has 64 points and gives 6 bits of data out at a time.
- the reason they call it a constalation is when you look at the signal on a vectorscope it looks like a constalation of stars (although symetric not random).
(1) Wrong. QAM is QAM…all carriers are analog in nature but are not considered a ‘digital signal’ until they are modulated using a digtal signal.
(2) Correct. A digital signal is one that carries digital information.
Therefore AM, FM, PM, QAM, can be considered analog or digital. One of the earliest forms of digital FM was FSK (Frequency Shifted Keying). For example a 5KHz signal is the baseline carrier. We shift down to 4KHz equal logic 0. We shift up to 5KHz equal logic 1. All the older carriers AM, FM, etc had fairly signifcant weaknesses, so newer carrier techniques were (and still are being) developed to: (a) make the carrier less susceptable to interference, and (b) get more information through a given size (bandwidth) carrier. [:)]
Not wrong. QAM is a modulation technique for digital signal. I’m not saying all digital signal is QAM but it’s the most common type used for high bandwidth carriers. And it’s Phase Shift Keying.