Why is DC current used for traction in railway locomotives? Why not AC?

Question

Why is DC current used for traction in railway locomotives? Why not AC?

Answer 1

More power from dc with less voltage

Answer 2

Your assumption is incorrect. By far the greatest proportion of electric railway locomotives in the world - and indeed multiple units - are powered by AC current. AC has been used for many years as it is far more efficient than DC. In the UK the old London, Brighton and South Coast Railway started a programme of overhead electrification of its lines at 6600V AC well before the 1st World War  and converted a considerable part of its suburban network. On the grouping in 1923, however, it was discontinued in favour of the 660V DC electrification already under way on other major parts of the new Southern Railway. However, other companies had also opted for AC overhead electrification (the London and North Eastern Railway, for example, on its suburban routes), albeit at varying voltages and AC was soon adopted as the norm. AC electric locomotives have been in use here since the 1950s (the answer which suggests such did not come into use until the 1990s is totally incorrect).  In fact nearly all railway electrification, wherever in the world is now on AC. DC is only used by a few metro systems - here in the UK on the London Underground and on the vast system south of London extending to the Kent, Sussex and Hampshire coasts - where modern technology means that DC units can travel at up to 100mph. Elsewhere in the world, notable systems that use DC are the New York and Chicago systems and the Paris metro.

Answer 3

AC technology in Traction is a relatively new development in terms of railways. When electric trains were first introduced, AC technology was not completely deciphered or understood in the early years of electric locomotives. Hence existing rail systems still carry on the usage of DC for traction. The newer electric systems worldwide use AC as usage of high voltages (in terms of tens of thousands of volts) allows the use of low currents which means that the power transmission costs are reduced. However, converting an existing DC system to AC involves cost and cannot be achieved overnight. There are other challenges which come in the way.

In some countries, as the AC technology is becoming cheaper, existing rail systems which run on DC are  being converted to AC.

Hope this helps.

Answer 4

DC current is the most energy efficient way to power them and is lower in voltage.  AC currents only advantage is that it can be sent long distances with less loss.  Look at your automobile, you have an alternator that generates ac power and then converts it to DC for use in the vehicle.  Freestanding locomotives are diesel electric, where the diesel engine runs a generator to power the DC motors driving the train.  Cruise ships use similar technologies to drive motors that power the vessel.

Answer 5

Newer rail technology uses only AC.

Main problem with DC is higher transmission loss than AC.

Answer 6

Good question, AC is more efficient, but harder to control. The newer locomotives are mostly AC.

I am guessing that your question referring to traction means the traction motors and not overhead lines?

DC is easier to control to the traction motors and until microprocessors were made that were fast and reliable enough to work, it was not feasible. The power to traction motors needs to be modulated instantaneously in order to provide maximum traction control.

We have had AC locomotives for quite a few years now, and the first series that I was on were made by EMD, the SD-70 MAC and they werent nearly as good as the newer series. As a matter of fact I hate seeing them in my consist.

They had serious traction control issues and were no improvement over what they were replacing.

It was just a matter of available technology being adapted to fit the need.

added later: I tried and tried to cleverly put an ACDC song title in my answer, somethign like "AC does the dirty deeds dirt cheap" but I just couldnt think of anything LOL.

Answer 7

Rainy weather.

Answer 8

AC locomotives have been used since the early 90's.  General Electric makes several versions with the CW-60 AC being the most powerful at 6000 horsepower.  EMD makes the SD 70, 80, and 90 MAC.  The MAC stands for modified, alternating current.  These locomotives are more efficient because the traction motors use less moving parts to repair.  The AC traction motors are more reliable and have eliminated ground relay problems.  GE's newest version is a fuel efficient version that uses a 12 cylinder diesel engine rather than a 16 cylinder engine.  This engine also has the ability to shut itself off when it is not needed and restarts automatically.

Answer 9

they do use a/c current.

Answer 10

Firstly I think the asker is talking about AC -v- DC Motors rather the means of transmission. Older trains take AC or DC from the supply, and convert to DC to feed to Syncronous motors.

Newer trains take AC or DC from the supply, convert to DC - then invert it to Variable Frequency AC to drive Asyncronous Motors

AC Motored trains have been around since the 1960s in Europe, The Swiss BLS and Deutsche Bahn both experimented with the technology.  In the UK the first AC motored units were the Class 465 networkers and the Class 323.

Now I'll quote 'Cathode Ray' From UK Railway

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Whilst the three phase [A] motor is extremely simple, rugged, robust and

has a high overload capability, to exercise variable speed control is

electrically complex. Traction engineers realised very early on in the

development of electrically propelled trains that the three phase

motor was their holy grail becuase of its virtues, but controlling its

speed and torque over a wide range was generally beyond the

techological means until the mid 1960's. Numerous complex

electromechanical, electronic, valved and plain madcap ideas were

tried on rolling stock before then but most fell at the first hurdle

or required an accompanying road van full of spare thyristors.

The development of power electronics in the 60's allowed both the

voltage and the frequency applied to the stator windings to be

individually controlled at the voltage and current ratings required of

a traction drive and for the first time series production of three

phase motored EMU's and locos was feasible. Ongoing development shows

how technology marches on, on a DB BR120 loco there are three

inverters, each of which originally contained 120 thyristors. A modern

inverter of similar rating will generally use either 6 or 12 devices

(which are more likely to be IGBT's than thyristors).

In an asynchronous motor the rotational speed is determined by the

frequency applied to the stator windings and the torque is

proportional to the voltage applied to the stator (to be technically

correct the applied voltage gives rise to a current flowing in the

stator windings and torque is proportional to this current - in the

early days of such drives it was much simpler to make a voltage source

inverter than a current source inverter).

A typical EMU converter will generally produce an output of between

0-200Hz and 0-400V, although values of 0-300Hz and 0-600V are also

common.

At starting a voltage needs to be applied to the stator which will

allow the production of high stator currents (and hence high torque)

but at a very low frequency. It is almost impossible to ecomonically

and directly drive the power electronics at frequencies below say 5 to

10Hz at the voltage and current levels required, thus another method

of creating these low frequencies is needed.

To achieve the low speed frequencies and to achieve finer torque

control over the enire speed range, most inverters use pulse width

modulation whereby the mark-space ratio required to synthesise the low

frequencies is overlaid onto the basic switching frequency of the

inverter.

The noise ['Gear Changing'] to which you are referring is typical of that produced by

GTO based converters which use low basic switching frequencies. The

sequence of musical tones [produced by Siemens locomotives] to which you and Arthur refer to is the

pulse width modulation gradually being widened over one basic

switchning frequency then the switching frequency being changed and

the PWM again being widenend over that, new, frequency range.

Believe it or not, this is given the term of "gear changing" - yes I

know, we electrical engineers owe debts of gratitude to our mechanical

bretheren. It can also be heard on Class 323's and 465's as well as

the JLE stock here in the UK.

In short, it allows fine control of torque and speed without having to

directly drive the converter at very low frequencies. It also has one

other ace up its sleeve. No matter how hard you try, some of what is

going on electrically at the output of the inverter will end up being

reflected back onto the input, be it AC or DC. Some of the frequencies

or their harmonics on the motor side could cause interference to other

railway infrastructure such as track circuits. There are various ways

of tackling this problem without going to the bother of having every

coach full of filtering components, the most sucessful of which is to

operate the converter at a high enough switching frequency (such as

4kHz) such that the junk coming out is easy to filter out and if it

does escape puts at risk very little equipment and this is why modern

drives use IGBT's - they are capable of handling these frequencies

whereas GTOs are not.

If you are building a thyristor based inverter you can use gear

changing to your advantage by simply missing out the basic switching

frequencies that either they or their harmonics would cause

interference to other equipment. If the switching frequencies of the

323 are examined, Holec cuningly conceived a notched pattern which

allowed the drive to miss all of the forbidden frequencies in the

equally forboding specification BR1914 (see note 2), which governed

what was and wasn't allowed in terms of interference currents and

hence the unit was the first three phase drive train in the UK to have

a nationwide safety case.

Ray

Note 1: Just about everything electrically propelled these days uses

three phase drives. There are some notable exceptions such as a

sizeable chunk of North American loco production for backward

railroads, as well as those produced in some "lower tech" economies,

although I'm struggling to google many. A colleague was involved with

the production of some 25kV freight locos for Azerbaijan, built in

China, where most domestic output was for three phase drives. Bharat

Heavy Electricals of India will still make DC drives for

refurbishment, but its current product line is all three phase.

Presumably all electric rolling stock production in North Korea is

three phase but Kim il Jong just doesn't want to tell the world about

it?

Note 2: I'm sure somebody in the BR specs unit gave this specification

the number 1914 out of a dry sense of humour on the basis that:

A; it wouldn't last long

B; the other side would back down first

C; our best generals are working on it

D; our lads will be home by Christmas

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For an example of the gear changing - http://www.youtube.com/watch?v=pg30TG1dY...

Answer 11

You might want to have a look at two of the major manufacturers before you ask this question.

General Electric Locomotive Division

and Electromotive Division of General Motors Corporation, (AKA: EMD)

The newer model locomotives, 1990 and newer use an alternator in their system to generate the DC Power needed for the main traction motors.

General Electric with their new Dash 9 AC4400CW are a Hybrid.  Infact all Diesel Electric Locomotives since 1930 have been Hybrids!  Bet you did not know that.

The only ones that were not were the Krauss Maffei Locomotives.  These were Diesel Hydraulic just like your automatic transmission in your car.

EMD's Line-Up are the SD70MAC, SD75MAC and now their new SD90MAC-T

Because they are using AC to DC power they generate more than 4000 horsepower.  

The other reason for using DC at the Traction Motors is a device called a Dynamic Brake.  

This device allows the polarity to be reversed in the traction motors, and act as a brake during down hill operations.  This almost elminates the need for physical braking to control train speeds down hill.  

I hope that answers most of your question!