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

Rainy weather.

More power from dc with less voltage

DC current is the most energy efficient way to power them & is lower in voltage. AC currents only advantage is that it can be sent long distances with less loss. Look at ur automobile, u have an alternator that generates ac power & then converts it to DC for use in the vehicle. Freestanding locomotives r 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.

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 r reduced. However, converting an existing DC system to AC involves cost & cannot be achieved overnight. There r other challenges which come in the way.

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

Hope this helps.

Good question, AC is more efficient, but harder to control. The newer locomotives r mostly AC.
l am guessing that ur question referring to traction means the traction motors & not overhead lines?

DC is easier to control to the traction motors & until microprocessors were made that were fast & 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, & the first series that l was on were made by EMD, the SD-70 MAC & they werent nearly as good as the newer series. As a matter of fact l hate seeing them in my consist.
They had serious traction control issues & were no improvement over what they were replacing.
It was just a matter of available technology being adapted to fit the need.

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

Firstly l think the asker is talking about AC -v- DC Motors rather the means of transmission. Older trains take AC or DC from the supply, & 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 & Deutsche Bahn both experimented with the technology. In the UK the first AC motored units were the Class 465 networkers & the Class 323.

Now l will quote 'Cathode Ray' From UK Railway
------------------------------------------------------------------
Whilst the three phase [A] motor is extremely simple, rugged, robust &
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 & torque over a wide range was generally beyond the
techological means until the mid 1960's. Numerous complex
electromechanical, electronic, valved & 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 is allowed both the
voltage & the frequency applied to the stator windings to be
individually controlled at the voltage & current ratings required of
a traction drive & for the first time series production of three
phase motored EMU is & locos was feasible. Ongoing development shows
how technology marches on, on a DB BR120 loco there r three
inverters, each of which originally contained 120 thyristors. A modern
inverter of similar rating will generally use either 6 or 12 devices
(which r more likely to be IGBT is than thyristors).


In an asynchronous motor the rotational speed is determined by the
frequency applied to the stator windings & 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 & 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 & 0-400V, although values of 0-300Hz & 0-600V r 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 & current levels required, thus another method
of creating these low frequencies is needed.


To achieve the low speed frequencies & 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 u r 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 u & Arthur refer to is the
pulse width modulation gradually being widened over one basic
switchning frequency then the switching frequency being changed &
the PWM again being widenend over that, new, frequency range.


Believe it or not, this is given the term of ''gear changing'' - yes l
know, we electrical engineers owe debts of gratitude to our mechanical
bretheren. It can also be heard on Class 323 is & 465 is as well as
the JLE stock here in the UK.


In short, it allows fine control of torque & 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 u 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 r 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 & if it
does escape puts at risk very little equipment & this is why modern
drives use IGBT is - they r capable of handling these frequencies
whereas GTOs r not.


If u r building a thyristor based inverter u can use gear
changing to ur 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 r 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 & was not allowed in terms of interference currents &
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 r 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 l am 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 does not want to tell the world about
it?


Note 2: l am 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 would not last long
B; the other side would back down first
C; our best generals r working on it
D; our lads will be home by Christmas
---------------------------------------------------

For an example of the gear changing - http://youtube.com/watch?v=pg30TG1dYWk

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, & 90 MAC. The MAC stands for modified, alternating current. These locomotives r more efficient because the traction motors use less moving parts to repair. The AC traction motors r more reliable & have eliminated ground relay problems. GE is 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 & restarts automatically.

they do use a/c current.

Newer rail technology uses only AC.

Main problem with DC is higher transmission loss than AC.

Your assumption is incorrect. By far the greatest proportion of electric railway locomotives in the world - & indeed multiple units - r 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 & South Coast Railway started a programme of overhead electrification of its lines at 6600V AC well before the 1st World War & 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 & North Eastern Railway, for example, on its suburban routes), albeit at varying voltages & 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 & on the vast system south of London extending to the Kent, Sussex & Hampshire coasts - where modern technology means that DC units can travel at up to 100mph. Elsewhere in the world, notable systems that use DC r the New York & Chicago systems & the Paris metro.

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

General Electric Locomotive Division

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

The newer model locomotives, 1990 & 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 r a Hybrid. Infact all Diesel Electric Locomotives since 1930 have been Hybrids! Bet u did not know that.

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

EMD is Line-Up r the SD70MAC, SD75MAC & now their new SD90MAC-T

Because they r 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, & act as a brake during down hill operations. This almost elminates the need for physical braking to control train speeds down hill.

l hope that most of ur question!