AC/DC Motors
- Thread starter Kris94
- Start date
AC is Alternating Current which varies at 60 hz (cycles) per second (tho not all over the world - some countries use 50 Hz and possibly other frequencies). AC reverses the electron flow down the wire 60 times a second. On airplanes it used to be at 400 hertz (and still might be) since the higher the frequency the lighter transformers could be. Household wiring is AC. AC is best for long distance power transmission.
DC is Direct Current and does not vary and the electrons only travel down the wire in one direction. Batteries are DC.
Most model trains today run on DC but Lionel and I think Marklin still run on AC (there may be others).
Ben
DC is Direct Current and does not vary and the electrons only travel down the wire in one direction. Batteries are DC.
Most model trains today run on DC but Lionel and I think Marklin still run on AC (there may be others).
Ben
So AC locos are more practical for heavy haul applications and trains that require additional power to carry and in mountain areas. While DC locos are most useful for mid weight trains and on flat lands or where traction power and effort isn't vital or as important. Someone help me please?
AC Motors and Generators are also brush less, they send current through the magnetic fields of the outer coils unlike DC which sends current through the inner coils through brushes.
AC motors and generators spin easier than DC, that's why cars use an Alternator generators rather than DC generators, DC would put out lots of loading on the engine.
where I live our current is 50/60hz cycles, 240vac 50hz and 415vac 60hz.
Cheers.
AC motors and generators spin easier than DC, that's why cars use an Alternator generators rather than DC generators, DC would put out lots of loading on the engine.
where I live our current is 50/60hz cycles, 240vac 50hz and 415vac 60hz.
Cheers.
So why does GE still make DC locos?
The maximum traction force of the locomotive has linear dependence from its weight, so sometimes to put greater loading on loco is useful (unless its weigth is dangerous for the rails).
AC motors needs a current, that has a frequency, that is dependent from loco's velocity. So an AC-DC-AC converter used. It reduces loco's total efficiency in comparation with AC-DC converter + DC motors.
Another disadvantage of DC-motors is short range of their regular rotation frequencies. At low frequency they tend to overheat, at high frequency they tend to get an arc between brushes and to be burned down.
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So what's an advantage of a DC motor and a disadvantage of a AC motor because it seems like DC is bad but AC is good.
johnwhelan
Well-known member
DC is sometimes preferred for long distance power transmission, currently in Canada the newer long distance power lines are DC.
There are technical considerations such as getting two power networks in phase on the AC side which can complicate things when you connect them.
Sometimes its simply what is available locally and is the cheapest.
Cheerio John
There are technical considerations such as getting two power networks in phase on the AC side which can complicate things when you connect them.
Sometimes its simply what is available locally and is the cheapest.
Cheerio John
Don't assume that because your line frequency is 60hz that 'AC' when referring to locomotives - which is most likely what we're referring to in a thread on this forum - is 60Hz.
In most cases of modern diesel-electric 'AC' locomotives (eg SD70ACe, SD90MAC, AC4400, ES44AC) the frequency is variable, and part of the torque control circuits - and this is one of the advantages of a modern AC locomotive, there are simply more ways that the amount of tractive effort can be controlled.
In fact, building a static frequency AC locomotive is difficult, since an AC generator will produce a frequency that is relative to it's rotational speed, and the traditional method of controlling voltage/current from a diesel-electric is to vary the output shaft speed with a throttle control. Thus to build a static-frequency AC motored locomotive the usual method is to rectify the generator output to DC, then use a DC->AC inverter to get an AC signal at the required frequency. This is obviously a very lossy process, and as such the advantages of having a AC motor (in this case, mostly just flashover reduction) are reduced by losing so much of the power output from your prime mover.
Modern motor designs allow for better variable-frequency control, which allows you to gain the advantages of an AC motor, without losing too much power, and also gaining the advantages of a third variable on the electric circuits for the tractive effort control computer to use.
DC motors are cheaper and simpler, but they are more prone to flashover, and on a modern system, have fewer ways that tractive effort can be controlled.
Even in fixed-frequency AC diesel-electric and electric locomotives 60Hz isn't guaranteed by any sense, for example, the NEC runs at 12.5kV/25Hz except for CT to MA which is 25kV/60Hz, so all Acela have to cope with both 25Hz and 60hz at two different line voltages. Likewise any AEM-7, HPP-8, and ACS-64.
Wessex_Electric_Nutter
Active member
Theres a lot more than that, initially, both types of motors were used to test the differences, but railways favored the DC option because of the simplicity of control that could be done at the time. (To simplify things, I am talking about electrified routes) DC motors require just usually a switch, set of resistances and the associated shoe gear/pantograph/compressor or exhauster (brakes) and a motor generator or alternator for the auxiliaries.
In this way, earlier trains used contactors to select the resistor thats in the circuit. The trouble with working off a constant supply is that if you apply all the current across the motor, the motor would heat up, burn out and catch fire. So, the resistors are there to asorb some of the current, IIRC, these worked on volts. Previous to the stock around 1920 or so, most older trains had to have banks of resistors or resistors individually selected by hand, so the driver would need to know when to take out the resistors and select the correct grouping (Series where the motors are wired together or parallel where the motors are wired independently of each other).
Later developments allowed the automatic switching of the resistors (Checking the load across the traction supply) and changing the switch themselves from electro mechanical to electro pneumatic supply. Much to this reason I believe is why DC was preferred as it was more efficent to deal with.
Traction control then got to a stage where inefficiencies of DC were showing to be a problem, like Thyrislor control which chops up the current which is then fed to the motors. In which way, it works by having the current on for a specific time frame and turned off.. so if you have 1000v to the motors on for 25% of the time during 1 second, it would be 250v for one second (averaging 250v.. again, simplifying things here). Which found a slight problem, its the start of solid state control, but how do you turn the circuit off? The answer was to reverse the current, which is where AC started being used again, because it was found that AC was more efficient and DC wasn't that particular tolerable to an unstable supply as you have to have various bits (capacitors) to smooth out the ripples caused in a supply that is... shall we say, unstable.
AC I believe or though maybe wrong, is more tolerable. Although the way AC works is by cycling the feed and return poles (live and neutral) certain times per time frame. Plus, 3 phase supplies really help well with developing a lot of power, 3 phase is where you have 3 live poles and a ground at the minimum. Some supplies have 3 poles, a neutral and an earth. This becoming 3 phase. The big problem with 3 phase, you have to keep each phase separate, otherwise the voltage effectively doubles the voltage, which makes it a bit more hazardous, although each supply can be connected to a common earth.
Therefore, DC which is usually single phase only allows a certain amount power, with AC, you can have 3 more times, although 3 phase is considered to be the most efficient.
With feeding in the 3 seperate supplies, you can build traction equipment lighter, the motors lighter and more robust and you don't need to maintain them to such a high degree as with DC. Because AC works different to DC, you don't need brushes, brushes are required in most DC motors, because in order for the motor to turn, you need brushes to bridge the connection from the wire to the motor and that the motor must turn so that the current is always going from one side to the other. If you had a solid ring connecting the 2 sides it would short out, so you must also have a gap in the ring or have 2 rings which have the pole/coils/whatever insulated. With AC, the design allows usually for the motor to be perminantely connected to the poles, but a further development is whats known as a "Squirrel cage" motor, which has a rotor which isn't connected to the supply. Therefore, the magnetic field (as motors require a magnetic field) is induced by electrical currents going across the rotor to turn.
So, if you need overcomplicated motors but cheap to maintain and lightweight as the design goals for most units (lighter trains = lower access charges), then you would usually go for AC. But for some supplies, like locomotives, DC maybe considered better as crews maybe familiar with how DC works. Another point I almost forgot, electrical braking, AC can aparently provide better electrical braking, whereas DC fades at a certain speed and then pretty much becomes ineffective at slow speeds.
http://www.railway-technical.com/drives.shtml This may help a lot to see the difference between the 2, also check out traction supply; http://www.railway-technical.com/tract-01.shtml
And yes, the reasons get a heck of a lot more complicated than that with the choice of AC or DC. Particularly the supply, which believe it or not, its easier to go from DC to AC than for AC to AC. Why? Because the AC supply (single phase) needs to go to DC and then back to AC. And that goes into the rounds of "dual voltage trains", particulary where I live, wheras if you want to go north of the Thames, its all AC, south is all DC.
Hey Kris,
Your original post was
You didn't specify it to be trains only, we thought you where asking what's the difference between the two so we tried to give you examples that we know.
Cheers.
They are, except one thing. DC motors are more effective, then DC->AC inverters + AC motors. In normal mode.
Wessex_Electric_Nutter
Active member
It does refer to trains, except the terminology used here (where I live) is completely different to where you live. Locomotives work on almost the same principle as multiple units anyway, just the equipment has to be upscaled to suit the demands of the larger motors. So its a bit of a narrow minded approach to say that multiple units and locos don't share the same sort of equipment, they do!
Diesel locos are a bit simpler I believe to deal with, because the generator/alternator can control the current, the trouble is, as pointed out, with DC, the current degrades over the length of the wire, so you may not get 99% of the output (hence why DC substations are closer together and AC ones are miles apart). AC does degrade but at a slower rate than DC.
Plus, for DC, you still need resistors, although not as many, If you ever go around a loco, you wlll see them, in fact, the loco I worked on for a while (a Class 45) had about 4 per side if I recall correctly and I believe these were only used for starting the loco as I am not aware of the loco having been fitted with electrical braking of any sort.
What would be considered normal mode.