AC/DC Motors

[AJ_Fox]

This may be somewhat repetitive to other posts here, but AC electrical power is alternating current which means it is supplied through a sine curve, as opposed to a direct current. From what I've read and heard, it is more effective and resilient than direct current, but they both seem to work well. AC is newer technology.

Cheers

AJ

 

[maruffijd]

Since this thread was at the top of the list, I'll answer one of the old comments above. Aircraft still do run on 400Hz power, both from ground support connections and on board AC generators.
My source of this info is I worked as an Aviation Electronics Technician on the McDonnell-Douglas F/A18 hornet for 14 years during my run in the U.S. Navy.

 

[Kris94]

I still love my DC engines. Don't know why though. Just seems more appealing to me.

 

[SantaFebuff]

DC can output full torque at slowest speeds. That makes DC locomotives better for freight drags. AC motors are more efficient and have more overall power they can give out, making them better for everything else.

^ pulled from above posts

Amtrak units are DC. (because they are older)
NS used to be a DC-only railroad. (they ordered ES40DCs, but then bought ES44ACs and SD70ACe)

Cheers,
Joshua

 

[PWeiser]

Just to throw a few more twigs on the fire (all for purposes of light, not heat, of course)...

Since weight was a plus rather than a minus (as it is on aircraft) and some other reasons, most early AC electrifications used lower frequency - 10-25Hz. As the country (US, for example) became electrified at 60Hz and 3-phase (instead of 2) this caused a little trouble; I believe most current electrifications are commercial-frequency (50 or 60Hz) but two-phase because 3-phase requires two wires, two pantographs, etc.. I also seem to recall that some very new diesel locomotives use 3-phase traction motors, though it might be 3-phase alternators at the prime mover (which would make more sense). 3-phase motors are very efficient, except that their speed tends to be very difficult to control continuously. (Maybe modern electronics have solved that problem.)

Electrifications have to avoid interfering with the track circuit signal frequencies, too, since with only one contact wire the ground return has to be through the rails - shared with signals.

And one other comment, from the old-old days of model railroading before CTC: One thing that's hard in the model world is starting those small DC motors smoothly - even when there aren't a bunch of not quite perfectly quartered connecting rods to move around. Flywheels, and increasing the number of brushes, help run at low speeds - but adding a little nudge in the form of a square- or sawtooth-wave frequency sure does help get off the dime that first time in the morning!

 

[Kris94]

I thought that was AC for drags. I see a lot of AC44 and ES44AC on drag and heavy trains and DC for lighter fast freights.

 

[BLACKWATCH]

Kris, the railway did not start or end in the USA, if you really want to know about railways, try looking at the land that invented the darn thing.
If you think that "AC for drags" then you ain't got a bloody clue.

Sit back lad, and take a look at the 'Woodhead' route, as with all things, the USA was NOT the first or the "be all & end All" of
anything, America is just the "Land that copied all, then claimed it as their own".
The USA has nothing of it's own, it just takes what the rest of us have done, then sticks it moniker on it.

People say the USA has taken this, that & the other & improved it .....
no they haven't, they have taken what the others have invented & basterdised it, in my world, that's called ..................

Interlectual robbery.

 

[Kris94]

Kris, the railway did not start or end in the USA, if you really want to know about railways, try looking at the land that invented the darn thing.
If you think that "AC for drags" then you ain't got a bloody clue.

Sit back lad, and take a look at the 'Woodhead' route, as with all things, the USA was NOT the first or the "be all & end All" of
anything, America is just the "Land that copied all, then claimed it as their own".
The USA has nothing of it's own, it just takes what the rest of us have done, then sticks it moniker on it.

People say the USA has taken this, that & the other & improved it .....
no they haven't, they have taken what the others have invented & basterdised it, in my world, that's called ..................

Interlectual robbery.

Im not an idiot. I know the railroad didn't start in America. I never said anything was started here at all!! So I don't know why you are ranting to me just because I'm American and that's our reputation and perception to everyone else. So if you think I'm like everyone else, then you dont have a bloody clue. Plus if you're going to blast at least stay on the damn topic!! We were talking about AC and DC motors and what they are and how they operate!! So don't talk about who started what because no one was engaging into such. That's politics and you're opinion which should be discussed elsewhere but not on these forums!! That's actually a violation of the COC. No place on this forum period.

 

[jjanmarine3]

Hi guys, interesting subject.
I worked on GM and GE diesel locos which all had DC traction motors , but here is an AC v DC extract concerning locomotives which sums things up nicely for me ( sorry if it is a bit long to read ) Cheers

AC TRACTION​

The AC (alternating current) Drive, also known as Variable Frequency Drive, has been the standard in industry for many years. While it has been used in locomotives for over two decades (especially in Europe), it has only been recently that the price of the drives has allowed them to be used in most of the new diesel-electric locomotives in the United States.​

AC traction for locomotives is a major improvement over the old DC systems. The primary advantages of AC traction are adhesion levels up to 100% greater than DC and much higher reliability and reduced maintenance requirements of AC traction motors.

The tractive effort of a locomotive (whether AC or DC) is defined by the equations:

Tractive effort = Weight on drivers x Adhesion
Adhesion = Coefficient of friction x Locomotive adhesion variable​

The friction coefficient between wheel and rail is usually in the range of .40 to .45 for relatively clean, dry rail in reasonable condition and is essentially the same for all locomotives. The locomotive adhesion variable represents the ability of the locomotive to convert the available friction into usable friction at the wheel rail interface. It varies dramatically from about .45 for old DC units to about .90 for modern AC units. This variable incorporates many factors including electrical design, control systems, truck type and wheel conditions.

First generation DC locomotives such as SW1200s, GP9s, SD40s, and GE center cabs typically have adhesion levels of 18% to 20%. More modern units with adhesion control such as SD60s and Dash 8s have adhesion levels of 25% to 27%. The newer AC traction units such as the SD80MAC and the C44AC are usually rated at 37% to 39% adhesion. Thus, the newer locomotives have about twice the adhesion of the older units and the Class I railroads are, in fact, typically replacing two older units with a single new AC unit.

There are three primary reasons that AC traction offers so much more adhesion. First, in a standard DC drive, if wheel slip occurs, there is a tendency for the traction motor to speed up and run away, even to the point of mechanical failure if the load is not quickly reduced. As the wheel slippage increases, the coefficient of friction also drops rapidly to a level of 0.10 or less, and because all the motors are connected together, the load to the entire locomotive must be reduced. Therefore, maximum adhesion is obtained by operating at a level with a comfortable margin of safety below the theoretical maximum. More modern DC systems incorporate a wheel slip control which senses the beginning of a slip and automatically modulates the power in order to retain control. This allows the locomotive to operate safely at a point closer to its theoretical maximum.

The AC system, however, operates in a very different fashion. The variable frequency drive creates a rotating magnetic field which spins about 1% faster than the motor is turning. Since the rotor cannot exceed the field speed, any wheel slip is minimal (less than 1%) and is quickly detected by the drive which instantly reduces load to the axle.

Next, the DC locomotive typically has a number of throttle settings with a set power level for each one. While this sytem is simple and effective, it does not produce a constant motor torque since power is the product of torque and speed. Therefore, the tractive effort varies significantly for each throttle setting depending on speed, making it impossible to obtain maximum adhesion.

The AC locomotive, however, can control to a specific motor torque level allowing the tractive effort to be essentially constant at the higher range of available adhesion. Ths fast acting wheel slip control can counteract any wheel slip so that the torque level can be set close to the upper limits.

The third way that AC traction provides improved adhesion is through weight transfer compensation. When a locomotive is pulling a load, weight tends to transfer from the front axle to the rear axle of each truck. At maximum tractive effort, the weight on the lead axle may be reduced by about 20%. Since the tractive effort is proportional to the weight on drivers, then in a DC system where the motors are fed from a common source, the tractive effort will be determined by the lightest axle. Thus, in effect, the equivalent locomotive weight is reduced by about 20%. With an AC system, however, the drive is able to compensate for the weight transfer. When the lead axle goes light, the AC drive system will reduce power to that axle and apply more power to the rear axle without incurring wheelspin.

The combination of eliminating wheel slip and compensating for weight transfer gives the AC traction system an adhesion of 37% to 39% versus the 18% to 20% of the older DC systems. Therefore, a locomotive with AC traction can provide the same tractive effort as a DC locomotive weighing twice as much or can give twice as much tractive effort for the same weight.

GE and EMD added AC traction to their mainline units and were then able to replace two older DC units with one new AC locomotive. Republic locomotive took a different approach and decided to make a lighter, less costly unit for industrial switching. The DC powered SW9/SW1200, produced in large quantities from 1951 to 1965 and used for heavy yard switching as well as branch line service, was taken as the performance standard. At 230,000 to 240,000 pounds these units are typically rated at about 40,000 pounds tractive effort continuous (somewhat higher intermittent but limited by traction motors and generators). The AC traction RX500 at 144,000 pounds and a conservative 35% adhesion level is rated at 50,400 pounds tractive effort continuous.

With AC traction, it is also important to consider braking. As with traction, braking is a function of weight on drivers. Therefore, when using standard friction braking (tread brakes) the braking capability of the locomotive (excluding train braking) is proportional to the locomotive weight. With AC traction, however, the braking can be much higher because the drive system in braking acts just like the drive does in traction thus eliminating wheel slip. The drive converts the motors to generating mode (dynamic braking) and the electricity produced is dissipated in the braking resistors. Thus the motors are slowing the locomotive without using the air brakes. Again, the adhesion levels are much higher so the locomotive can again be significantly lighter for the same amount of braking. The dynamic braking in AC traction locomotives also allows full braking down to zero speed, unlike DC dynamic braking.

All in all, the AC traction locomotive offers about twice the amount of adhesion as a DC unit. Therefore, a modern lightweight AC locomotive such as the RX500 can provide as much or more tractive effort than an old style DC unit like the SW1200 which weighs 60% more.

 

[Kris94]

Wow!!:udrool: Thanks for the info. Didn't know that. I guess AC is better than DC. But you didn't cover the Dash 9 or ES40DC or ES44DC

 

[jjanmarine3]

Wow!!:udrool: Thanks for the info. Didn't know that. I guess AC is better than DC. But you didn't cover the Dash 9 or ES40DC or ES44DC

I don't know much about them, GE manufactured them, from what I gather the dash 9 was replaced by the ES40DC/AC and the ES44DC/AC .They all used the same diesel engine and were supplied equipped with AC or DC motors depending on what the customers in North America wanted. I guess their preferences had something to do with training, cost and maintenance issues, I won't go into comparing because I don't know enough about it.

 

[817TAYLOR]

What does AC stand for? What does DC stand for? What are the differences between the two. Also advantages and weaknesses of both would help also. Thank you.

AC/DC = A Kick Ass Rockband
XD
Actually Ben Dorsey Has It Just About Summed Up

 

[H222]

That's actually a violation of the COC. No place on this forum period.

:hehe: Coming from you :hehe::hehe::hehe::hehe:

Jamie

 

[Wessex_Electric_Nutter]

With AC traction, it is also important to consider braking. As with traction, braking is a function of weight on drivers. Therefore, when using standard friction braking (tread brakes) the braking capability of the locomotive (excluding train braking) is proportional to the locomotive weight. With AC traction, however, the braking can be much higher because the drive system in braking acts just like the drive does in traction thus eliminating wheel slip. The drive converts the motors to generating mode (dynamic braking) and the electricity produced is dissipated in the braking resistors. Thus the motors are slowing the locomotive without using the air brakes. Again, the adhesion levels are much higher so the locomotive can again be significantly lighter for the same amount of braking. The dynamic braking in AC traction locomotives also allows full braking down to zero speed, unlike DC dynamic braking.

That resource is slightly biased, for that very simple reason, AC motors is more often used for braking, that is true, because the electronics can control it (although AC still has areas where the motors, whatever you do, will just either spin, or slip.). However, electrical braking or dynamic braking you yanks have called it (rhenostatic here - I think I spelt that correctly, feel free to correct me) has been in use on DC motored trains for a long long time, in fact, earliest I've known it to be used, is in the 1930s on London Underground's C and P stocks, which used metadyne equipment, which essentially was a big failure. Don't ask me how it works, I have absolutely no idea. They eventually became CO and CP (C meaning 'Converted') which used Pneumatic Camshaft Motor (PCM) - or should it actually be "Pneumatic Camshaft Control"? And thus lost their electrical braking capabilities. Later stock featured 2 camshafts, one for accelerating and one for braking, if you ever been on the Circle line, all that ticking and clicking is the camshaft going through its steps. Both accelerating and braking, depending where the controller is. (Although lots of trains have PCM, it seems to be much louder on London Underground stock.) I also believe they use the same resistors to accelerate and to brake, why duplicate the equipment?

The trouble is, AC has one big advantage over DC, that is, AC is more consistent with braking, DC starts to fade quickly from about 20mph or wherever the gearing causes the effect to happen, therefore, you start loosing electrical braking from that point until its pretty much useless, like around 3 to 10 mph, where nothing happens. Again, you'lll hear this because that all familiar noise of the motors slowing down will just fade to nothing while the train is still moving!

 

[Kris94]

So then what big advantage does DC have or AC? Or does it have any?

 

[H222]

AC has one big advantage over DC, that is, AC is more consistent with braking, DC starts to fade quickly from about 20mph or wherever the gearing causes the effect to happen, therefore, you start loosing electrical braking from that point until its pretty much useless, like around 3 to 10 mph, where nothing happens. Again, you'lll hear this because that all familiar noise of the motors slowing down will just fade to nothing while the train is still moving!

^^^^^^^^^^

 

[jjanmarine3]

I know on the diesels that I know with DC traction motors the dynamic braking effort is most efficient at 30 KPH and is controlled by the driver using the braking rheostat , any speeds above or below that and the braking effort decreases as stated by Wessex , the loud noise one hears are the grid blower motors ( driven by DC power from the motors that are actually coupled as generators during rheostat braking ) drawing air through the resistors or grids to cool the resistors. The diesel engine then only runs at notch 3 level revolutions just to cool the motors by the tr. motor blowers. Interestingly - should the driver apply too much independant ( locomotive ) brake during dynamic braking ( especially in wet conditions ) the wheels will lock ( brakeslide ) and an alarm buzzer is activated and a white light burns on the panel.
I do agree that on AC regen or dynamic braking there is a much wider braking spectrum through the different train speeds.

 

[JCitron]

jjanmarine93,


"There are three primary reasons that AC traction offers so much more adhesion. First, in a standard DC drive, if wheel slip occurs, there is a tendency for the traction motor to speed up and run away, even to the point of mechanical failure if the load is not quickly reduced. As the wheel slippage increases, the coefficient of friction also drops rapidly to a level of 0.10 or less, and because all the motors are connected together, the load to the entire locomotive must be reduced. Therefore, maximum adhesion is obtained by operating at a level with a comfortable margin of safety below the theoretical maximum. More modern DC systems incorporate a wheel slip control which senses the beginning of a slip and automatically modulates the"power in order to retain control. This allows the locomotive to operate safely at a point closer to its theoretical maximum."

This would explain why the New Haven Railroad discontinued electric passenger service to Danbury, CT. There was an issue with leaves on the tracks which caused a lot of wheel slip. The New Haven operated on AC, but at an extremely low frequency, and the traction motors would kick out when the tracks got wet and slippery from the leaves. I wonder if this would be electrified again if Metro North used the standard 25kv off of the NEC.

If you look at Bing Bird's Eye view of Danbury, you can still see the loop and catenery poles still in places along the line.

Kris, other than answering your current question regarding the different diesel electrics which I think is already covered in various threads, one of the other reasons why DC wasn't used for very long distance, until recently, is because DC current is like a stream of water running through a hose. The longer the distance, there is less and less power due to the increase in resistance over the distance traveled. Eventually, if the line is too long, there is very little current) to drive the motors on the other end. DC transmission lines get away with this today because there are electronic switches that act as pumps to push the current along the way.

AC current works almost like a rope does when it is whipped. The ripples of the current will travel over a longer distance, and with the use of switching equipment, acting as pumps at various points along the line, the current and voltage can be kept up at their proper level over the whole distance.

Getting back to the New Haven Railroad. The New Haven was electrified by Westinghouse during the early 1910s. The voltage until the recent Boston extension of the electrification, was 11kv AC, with a frequency (I'm trying to remember), was at 10 or 12hz. Today the line is 25kv, and is pulled off of the mains. Back in the New Haven's time, they had their own power plant near Bridgeport, actually at Coss Cobb. The reason for this, if I recall was due to an experiment by George Westinghouse to prove that AC was better than DC. Up to that time, most service was DC because each municipality had a power station of its own. The streetcar companies used 600V DC, and most still run on that today. The same with third-rail electric trains today. The DC is good for quick stops and starts, and being a relatively short run, there's no need for AC service.

John