AC or DC?

Robert Wightman has been looking into the question of AC versus DC traction motors, and posted the following long comment.  I have moved it to its own thread.

At the end of the note, Robert asks that people send him additional info if they have it.  I have removed his email address to avoid harvesting by web crawlers, but if someone needs this, just let me know in a comment and I will pass on the address in a private response.

I am trying to find out more information for a comparison of AC versus DC traction motors, diesel versus electric propulsion and self propelled versus locomotive hauled trains.

Jared Says:
April 28th, 2009 at 3:19 pm

B) “Looking through Bombardier website, really enjoying the PRIMOVE Catenary-Free Technology. Is this something that could be considered in the upcoming TC or in future legacy track replacements? Over head wires are as much an intrusion as billboards.”

The one problem I have with this system is that it uses an air gap transformer to do power induction. This causes two problems:

1) You have to use AC instead of DC for power distribution as DC won’t work with a transformer. This means that you can connect anywhere into the power distribution system but you need to carry a transformer on the car, more weight,

2) Air gaps are notoriously inefficient. This is why Linear Induction Motors have such low efficiency. I wonder what the power delivery efficiency is with this system.

I have been doing some research into super capacitors and energy storage systems and have found three websites that provide some info:

Number one is written in readable English, Number 2 has a lot of high level math as it is an engineering document but it still should give you some ideas that are useful. Number 3 is a product brochure from bombardier but it seems to be relatively conservative in its claims. (This is a situation when it is good to be conservative.)

1) NASA report on a bus with super capacitors as part of a hybrid system

2) Czech report on energy efficiencies for super capacitors and hybrid cars

3) Bombardier site for information on their MITRAC system.

I have also been researching advantages and disadvantages for AC traction motors versus DC traction motors and have fount this information comparing the AC and DC version of the SD70 diesel locomotive from EMD, I would appreciate any information anyone has one anything related to this especially firm values on cost comparison as my costs are of vague origin.

SD70 with DC motors

Cost about $1.5 million
• 4,300 THP and locomotive equipped with EMD’s 16-710G3C-T2 engine
• EPA Tier-2 emissions certified
• Tractive and braking effort capability

o 113,100 lbs continuous TE
o 163,000 lbs starting TE
o 86,850 lbs braking effort

SD70 with AC motors

Cost $2.3 million
• 4,300 THP and locomotive equipped with EMD’s 16-710G3C-T2 engine
• EPA Tier-2 emissions certified
• Tractive and braking effort capability

o 157,000 lbs continuous TE
o 191,000 lbs starting TE
o 106,000 lbs braking effort

Note that both vehicles use the same prime mover and weight essentially the same. The AC units will start larger trains and have better (regenerative) braking characteristics. Once the unit is moving then total horse power and NOT tractive effort is the speed limiter. The AC unit can only produce its continuous tractive effort up to 15 ft/s (about 10 mph) while the DC unit can maintain its lower value to 21 ft/s (about 15 mph. Once you get above those speeds there is no advantage to the AC unit except for lower un-sprung weight (lighter traction motors) and maybe marginally better fuel economy.

Note that the ratio for continuous tractive effort is 1.39:1 or about 39% better. For starting tractive effort it is 1.17:1 or 17% better. This is nowhere near the reported advantage one would expect from the supposedly better coefficient of friction that is supposedly to be available from AC motors, 0.40 compared to 0.25. These numbers are from EMD’s tests of the two units with a dynamometer car.

The railways that have spent the extra $750 000 for the AC units seem to be ones with a lot of hills. I would appreciate any info anyone can send me on this. Thanks in advance.

Robert Wightman

9 thoughts on “AC or DC?

  1. There are two other advantages that AC traction motors offer, and neither of them is directly performance related.

    AC traction motors, by virtue of their brushless design, require less maintenance than DC motors. This will cease to be an advantage once large brushless DC motors become prevalent/commonplace, but how long may that be? We’re still waiting for widespread acceptance of small brushless DC motors.

    The other advantage is that it is much harder to overheat an AC traction motor. I’m not going to pretend to understand why, but DC traction motors have short-term ratings (they can produce X kW for 1 hour, Y kW for a half-hour, etc.), whereas AC traction motors are capable of providing their peak output for extended, and nearly infinite amounts of time.

    Of the two additional advantages, only the first one is really applicable to transit applications. The second is very handy however for those same railroads operating in mountainous areas – they can now put a bare minimum of power onto their trains and still get them over the road.

    Toronto, Ont.


  2. As I understand it, the really big advantage with AC traction motors (induction in particular) is the significantly lower maintenance costs with them. Though I don’t have specific numbers, just the simple though experiment that compares a rotor that requires brushes involves parts that rub against each other and will eventually wear out. As the brushes (made of carbon/graphite) wear, they give off dust that is conductive, meaning that in addition to the replacement of the worn brushes, cleaning becomes a very important and frequent aspect of the maintenance.

    What the brushes contact will also wear, though not nearly as quickly as the brushes as they are made of graphite which is soft and has a lubricating effect. AC motors that use slip rings and brushes will have some wear, which is why an induction motor is better as it eliminates this wear altogether, leaving only bearings on the shaft to be a component subject to wearing out. True that an air gap has inefficiencies, but the gap between the rotor and the stator in a rotary induction motor is as minimal as possible and not subject to weather (as the gap in a linear induction motor).

    In the case of DC motors, the brushes are not contacting slip rings, but are contacting a commutator. This is a ring with breaks in it, the number of breaks being related to the number of poles in the motor. Not only are the brushes wearing against a copper surface, but the surface has breaks in it, which I believe leads to faster wear on the brushes and more conductive dust. In addition to that, every time a brush crosses from one section of the commutator it is like throwing a switch as current is turned on in one pole’s coil and turned off in the previous one. Coils have inductive properties, which attempts to sustain the flow of current by any means necessary (read: a high voltage is induced – this is how an ignition coil works in a gasoline engine). This means that arcing occurs at the brushes as the rotor spins (operate a power drill in a dark room – carefully! – and you will see this effect). That arcing takes its toll on the brushes and the commutator and shorten their life even more.

    Having done some light repair on household type motors, I can testify to the difference of what you get into with DC motors compared to AC motors, so I can only imagine the difference made by a motor of several hundred horsepower that operates for long periods every day.

    I hope this helps.

    Steve: AC propulsion is now the standard for transit vehicles for the reasons you describe as well as the lower weight.


  3. Most of the biggest pro/con arguments have been brought up but I’d like to add one more note:

    The railways that operate on a lot of hilly terrain also experience another significant benefit of having AC motors: You can pretty much stop and hold trains or inch it on a grade on the motors and this isn’t possible with DC motors. If you’ve got a propulsion package designed with this purpose in mind feeding the traction motor, you can drop the frequency of the current and add some duty cycle control on top of that too if necessary, so that the traction motor effectively becomes a constant torque motor to hold the train in place, without grossly overloading it due to locked rotor current.

    This can’t be done with a DC motor because when it’s armature’s stopped, there’s no back EMF to oppose the flow of current and it’s a short circuit across whatever circuit’s feeding it and if you’re a railfan, you hope you’re nearby (but not too close) with a video camera to catch the show.


  4. I forgot how much of a problem commutators are. They get clogged with dirt, moisture and carbon from the graphite brushes which are needed for current transfer. If the the graphite builds up to a high enough level then the commutator flashes over and shorts out. To prevent this the commutator must be cleaned by cutting out the crap from between the copper slots. This is time, labour and money intensive. On railway locomotives in the spring or during high water they would sometimes also have fish in them.

    Squirrel cage induction motors have only a rotor that looks like the hamster cage on those stupid TV adds, hence the name. They have no electrical connection to them so there is nothing to maintain on the rotor except the bearings. They are about 1/3 of the weight of a DC traction motor so are much less damaging of track. Because of this open internal cage there is really no where for heat to build up as in the wire wound armature of a DC motor. The magnetic field in the squirrel cage is produce by electric induction from the stationary field windings, transformer effect. Because of this the rotational speed of the motor is slightly less than the rotation speed of the electromagnetic field. Maximum torque occurs when the rotational speed of the cage is between 90 and 95% of the magnetic field. This difference is called slip and is necessary for the motor to work. If the slip is too great then the motor does not work. They were useless in traction applications until the development of variable frequency inverters which are cheaper to make if you feed them DC current.

    I have done some more research into transit systems and discovered that Philadelphia has 18 rebuilt PCC’s with AC traction motors and solid state drives.


  5. For those who are interested Rochester NY is looking at putting in a LRT line and one of the options for vehicles that they are looking it is high floor articulated cars from San Jose made by the UTDC for only $325 000 US each. Anyone want to buy a few.

    The web site is:

    Steve: The San Diego cars were the only sale (of streetcars) that UTDC made outside of Toronto.


  6. David Aldinger Says:
    May 7th, 2009 at 9:51 pm

    “Rochester? I thought light rail was pretty much a dead issue there. After all, didn’t they fill in part or all of their old downtown subway?”

    They abandoned it one June 30, 1956. From old pictures it looked like it was a street car – interurban line. Part of it was used until 1998 to bring newsprint to the local newspaper. I did more checking and the article that I read was from 2002-2003. There does not seem to be anything more recent on their website.


  7. With regards to the old Rochester subway, I believe that there was a proposal to continue the subway operation using ex-TTC Peter Witts. In fact, the very purpose of the subway was to take streetcars and interurbans off downtown streets. The interurbans were already long gone when the subway was abandoned.


  8. In your comparison of AC vs DC systems you have compared the prime movers and the earlier comments suggest you are looking at the energy transfer system.

    The details of the Prime movers are correct but not essential in understanding the benefits that the energy transformer systems provide.

    To elaborate:

    A DC overhead system can power both DC and AC prime movers (Motors) as the DC system voltage can be converted to AC, locally on board a vehicle.

    The main difference is in the overhead distribution system and conversion on the train.

    As the overhead wire is a single wire (there are some exceptions but for simplicity I won’t go into this) it is single phase AC which is derived from typically 3 phase 25kVac. The AC vehicle powered vehicle therefore has to carry a transformer to convert the high voltage to a lower system voltage on board. This transformer is quite heavy and therefore for smaller trains carrying smaller passenger capacities it is inefficient in terms of weight per vehicle. This tends to be one of the reasons why only commuter rail and high speed vehicles are AC powered. However carrying around the transformer means that the supporting trackside infrastructure is less complex and has reduced costs; which is ideal for longer distance train journeys.

    For DC systems the transformer is located at the side of the track every 2.5km and rectified to supply DC to the overhead or third rail. This means the vehicle is very light and energy efficient as it doesn’t carry much equipment (Hence the name light rail) and for short distances it represents a good compromise on weight/cost vs passenger capacity. The vehicle also tends to be more reliable as there is less equipment to fail.

    Another important issue is the regenerative ability of the vehicles into the power system. AC vehicles can do this fairly simplistically, whereas DC vehicles require another vehicle which can receive the excess generated current.

    In essence the differences between the energy transfer systems is the location of the transformer:

    DC power system : Transformer and rectification at trackside.
    AC power system : Transformer and rectification in vehicle.
    Primove: Transformer ‘secondary’ and rectification in vehicle. Primary in track.

    Primove looks a promising supplementary power system combined with supercaps but I can’t see this being a high current application; although I am prepared to be corrected as Bombardier are providing little details on the system

    Hope that helps


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