AC vs DC traction motors?

[5292nate]

Can someone please explain to me what the difference between AC and DC traction motors is, and how each work? I can't quite understand. What makes AC traction motors have more tractive effort? I'm clueless on this.

Thanks!

Nathan

[buzz456]

Can someone please explain to me what the difference between AC and DC traction motors is, and how each work? I can't quite understand. What makes AC traction motors have more tractive effort? I'm clueless on this.

Thanks!

Nathan

This ought to keep you busy for five minutes or so.
http://www.reliance.com/prodserv/motgen/b7096.htm

Then do this:http://www.republiclocomotive.com/ac_traction_vs_dc_traction.html

[Rich_S]

Can someone please explain to me what the difference between AC and DC traction motors is, and how each work? I can't quite understand. What makes AC traction motors have more tractive effort? I'm clueless on this.

Thanks!

Nathan

Nathan, I can tell you AC traction motors use induction and DC traction motors have four brush holders that hold (depending on locomotive either three or four) carbon brushes and these carbon brushes transfer the current to the commutator plates. The commutator plates are wired to the armature. A DC traction motor has four cables that use approx. 1600 AWG cable, two cables are used for the field and the other two cables are used for the armature. A AC traction motor only uses 3 cables that are approx. 700 AWG cable. Dynamic braking is also handled differently depending on weather you have AC or DC traction motors.

In very simple terms, both AC and DC traction motors work the same, they use magnetism. You've heard the old saying that opposites attract, this is true with magnets, but traction motors do not contain magnets. When you wrap wire into coils and send a current through the coils you develop a magnetic field and depending on current flow, the field is either called North or South. An example, think of a clock (not a digital clock, but a clock with numbers on the face) and lets say the numbers represent the field of the DC traction motor and the big hand represents the armature of the DC traction motor. Now if you ran a current one direction through the number 12 and ran a current in the opposite direction through the big hand as it was passing the number 11, the big hand would be attracted to the number 12. If you kept doing this with the numbers around the face of the clock in sequential order, the big hand would continue to be attracted to the next number. This is a very simple explanation of how a DC traction motor works. An AC motor works pretty much the same way, except the current for the armature is induced into the armature, there are not any brushes in a AC motor. Think along the lines of how a transformer works, as in the primary induces a current into the secondary windings. One more difference between AC and DC traction motors, to make a DC motor turn faster you just give it more voltage and current. To make a AC motor turn faster, you have to change it's frequency. That is why locomotives with AC traction motors have both rectifiers and inverters. Hopefully this sheds some light on how AC and DC traction motors work?

Rich S.

[BillS]

Rich_S, that was a really excellent explanation of a/c, d/c traction motors. It's amazing the amount of various knowledge of the people that visit this site!

Other than building excellent routes are you, or were you, an electrician, electrical engineer or something similar?

BillS

[5292nate]

Fantastic explanation, Rich!!!

[Rich_S]

Rich_S, that was a really excellent explanation of a/c, d/c traction motors. It's amazing the amount of various knowledge of the people that visit this site!

Other than building excellent routes are you, or were you, an electrician, electrical engineer or something similar?

BillS

Bill, Thank you for the compliment and yes I'm a locomotive electrician.

Rich S.

[BNSFdude]

Ah, a fun job indeed to have! I'm an apprentice to Ron Erickson who started out as an electrician and moved on to Airbrakes and has been doing it for 50 years, though retired now. He works extensively with the railroad museums in MN, and the GNRHS. Man fills my head up every time we meet.

[1225fan5358]

Ah, a fun job indeed to have! I'm an apprentice to Ron Erickson who started out as an electrician and moved on to Airbrakes and has been doing it for 50 years, though retired now. He works extensively with the railroad museums in MN, and the GNRHS. Man fills my head up every time we meet.

half the engines in our yard are an electricians nightmare.

[dfcfu342]

Can someone please explain to me what the difference between AC and DC traction motors is, and how each work? I can't quite understand. What makes AC traction motors have more tractive effort? I'm clueless on this.

Thanks!

Nathan

Rich gave a good explanation of the differences in how the motors work, but its more important how they work differently in real world situations. As a motor spins faster, it produces a back EMF (electromagnetic force) that opposes the incoming current and begins to cancel it out. This is the reason amperage decreases as speed increases; the motors push back. This seems counter productive but it is actually very important. If the motors didn't produce this back EMF it would require astronomical currents to maintain the power output that is desired at high speeds and amperages would push very high and things would get hot very quickly (remember those brushes Rich was talking about in DC motors?). Now, when a locomotive is traveling at low speeds the traction motors are also spinning at low speeds and producing relatively little back EMF. This results in the high currents you read on the display. These high currents produce a LOT of heat and can actually melt the brushes and destroy your expensive DC traction motor. These are the situations where the AC traction motor really shines; there are no brushes to melt. As train speed begins to drop, more tractive effort can be applied by AC traction motors and there is no worry about anything melting and turning into a very large paper weight.

However, the most important feature of AC traction is that it is computer controlled by a much better system than is implemented on DC traction systems. By implementing precise computer control, the field is rotated exactly 1% faster than the armature field. This almost eliminates wheelslip. Because the armature can only spin 1% faster before it catches up to the field being applied to it, it can be caught very quickly by the software and stop the slip. Pretty neat huh? This is where those fancy high adhesion algorithms you hear about come into play as the computer decides when enough traction has been attained to return the wheel to full driving force and reset the field back out to 1% ahead. This whole process can happen in tenths of a second meaning almost zero loss in tractive effort for that axle.

This point eludes to the other major advantage of AC traction: The motors are all operated independently. In an AC system if one axle loses traction you can simply reduce the output of that motor until traction returns and then return the motor to full power. This can be done while the other five motors continue to pull at full effort. In a DC system if one axle loses traction, the power has to be cut to all of the motors until the slipping axle regains traction. Because of this, a DC system can only put down as much traction as the weakest axle can maintain. This becomes important in high power, low speed situations.

In a high power, low speed situation the trucks on the locomotive will actually rotate rearward and lift the front axle. This works the same way as it does in the vehicle you drive: take off from a red light hard and the front of your vehicle lifts. When this happens on the trucks of a locomotive, weight is lifted off the front and middle axles and applied to the rear axle. When you lift weight off the front axle, you reduce the amount of traction available and that axle begins to slip long before it would if the full weight was available to it. This front axle becomes your weakest axle and in a DC system where all motors must follow the weakest axle, you loose a lot of tractive effort across the entire system to keep the weakest link happy. In an AC system where you can control the axles independently, you simply remove power from the leading axle and pass it back to the rear axle. Because this rear axle has extra weight applied to it, it can handle much more power before it slips. By shuttling the power backwards along the truck as more power is applied, AC traction can maintain full tractive effort with minimal sand application when a DC system would require lots of sand and still have tractive effort reduced by the leading axle.

Combine no worries of melting, great slip control, and maximum tractive effort at near zero speeds and you get a locomotive that is perfectly suited to mountain dragging.

Hope you kept up with all of that.

[Rich_S]

These high currents produce a LOT of heat and can actually melt the brushes and destroy your expensive DC traction motor.

Heat is an issue, but it is controlled by the traction motor blowers. The bigger issue is when the motor begins to stall the air ionizes which leads to a flash-over and flash-overs will destroy the brush holders and commutator plates. GE Dash 9 and EVO locomotives monitor for flash-overs and if to many occur, the computer will cut-out that axle.

This point eludes to the other major advantage of AC traction: The motors are all operated independently. In an AC system if one axle loses traction you can simply reduce the output of that motor until traction returns and then return the motor to full power. This can be done while the other five motors continue to pull at full effort. In a DC system if one axle loses traction, the power has to be cut to all of the motors until the slipping axle regains traction. Because of this, a DC system can only put down as much traction as the weakest axle can maintain. This becomes important in high power, low speed situations.

In older locomotives with DC traction motors like the SD40-2, and SD50 this is true. The EMD SD60 and following 70 series EM2000 computer made major leaps into wheel slip control on DC motors. Like GE Dash 9 locomotives, the 70 series has speed probes to monitor wheel speed and the computer can regulate traction motor current. One other point is the SD60, SD70, SD70M and SD70M-2 no longer make traction motor transition, they make Alternator transition. GE Dash8, Dash 9 and EVO's do not make transition at all, their main alternator can handle the current demand of low speed operation. I would say DC traction has probably peaked with the GE ES40DC and the EMD SD70M-2 and AC traction is the way to go, but along with that is a whole new set of problems. One of the major problems is the AC motor itself, it turns into a giant capacitor after being loaded. Both GE and EMD have developed circuits to discharge the motors, but in a few years when these circuits begin to fail, look out.

Regards,
Rich S.

[BillS]

This is amazing! It's like a correspondence course in Locomotive Electrics.

BillS

[mrennie]

Absolutely fascinating stuff!

I graduated with a degree (B.Sc. Hons) in Electronic & Electrical Engineering, but I haven't done anything even remotely related to motors and circuits since then (I was bitten by the software bug in my second year). This discussion brings it all back! Perhaps I should have stuck with the electrical engineering side.

Might I suggest that someone write an article on this for RSC's Engine Driver? You could collaborate on it - maybe one could write a draft and others could review it. Just an idea.


Mike

[dfcfu342]

Heat is an issue, but it is controlled by the traction motor blowers. The bigger issue is when the motor begins to stall the air ionizes which leads to a flash-over and flash-overs will destroy the brush holders and commutator plates. GE Dash 9 and EVO locomotives monitor for flash-overs and if to many occur, the computer will cut-out that axle.

Interesting I had never considered ionization channels. I was always under the impression that the short time ratings at high amperages were to control heat and prevent any welding of brushes and plates. If that isn't an issue than why the short time ratings instead of a "do not pass this point"?

In older locomotives with DC traction motors like the SD40-2, and SD50 this is true. The EMD SD60 and following 70 series EM2000 computer made major leaps into wheel slip control on DC motors. Like GE Dash 9 locomotives, the 70 series has speed probes to monitor wheel speed and the computer can regulate traction motor current. One other point is the SD60, SD70, SD70M and SD70M-2 no longer make traction motor transition, they make Alternator transition. GE Dash8, Dash 9 and EVO's do not make transition at all, their main alternator can handle the current demand of low speed operation. I would say DC traction has probably peaked with the GE ES40DC and the EMD SD70M-2 and AC traction is the way to go, but along with that is a whole new set of problems. One of the major problems is the AC motor itself, it turns into a giant capacitor after being loaded. Both GE and EMD have developed circuits to discharge the motors, but in a few years when these circuits begin to fail, look out.

Regards,
Rich S.

While that is true that the newer DC's have gained a form of independent control it still pales in comparison of what's possible with independent control on AC's. It's almost a moot point as the ES40DC can only maintain a maximum of 26% adhesion while an ES44AH can manage a whopping 50% adhesion. One ES44AH can pull a train up a hill as well as two ES40DC's. The same applies to dynamic braking and the ability to extend dynamic breaking down to almost a dead stop, much farther than the best extended range dynamic break.

I'm waiting for one of the discharge circuits to fail. It should make for a fantastic show

[BNSFdude]

Believe it or not, the discharge circuits, I believe, are not checked on a regular (92 or 126 day) basis.

[Rich_S]

Interesting I had never considered ionization channels. I was always under the impression that the short time ratings at high amperages were to control heat and prevent any welding of brushes and plates. If that isn't an issue than why the short time ratings instead of a "do not pass this point"?

No, you misunderstand, heat and burning the armature plates are issues if the motor is stalled or run at very slow speed for to great a period of time and that is why there is a short term rating. But that is also why the traction motors have blowers to help dissipate the heat. The point I was getting at is another problem under heavy load is a flash-over. You are 100% correct that AC motors do not have a short term rating and can be stalled indefinitely.

The same applies to dynamic braking and the ability to extend dynamic breaking down to almost a dead stop, much farther than the best extended range dynamic break.

Dynamic braking on the new AC's are handled completely differently than on DC locomotives. The grids now are just used to dissipate the extra current produced by the main alternator. Like you mentioned with the field rotating 1% faster in power, in dynamic braking, the field is rotating slower than the armature.