Why AC locomotives have twice as much adhesion than DC locomotives

[wwillson]

A post in a previous thread discussed the increased adhesion of AC locomotives. I have heard the term and understands that it means AC locomotives have more tractive effort for the same weight, but I didn't know why.

This is a great article that clearly explains the why, in case you were curious.

AC Traction vs DC Traction - Greenville, South Carolina - Republic Locomotive

The AC (alternating current) Drive, also known as Variable Frequency Drive, has been the standard in industry for many years.

www.republiclocomotive.com

 

[ctechbob]

A post in a previous thread discussed the increased adhesion of AC locomotives. I have heard the term and understands that it means AC locomotives have more tractive effort for the same weight, but I didn't know why.

This is a great article that clearly explains the why, in case you were curious.

AC Traction vs DC Traction - Greenville, South Carolina - Republic Locomotive

The AC (alternating current) Drive, also known as Variable Frequency Drive, has been the standard in industry for many years.

www.republiclocomotive.com

Basically boils down to better control over the motors, not that they magically have more friction at the wheel/track interface.

Interesting read though, never would have thought the difference would have been that big.

 

[supton]

Interesting.

The middle point seems off. The older DC control systems use what, 8 settings? seems like they did that because they could and it got the job done. A different controller could offer more settings. More importantly, it's a controller--why not change to one that offers constant torque? It'd be more complex but surely the AC drive ones aren't simple either.

But I think I could see how it works better... if the wheel speeds up and the drive is no longer leading, applied force drops. It's a bit like current flow: in order to have current flow (in most circuits), you have to have voltage difference. Remove the difference and current doesn't flow. Here, if the wheel speeds up and controller is still forcing a magnetic field but in a spot it thinks the wheel should be in, well, the field doesn't result in a force applied to the wheel. Negative feedback without the drawback of the delay time of the controller (surely a DC controller could do the same, 'cept it has to first sense it, then respond).

 

[Torrid]

Basically boils down to better control over the motors, not that they magically have more friction at the wheel/track interface.

Interesting read though, never would have thought the difference would have been that big.

I’ve pulled trains from a stop that I would never thought would have been possible. Almost stalled up a hill where we got to 0.5 mph and the AC powered right through it. All my early experience with locomotives was DC and if you got much under 10 mph in high tractive effort they wanted to wheel spin. That and with direct current, short time ratings would heavily come into play because of heat. I always heard ACs were superior but I didn’t realize how much more capable they were until I went to a Class 1 railroad.

 

[Torrid]

Interesting.

The middle point seems off. The older DC control systems use what, 8 settings? seems like they did that because they could and it got the job done. A different controller could offer more settings. More importantly, it's a controller--why not change to one that offers constant torque? It'd be more complex but surely the AC drive ones aren't simple either.

But I think I could see how it works better... if the wheel speeds up and the drive is no longer leading, applied force drops. It's a bit like current flow: in order to have current flow (in most circuits), you have to have voltage difference. Remove the difference and current doesn't flow. Here, if the wheel speeds up and controller is still forcing a magnetic field but in a spot it thinks the wheel should be in, well, the field doesn't result in a force applied to the wheel. Negative feedback without the drawback of the delay time of the controller (surely a DC controller could do the same, 'cept it has to first sense it, then respond).

The ACs use the 8 notch setup too. It’s retained for multiple unit operation. The controls themselves between AC and DC are basically identical. Because the actual commanded throttle positions are identical you can chain ACs and DCs together and control them together.

I’m not sure I understood what you mean by the effort part, but you do see effort drop as speed increases. It’s not so much a design thing but how electricity responds as the traction motor speed increases. Watching tractive effort when braking is a huge part of how we operate. If we take air to slow down and are still in throttle(you should be if you don’t want a rough ride) you’ll see effort increase. At this point it’s time to reduce throttle.

 

[mk378]

but you do see effort drop as speed increases.

When under way, the amount of torque that can be applied to the wheels becomes limited by the prime power horsepower available from the diesel engine. Horsepower is speed x torque, so as road speed increases torque must decrease.

At zero speed, theoretically an infinite amount of torque is possible, except of course the wheels will spin.

 

[dnewton3]

I won't go into the weeds of the world of physics, but there are better ways to define that Republic formula, as it ignores a few things and also creates a circular reference. Rather, I'll just stick to the simple result of desired output ...

What this AC technology is doing is allowing greater control of the resultant thrust. The theoretical thrust force limit is no different between DC and AC; it's just that AC can get a lot closer to that limit without losing control. In their formula they introduce a "locomotive adhesion variable" ... (aka technology). The AC "tractive effort" isn't better by formula, it's better because it is controlling thrust closer to the true inherent maximum at each individual wheel incidence, by means of improvements in technology such as frequency drivers, sensing components, etc. Merely it is the improvement in control of variables that makes the output "better".

Or, in simple terms, it's "traction control" for a massive 200 ton vehicle.

 

[4WD]

This is similar to other modern power systems managing severe loads … The introduction of IGBT/VFD has added amazing finesse converting from static to dynamic loads - and does an amazing job on the braking side as well …

 

[mk378]

The point that Republic was ultimately trying to make is that their new AC model can pull more at zero to slow speed than the former DC one, despite the locomotive weighing much less.

 

[SubieRubyRoo]

A post in a previous thread discussed the increased adhesion of AC locomotives. I have heard the term and understands that it means AC locomotives have more tractive effort for the same weight, but I didn't know why.

This is a great article that clearly explains the why, in case you were curious.

AC Traction vs DC Traction - Greenville, South Carolina - Republic Locomotive

The AC (alternating current) Drive, also known as Variable Frequency Drive, has been the standard in industry for many years.

www.republiclocomotive.com

Thanks! So effectively, AC locos have built-in fancy traction control like we see in high-level drag racing that watches wheel speed and curtails output accordingly, so the power is always maximized to the ultimate limit of traction. Good stuff!

 

[shortyb]

Being a die-hard railfan, and located about 15 miles away, I've been to Republic dozens of times. Seen some pretty cool old locomotives there for repair and maintenance. NS has a yard nearby the passenger terminal for Amtrak and some of the remaining high hood road GPs for Western NC line work come in for attention. Neat place.

 

[Dave Hess]

The ACs use the 8 notch setup too. It’s retained for multiple unit operation. The controls themselves between AC and DC are basically identical. Because the actual commanded throttle positions are identical you can chain ACs and DCs together and control them together.

I’m not sure I understood what you mean by the effort part, but you do see effort drop as speed increases. It’s not so much a design thing but how electricity responds as the traction motor speed increases. Watching tractive effort when braking is a huge part of how we operate. If we take air to slow down and are still in throttle(you should be if you don’t want a rough ride) you’ll see effort increase. At this point it’s time to reduce throttle.

Nice technical info. Which manufacturer of locomotives do you operate ?

 

[Torrid]

Nice technical info. Which manufacturer of locomotives do you operate ?

Mostly GE. There are some EMDs but my understanding is that the EMDs with my current railroad are all rebuilds. We end up on multiple railroads engines since they kind of get passed around as they move about, but for road use I think GE has the vast majority of the market. You do see a lot of EMDs in switching service.