• Why the need for AC ?

  • General discussion about locomotives, rolling stock, and equipment
General discussion about locomotives, rolling stock, and equipment

Moderator: John_Perkowski

  by Lehigh Valley Railroad
Traction motors?

I see need GM's and GE's, any advantages?


  by DutchRailnut
DC traction motors were at limit , no more possibillities of cramming more HP in space alloted for Traction motors.

No Carbon brushes and commutators to maintain, AC traction motors have a few small carbon brushes and sliprings, but AC has power circuits on Stator, while DC motors need high power to both stator and rotor.

better control of slip and creep of wheels, due to each motor (on GE) or each truck(on EMD) basicly being independent locomotive

unlimited Dynamic brake from track speed all way down to 0 miles per hour.

  by Railpac
Also, with AC motors, you can crawl up a steep grade at 5 mph and not have to worry about burning up the motors. With DC, trying to run at that speed under load, you will burn it up real quick.

  by rdganthracite
Also AC motors can be sealed. So they would not short out while moving through high water, or from snow being blown up into the motor. In fact that is one of the reasons that CP was so interested in AC motors early on.

  by FDL4ever
One other thing that I haven't seen posted- traction control is a whole lot more effective with AC motors than with DC. The computer can run the inverters so that the motors are "trying" to run only slightly faster than track speed, yet are delivering 100% torque. With DC motors, the motor will whing up to a very high RPM the minute traction is lost- AC motors don't do that. They try to run EXACTLY one speed- the speed determined by the AC supply frequency.

  by locomotiveguy
I got to do some switching at the round house with an AC locomotive once. You have to get used to it. It dosn't jerk out of the hole, like a DC unit. It comes out smouth, then builds up torque. You have to be prepared to throttle off. Cause when the speed gets about 3 MPH, it takes off. But on the whole I liked it. I could control it better than the DC's.

  by Hostler
I work for a industrial electrical supply firm in NE New Jersey. We sell a lot of 3 phase AC Inverter drives to the industry. Our units run from fractional up to 500 HP. You'll find that the AC Drive/motor combo has the ability to maintain full torque at zero speed, if the motor is so designed, this can be done practically on a continuous basis. This principle is used in the paper trade for winders and tensioners with little or no speed but up to full constant torque. Since all drives are processor controlled (they have comuter chips that set the parameters and control operation) and can be programmed on how they function, rapid lurching start ups are almost non existant. All drives have programmable ramp up speeds, that's why you may see a delay 'til the units takes off. Part of this ramp up also limits the inrush currents. Also AC Drives can provide 150% torque at zero speed. The ramp up will always occur smoothly and consistently. Wheel slippage is easier to control as well since drives can either sense back EMF to control speed and/or use feedback devices on the motors to detect slippage as a no load condition. Also the drives can talk to each other to synchronize their operation. Drives are very intelligent and can be run by a master computer that can modify parameters on the fly depending on various conditions. As you can see, AC drives/motors offer tremendous control that DC drives don't achieve. You might have noticed on Freights that one AC drive units can handle what it took 1-1/2 or two DC units to do. Like in locos, industrial units can use braking resistors (dynamic braking) to off-load currents from high inertia loads that are ramping down in speed or coming to a stop.


  by arnstg
So why did UP go out and buy over 1000 SDM's instead of SD70MAC's?

  by Justin B

Also, AC unit really shine when they are in low speed run-8 drag service. But with lighter faster intermodal, van and manifest frieght, DC units can do the job well enough not to justify the higher initial cost per unit. That's why out here you will never see any BNSF SD70MACs, just Dash-9s. :(

  by Hostler
As to why a certain railroads buys AC vs DC can also be the roads preference and railroads have a lot of experience with DC drives. I'm sure that reliability has been an issue in the past and the maintenance staff and crews have been trained over the years on DC drives. AC drives are relatively new, only starting to show up on industrail market just over twenty years ago while DC units go back decades. AC drive reliability has increased over the years and their costs have dropped dramatically. In the industrial sector drives undergo upgrades practically on a yearly basis, they get smaller, cheaper and more reliable. Today's drives work much better with main computers than just few years ago. I think you find more AC drives showing up in the future, especially as their cost to manufacture drops. I think you see both drives continue for some time as railroad perference, needs and loco usage will determine which drive would be better for them.

  by FDL4ever
arnstg wrote:So why did UP go out and buy over 1000 SDM's instead of SD70MAC's?
Short-sightedness? :P

No, DC is still cheaper to buy up-front right now and the difference in maintenance cost isn't always enough to sway in AC's favor. It won't be that way in another few years though.