Yes, AC motors have a maximium continuous HP and a maximum contiuous torque (current carrying) rating, just like DC. But they can be overloaded or rather their short-time ratings are more generous in both time and current then dc motors. In other words, they can be abused more without showing damage. This is primarily because the rotating bars are excellent heat conductors and radiate the heat directly into the air without heating up insulation, as opposed to commutators, where the heat in the concucting wires must first go through insulation before it reaches either iron or air. This difference applies whether the motors are self-ventilating, simiply arranged for air to flow as caused by the fan motion of the rotating parts, air-blower (fan) cooled, or liquid cooled, the latter air-to-liquid cooled more precisely, liquid not touching the rotating parts themselves).
timzProbably no one here has a good idea how long it takes a locomotive prime mover to reach full power (20 seconds? Or more?)
If you wipe the throttle on a 40-2, you'll be at full load in about 25 seconds - and it's pretty linear. If you wipe the throttle on a Dash 8, it's 80 seconds, with about 2/3 of it happening in the last 20 seconds.
So, for those P42's, you'd be well past 20 mph before you got close to full HP.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
I wouldn't worry about 125-mph gearing cutting a diesel-powered train's acceleration. Probably no one here has a good idea how long it takes a locomotive prime mover to reach full power (20 seconds? Or more?) or how much power an AC transmission on a high-geared engine can handle at 10-20-30 mph. But I'm guessing no noticeable acceleration difference between 100-mph geared DC motors and 125-mph geared AC motors -- on the level, anyway.
beaulieuNo matter whether you are dealing with an AC motor or a DC motor, when you chose a higher speed gearing, you lose tractive effort.
This is true only for speeds below which full HP output can be maintained. This typically 8-12 mph for a freight DE loco and 20-35 mph for a passenger DE loco. Above that, you are on the HP curve and TE = HP/speed.
FWIW, I agree the drivers of AC propulsion for passenger locos are lower maint cost and reduced unsprung weight.
Amtrak's electric locomotives are not going to AC Drive for more tractive effort, rather they are doing so for lower maintenance costs, and lighter weight, plus possible greater motor speed range. An AC traction motor has a set amount of torque determined by the designer just like a DC motor. No matter whether you are dealing with an AC motor or a DC motor, when you chose a higher speed gearing, you lose tractive effort. Note that tractive effort is determined by motor torque, gearing leverage, and drive wheel leverage.
1. AC propulsion would lower the speed at which the locomotive can apply full HP to the rails, so you would get a theoretical boost to acceleration at very low speeds. That boost might not have any practical benefit since it the train might accelerate to that speed long before the diesel loads up to full HP. It also might cause acceleration greater than is comfortable (2-3 mph/sec).
2. It might matter on routes with steep ruling grades. For example, you might be running a third locomotive strictly because two will stall on the ruling grade. You could go with just two AC units in this case - provided your HP/ton is great enough to cover the existing schedule.
with the potential of AMTRAK ordering some AC high speed locos ( 125 MPH ) and motors ( 150 - 160 ) a question arises.
1. Does the high speed gearing cause a reduction in very low speed tractive effort or does the AC traction motors provide enough tractive effort to reach the limits of adhesion even with the HSR gearing? This question would seem to be important for trains to reach track speed quickly ?
2. since passenger locos usually have a high HP to tonnage rating does it really matter?
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