The overall horsepower of the locomotive is what is indicated, subject to various adjustments that others can elaborate on and depending somewhat on where it is measured/determined.
Whatever the horsepower, it is more or less divided equally between the powered axles. Thus a 4400 HP C-C locomotive would have about 733 HP on each axle. That does tend to explain why the high-horsepower B-B locomotives (3000 hp with ~750 on each axle) tended to be slippery.
I think you'll find few references to the horsepower at the wheels, however, as tractive effort is the measurement that counts.
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
Rusty,
That 4400 hp is the HP delivered to the control system. Most American manufacturers deduct the power used by secondary generators or air compressors. This is typically about 10%. The real shaft HP is about 4800-4900 HP.
The traction motors HP rating will vary as there are both AC and DC trction motors and associated control systems. The 'prime mover' can deliver enough watts of power to fry the electical system - that is why there is a load meter in engines and 'short time' ratings for how long the locomotive can run in the 'red line'. AC traction motors are much more rubust, but they do have limits as well.
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
Rusty521 wrote:What is the horse power of a traction motor? When people say an engine has 4400hp, are they meaning the traction motors making the power? I'm a die hard steam guy and will be honest and say I have no idea how the horse power rating is done for a diesal. Thanks in advance!
The maximum horsepower exerted by a traction motor is the net flywheel horsepower of the locomotive divided by the number of traction motors. For example, the horsepower of a D77 motor under a 3,000-hp SD40-2 is 3,000/6 = 500 hp. The identical D77 motor under a 3,000 hp GP40 is 3,000/4= 750 hp.
The horsepower exerted by the traction motor cannot in any circumstance exceed this number. A 3,000 hp locomotive does not create 4,000 hp at 1 mph, but still creates 3,000 hp.
Diesel-electric locomotives are rated in two ways: maximum horsepower of the prime mover, which is constant* at all locomotive speeds, and tractive effort of the locomotive, which decreases as speed increases.
RWM
How does "tractive effort" relate to the term "ability to overcome inertia"? That term I first learned in connection with pre-modern trolley cars, whose DC motors almost by definition are most efficient at pulling away from zero mph and tend toward least efficiency up to the speed limits customary for the cars, not much above 40-45 mph (unless you want a burn-out, I suppose).
Also, is any of this translatable to automotive terms? Would "torque" (pulling power) be the same as or similar to "tractive effort"? Would "speed" or "raw speed" or "acceleration" be relatable to the raw HP coming out of an array of diesel engines in one locomotive providing thousands of HP? I do realize that in discussing cars I'm discussing, usually, internal combustion; not traction in the sense we railfans know it. - a. s.
al-in-chgo wrote: How does "tractive effort" relate to the term "ability to overcome inertia"? That term I first learned in connection with pre-modern trolley cars, whose DC motors almost by definition are most efficient at pulling away from zero mph and tend toward least efficiency up to the speed limits customary for the cars, not much above 40-45 mph (unless you want a burn-out, I suppose). Also, is any of this translatable to automotive terms? Would "torque" (pulling power) be the same as or similar to "tractive effort"? Would "speed" or "raw speed" or "acceleration" be relatable to the raw HP coming out of an array of diesel engines in one locomotive providing thousands of HP? I do realize that in discussing cars I'm discussing, usually, internal combustion; not traction in the sense we railfans know it. - a. s.
Al, check out this link to Al Krugs website, this should answer your question in a easy to understand way. Also, poke around this site there is tons of good stuff.
http://www.alkrug.vcn.com/rrfacts/hp_te.htm
Railway Man wrote:The maximum horsepower exerted by a traction motor is the net flywheel horsepower of the locomotive divided by the number of traction motors. For example, the horsepower of a D77 motor under a 3,000-hp SD40-2 is 3,000/6 = 500 hp. The identical D77 motor under a 3,000 hp GP40 is 3,000/4= 750 hp.
The output of a traction motor on a diesel-electric loco would be less than that due to losses in the alternator and traction motors (neither are 100% efficient). The rules can be different with a hybrid locomotive i.e. traction motor output may no longer be limited by the prime mover (for short periods of time).
The output of a D77 in a straight electric could be significantly higher as it would be possible to operate the motor at maximum continuous current and voltage simultaneously. The diesel-electric can provide max current or max voltage (the product of the current times voltage is limited by the prime-mover). In the case of the GP40 vs. the SD40, the traction alternator in the GP40 supplies 2/3rds of the current of the traction alternator in the SD40 at max continuous traction motor current and thus can supply 3/2 the voltage (500x3/2=750).
Agreed.
I decided to stay away from the usual mention of the usual assumption that 18% of the horsepower at the flywheel disappears in transmission losses, since I'm not sure that the 18% is uniform at all times or is speed/time/temperature dependent, and from what I saw the original question wanted to know maximum possible horsepower, not actual.
On a side topic, it's hard to know how much information to offer -- too much is often worse than too little. An editor for a science magazine once told me that he classifies writing on a scale of 1 to 5. At the "1" end is pamphlets and advertising that a third-grader can understand but says almost nothing, and at the "5" end is detailed information understandable by perhaps 10 people in the entire world who are the experts that define that field. Most people want knowledge in the "2" range -- not too detailed but requiring very little specialized knowledge. He and I both liked information in the "3" range -- requiring some specialized knowledge but not a Ph.D. in that field. I try on this forum to get my answers into the "2.5" range, and I notice that when we start bringing in "4" information that we quickly limit the conversation to perhaps 2 or 3 of us!
Railway Man wrote:I decided to stay away from the usual mention of the usual assumption that 18% of the horsepower at the flywheel disappears in transmission losses...
erikem wrote:In the case of the GP40 vs. the SD40, the traction alternator in the GP40 supplies 2/3rds of the current of the traction alternator in the SD40 at max continuous traction motor current
It would if the SD40's motors were in parallel, which (as you no doubt recall) they wouldn't be.
Railway Man wrote:I decided to stay away from the usual mention of the usual assumption that 18% of the horsepower at the flywheel disappears in transmission losses, since I'm not sure that the 18% is uniform at all times or is speed/time/temperature dependent, and from what I saw the original question wanted to know maximum possible horsepower, not actual.
You're absolutely correct with regards to the maximum possible horsepower.
I would assume that the 18% figure is a near optimal value for a locomotive with DC motors, I would be very surprised if the worst case losses weren't much higher than that. Worst case would likely be high speed and light load.
timz wrote: erikem wrote:In the case of the GP40 vs. the SD40, the traction alternator in the GP40 supplies 2/3rds of the current of the traction alternator in the SD40 at max continuous traction motor currentIt would if the SD40's motors were in parallel, which (as you no doubt recall) they wouldn't be.
Yes and no....
When both locomotives are operating in full parallel and at the same current per motor, the traction alternator on the SD40 is producing 3/2's of the current of the traction alternator on the GP40. In order to maintain constant power output, the SD40's traction alternator needs to produce 2/3rds of the voltage of the GP40's traction alternator. Assuming the same gear ratio, this would mean that the GP40 is producing 2/3rds of the SD40's tractive effort at 3/2's the speed.
Maybe so-- but "at max continuous traction motor current" the GP40's alternator will be producing 4200 amps while the SD40's will be at 3150.
The traditional 82% assumed transmission efficiency for DC diesels isn't necessarily the maximum we can hope for from them-- it's supposed to be halfway conservative. In any case, SD40-2's were known to produce more than 2460 hp at the rail (tho for all I know maybe their 645E3s were stuffing more than 3000 hp into their alternators).
An SD40 will outpull a GP40 below 12-15 mph (for common 62:15 gearing). Above that speed, horsepower is the limiting factor. The GP will show a higher reading on the ammeter, and you better hope it's not raining-snowing-leaves on the rail.
A SD40 (6 axle, 3000 hp) and a GP38 (4 axle, 2000 hp) usually have about the same amp reading at a given speed.
Mike WSOR engineer | HO scale since 1988 | Visit our club www.WCGandyDancers.com
WSOR 3801 wrote:An SD40 will outpull a GP40 below 12-15 mph (for common 62:15 gearing). Above that speed, horsepower is the limiting factor.
Dunno if power matching was standard or an option on the GP40-- but most (or all) of them had it, didn't they? So EMD only claimed full horsepower from a 62:15 GP40 above 23 mph.
I believe the alternators in an SD40 or GP40 are capable of putting out the same power. A GP40 would not need 3000 hp for starting 4 traction motors. A GP40 has the TM's connected in parallel and does not make transition like the SD40 which starts in series-parallel ( 3 sets of 2 connected in series) and after some field shunting switches to parallel.
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