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GP20s and SD24s and E-units (Oh my!)

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GP20s and SD24s and E-units (Oh my!)
Posted by mrmckenzie0 on Thursday, July 22, 2021 2:44 AM

I'm curious why there is a discrepancy between these models, or rather a "discrepancy" since there's obviously a reason. The GP20 and GP30 are rated at 2000 hp and 2250 hp respectivly. The SD24 was able to get 2400 hp out of the 16 cylinder 567D engine. Now the obvious answer is that the SD24 had two extra traction motors, but 4 traction motor E-units had been rated at 2250 hp to 2400 hp for a decade, even using older motors. It would reason therefore that a prime mover that could generate 2400 hp with 4 traction motors that could put 2400 hp to the rails would be rated at 2400 hp, not 2000 hp, but clearly that wasn't the case. There's clearly something I'm not familiar with at work and I'm certainly no engineer. I would assume it has to do with the differences between low-speed freight operations and higher-speed passenger operations, and, from what I've read about the problems of the GP30/GP35 it's probably a limitation of an electrical nature. Are traction motors rated, ie what's the maximum horsepower/tractive effort you could squeeze out of a d27 motor vs a d47 motor (provided you had an appropriate prime mover) and such? The technical aspects of tractive effort/speed curves and whatnot are like a foreign language to me.

 

Also a minor side question that came to mind while looking up facts and figures, why the small 36" wheels on E-units? Wouldn't larger wheels be better on something meant to go fast, ie steam locomotives giant drivers? 

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Posted by SSW9389 on Thursday, July 22, 2021 3:43 AM

I think part of the answer is going to be with the traction motors in the electrical system. The D57 traction motor was starting to be used in late 1961. That's the same traction motor used in the GP30. The D67 traction motor became available with the GP35 in 1963.  That's a couple of electrical capability increases in a short time. And the GE 752 traction motor still had the capability edge on EMD's traction motor at this time. 

Ed in Kentucky

 

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Posted by BaltACD on Thursday, July 22, 2021 7:39 AM

E units had two prime movers per unit to attain their rated horsepower.  The SD & GP series of locomotives all had a single prime mover per carbody.

E units also had two generators to match the two prime movers.  SD & GP unit had a single generator.

Welcome mrmckenzie0

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Posted by timz on Thursday, July 22, 2021 3:42 PM

Guess you're wondering why the GP20 wasn't 2400 hp like the SD24?

No one here knows. Maybe EMD hadn't figured out a good way to limit the horsepower at 10-15 mph, so a 2400-hp GP would have a minimum continuous speed sround 16-17 mph?

Likely the GP20 and SD24 had the same traction motors? The motors would have current limits and a voltage limit, but I'm guessing they could handle 600 hp apiece.

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Posted by Overmod on Thursday, July 22, 2021 4:26 PM

timz
Guess you're wondering why the GP20 wasn't 2400 hp like the SD24?

We have discussed this here in the past.  As I recall the principal issue was practical traction-limited, a combination of increased propensity to slip and too much risk of motor overheating at slower speeds -- but don't quote me, look for it.

It may also be possible that with the GP35/40 in the offing, EMD had no reasons to sell GP20s with fragile higher output...

Certainly the competition by the early 1960s (C424/5 and GE U25B) had little issue with providing over 2400hp on four axles.

I'd be willing to bet that Preston Cook and maybe some others know an answer from inside EMD...

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Posted by Paul Milenkovic on Thursday, July 22, 2021 9:55 PM

timz

Guess you're wondering why the GP20 wasn't 2400 hp like the SD24?

No one here knows. Maybe EMD hadn't figured out a good way to limit the horsepower at 10-15 mph, so a 2400-hp GP would have a minimum continuous speed sround 16-17 mph?

Likely the GP20 and SD24 had the same traction motors? The motors would have current limits and a voltage limit, but I'm guessing they could handle 600 hp apiece.

 

Generally speaking, traction motors are a lot like steam locomotives that can supply as much horsepower as the prime mover (boiler in the analogy) can muster.

Especially DC traction motors have a rating in terms of the amount of current they can handle, which is more or less proportional to tractive effort.  If you could maintain the rated current at higher and higher speed, the traction motor applies more and more horsepower.

DC traction motors have a back EMF limitation -- to rotate them faster and faster, the electric transmission needs to apply more and more voltage across each traction motor.  They also have a mechanical upper limit on rotational speed where they "bird's nest" (fly apart into a wrecked tangle of copper windings), but generally speaking, the voltage limit that can be supplied by the electric transmission is reached well before the mechanical limit, that is, unless there is some hidden defect how the motor was manufactured.  Motor voltage to keep up with increased rotational speed can be supplied by transition (reconnecting the traction motors from series to parallel configuration) and field shunting (weakening), to trade off reduced tractive effort for reduction in the back EMF to get more speed.

The smaller wheels on the E-unit was, I believe, explained on an early thread that can be found by searching in posts by Bogie Engineer?  I think it had something to do with getting all the parts of the A1A truck to fit together.

As to the multitude of horsepower rating of nearly-the-same generation of EMD locomotives, the person most knowledgable would be oltmannd, who worked for Norfolk Southern in the acquisition of locomotives and training in their maintenance?  He knows from the user-of-diesel-electric locomotive side of things the specifics of how EMD wrung horsepower ratings, in keeping up with competitors, of the mid-generation diesel models, say, from GP20 through SD24 to GP and SD35s.  The 35-series were said to be particularly complicated in their transition and field-weakening steps to get 2500 HP out of the 567 (cubic inches per cylinder) diesel and the DC generator.

The GP and SD40s with their bigger 645 diesel and their use of an alternator followed by a silicon rectifier to power the traction motors were simpler.

If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
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Posted by beaulieu on Saturday, July 24, 2021 12:02 PM

At low speeds(under about 17 mph) a GP40's control circuits will only allow its alternator to provide about 1.5 MW of power to the 4 D77B traction motors (about 2000 hp the same as a GP38) due to heating of the traction motors and the limitations of the wheel slip control systems. The GP35 and to a lesser extent the GP30 suffer from having a older and simpler control system that relied on many steps of transition of the electrical power system to match power output to the needs of the Engineer versus the limitation of the D67 and D57 series traction motors. These control systems did not function well enough and were not rugged enough in regular day to day use on the railroads. The SD24 and SD35 with two additional traction motors and more locomotive weight did not suffer from most of the wheelslip problems and control circuit failures as the additional motors could handle the power at all but the very lowest of speeds(power divided by 6 versus by 4).

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Posted by Erik_Mag on Saturday, July 24, 2021 5:07 PM

beaulieu

The GP35 and to a lesser extent the GP30 suffer from having a older and simpler control system that relied on many steps of transition of the electrical power system to match power output to the needs of the Engineer versus the limitation of the D67 and D57 series traction motors.

The "many steps of transition" refers to several taps on the traction motor field windings. Tapping the field would allow for a higher power output from a DC series motor for a given motor speed and voltage.

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Posted by Overmod on Saturday, July 24, 2021 7:37 PM

Erik_Mag
The "many steps of transition" refers to several taps on the traction motor field windings. Tapping the field would allow for a higher power output from a DC series motor for a given motor speed and voltage.

For those that don't know motors, a little more explication might be desirable.

This type of motor has both a wound stator and a wound armature, and current flowing through them generates magnetic fields that interact to produce rotation and torque.  

However, as the armature rotates faster and faster it also tends to develop a reverse voltage (called back EMF or BEMF) that by Lenz' law opposes the direction of rotation.  Transition and field weakening work to reduce this, and by doing so allow the armature to spin faster for a given applied voltage.

The 'taps' Erik mentions might better be called 'shunts', as they remove (or perhaps better stated, reduce) magnetic strength in the stator without need for the heavy and expensive switches used to make formal transition (remember the 'hunting season'?)  The catch is that the amount of torque the motor generates is a function of current, and with lower current effectively flowing in the stator to produce torque, more has to pass through the brushes and armature windings to increase the field there.  Since little more than airflow effectively cools the spinning armature, and heat generation goes proportional to the square of current... there are some worries.

Occasionally mentioned was the somewhat daft experiment with field weakening coils on one of the North Shore Electroliners.  These worked altogether too well, leading to a speed at one point of 108mph on 28" wheels; at the end of the run someone was called upon to torch off the coils, which was done, and no one apparently called for field weakening again!

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Posted by BEAUSABRE on Sunday, July 25, 2021 5:35 AM

timz
Guess you're wondering why the GP20 wasn't 2400 hp like the SD24? No one here knows. Maybe EMD hadn't figured out a good way to limit the horsepower at 10-15 mph, so a 2400-hp GP would have a minimum continuous speed sround 16-17 mph? Likely the GP20 and SD24 had the same traction motors? The motors would have current limits and a voltage limit, but I'm guessing they could handle 600 hp apiece.

This would seem to answer the qustion - the limits of the D47 traction motor

Omaha GP20's, Union Pacific's GP9 turbocharging program - By Don Strack

 

"When EMD first offered the SD24 in early 1959, they did not consider a four-axle version of their 2,400 horsepower engine because the EMD marketing and sales people felt that the additional cost of the turbocharger was not warranted to gain a mere 250 horsepower over the GP9's 1,750 horsepower -- 2,000 horsepower being the limit of four D47 traction motors. However, the success of their turbocharger in the UP GP9s led to EMD offering the GP20."

It occurs to me that EMD was missing the point of UP's "Omaha GP20" program, it was to maintain the full horsepower rating when crossing the Continental Divide at or above a mile above sea level. Additional horsepower was great, but UP wanted to get the performance at altitude

 

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Posted by Overmod on Saturday, July 31, 2021 2:32 PM

By extension the GP30 was only 2250hp when the 'four-cycle competition' were all offering 2400hp -- note that FM would have happily built a C-24-4 but there were no takers, and C-24-5s only had four traction motors.

The nominal 'improvement' in engine horsepower between the GP30 and GP35 was the increase to 900rpm, which argues indirectly that the 2250hp restriction was also electrical.

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Posted by BEAUSABRE on Saturday, July 31, 2021 4:12 PM

GP30 had the 567D3 engine (835 RPM) and D57 traction motors

GP35 had the 567D3A engine (900 RPM) and D67B traction motors

 

 

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