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GP20 vs SD24

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GP20 vs SD24
Posted by VGN Jess on Monday, March 8, 2021 4:41 PM

From the Wiki on GP 20: "While the EMD SD24 with six axles was producing 2,400 horsepower (1,800 kW) with an engine of the same displacement (400 horsepower (300 kW) per axle), the four axle GP20 was limited to 2,000 horsepower (1,500 kW) (500 horsepower (370 kW) per axle) by the capabilities of the traction motors and the direct current traction generators of the time." What doesn't "compute" for me is: if the SD24 was in service BEFORE the GP20, why would the GP20 NOT have had the capabilities of the traction motors and the direct current traction generators of the SD24?

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Posted by SD70Dude on Monday, March 8, 2021 4:53 PM

You've answered your own question, the GP20 only has four motors instead of six.

The SD24's power output was limited only by what the diesel engine and generator could produce.  Had a more powerful engine been available it might have been rated at 3000 HP instead, using the same 500 HP per axle ratio as the GP20.

The GP20's power output was limited by what the traction motors then in use could handle. 

It is worth noting that most locomotives cannot use their full power output at low speeds, and lighter four axle or A1A trucked units are especially slippery.  As an example, the GP40 automatically derates to a maximum output of 2000 HP until you get above about 20 mph, no matter what throttle notch you are in.

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Posted by Erik_Mag on Monday, March 8, 2021 10:12 PM

In addition to issues with adhesion for 4 axle power, the other problem was dealing with the current and voltage limits of the traction motors and traction generator - there was a reason why Alco, EMD and GE started using traction alternators in the mid-60's. The GP-35/SD-35/DD-35 were EMD's last high power locomotive models that used traction generators, and the GP-35 needed several stages of field shunting in addition to series, series-parallel and full parallel transition to make it all work.

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Posted by BaltACD on Monday, March 8, 2021 10:14 PM

SD70Dude
...

It is worth noting that most locomotives cannot use their full power output at low speeds, and lighter four axle or A1A trucked units are especially slippery.  As an example, the GP40 automatically derates to a maximum output of 2000 HP until you get above about 20 mph, no matter what throttle notch you are in.

Not being able to use all the 'electrical power' generated in 4 axle locomotives was part of the reason the CSX got involved in building Mother-Slug locomotives.  The engine powered mothers at speeds less than 25 MPH powered would have their excess electrical power transmitted to the traction motors of the slug.  The slugs were carbodies of older 4 axle locomotives that were ballasted to the normal engine weight as the prime mover, generator etc. were removed.

In practical operation a 'near tonnage' train for a mother-slug most likely will never exceed 25 MPH as there isn't a sufficient amount of electrical power generated to reach higher speeds.

At the start, CSX tried to have the fuel tanks of the slugs operational and a tranfer pump system to transfer fuel to the mother.  It was problematical and was soon scrapped. 

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Posted by VGN Jess on Tuesday, March 9, 2021 2:44 AM

SD70Dude: Thanks. So, in essence the GP20 had more HP/Axle (500) because it came AFTER the SD24 (400), so the GP20 actually had more capable traction motors than the SD24, just less HP because of the number of axles powered?

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Posted by SSW9389 on Tuesday, March 9, 2021 5:59 AM

VGN Jess

SD70Dude: Thanks. So, in essence the GP20 had more HP/Axle (500) because it came AFTER the SD24 (400), so the GP20 actually had more capable traction motors than the SD24, just less HP because of the number of axles powered?

 
The first turbocharged geep producing 2000 horspower was rebuilt in late 1955 and two more were completed in the first half of 1956 for Union Pacific. See Don Strack's excellent article on the turbocharging of the GP9 here Union Pacific Turbocharged GP9s (utahrails.net)  It's more of how that horsepower was controlled than how it was generated. The first turbo geeps would have used the D37 traction motors. The D47 traction motors would have been developed as a result of testing those turbocharged Union Pacific GP9s. 
 
And on a side note EMD had 600 horsepower D37 traction motors used in the E9s starting in 1954. 
 
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Posted by SSW9389 on Tuesday, March 9, 2021 6:19 AM

The EMD Locomotive Reference Data dated January 1, 1959 shows two interesting models. They are the GP9 (Mod) rated at 2000 horsepower with the 16-567D (Mod) engine and the SD24 with the 16-567D engine. At that time EMD already had SD24 orders in hand from Santa Fe and Burlington, and was working with Union Pacific on refining the turbocharged Geep. Union Pacific's SD24 order would closely follow the Burlington and Santa Fe orders. 

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Posted by Backshop on Tuesday, March 9, 2021 6:20 AM

VGN Jess

SD70Dude: Thanks. So, in essence the GP20 had more HP/Axle (500) because it came AFTER the SD24 (400), so the GP20 actually had more capable traction motors than the SD24, just less HP because of the number of axles powered?

 

No, the SD24 didn't produce enough HP to fully utilize its traction motors.

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Posted by timz on Tuesday, March 9, 2021 10:03 AM

I'm guessing the SD24's traction motors were just as capable as the GP20's.

EMD started selling 2400 hp units with four traction motors in 1954.

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Posted by SSW9389 on Tuesday, March 9, 2021 10:10 AM

timz

I'm guessing the SD24's traction motors were just as capable as the GP20's.

If we look at all the Folio cards from the different railroads that owned GP20s and SD24s we will find that the majority of those units used the same EMD traction motor, the D47. 

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Posted by BEAUSABRE on Tuesday, March 9, 2021 2:09 PM

timz
EMD started selling 2400 hp units with four traction motors in 1954.

Tim, Wait a second. The GP9 was rated at 1750 hp, the GP18 at 1800 hp, the GP20 at 2000 hp and the GP30 at 2250 hp. The last gasp of the 567 engined BB's was the GP35, rated at 2500 hp. So far as I know. EMD never had a 2400 hp BB model...

 

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Posted by SD70Dude on Tuesday, March 9, 2021 2:21 PM

BEAUSABRE
timz
EMD started selling 2400 hp units with four traction motors in 1954.

Tim, Wait a second. The GP9 was rated at 1750 hp, the GP18 at 1800 hp, the GP20 at 2000 hp and the GP30 at 2250 hp. The last gasp of the 567 engined BB's was the GP35, rated at 2500 hp. So far as I know. EMD never had a 2400 hp BB model...

He's talking about the E9, which I had forgotten was introduced that early on.  Six axles but only four motors, and 2400 HP total from a pair of 12-567C engines.

I'd be interested to learn the detailed differences between the E9 and GP20 electrical systems, besides the obvious stuff like two generators instead of one. 

Or perhaps the GP20 design could have handled 2400 HP, but EMD and the railroads chose to derate it to save on wear, as a freight unit it would be expected to spend much of its life pulling hard at relatively low speeds, while a passenger unit like the E9 would be expected to run at higher speeds most of the time. 

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Posted by timz on Wednesday, March 10, 2021 10:41 AM

Judging by the continuous ratings, E-unit traction motors were rated for a bit lower amperage than F-unit motors. Continuous TE was around 10% lower for an E that was geared for the same top speed as an F. (In truth, they couldn't be geared for the exact same top speed.)

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Posted by Overmod on Wednesday, March 10, 2021 11:41 AM

timz
Judging by the continuous ratings, E-unit traction motors were rated for a bit lower amperage than F-unit motors.

What percentage of this reflects the different wheel size (which is what I presume to be the reason the gear ratios can' be the same)?

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Posted by timz on Thursday, March 11, 2021 11:37 PM

The 1956 Cyc says continuous rating for a 57:20 F7 is 27500 lb and for a 57:20 E8 is 27000 lb. Doesn't give the F9/E9 ratings.

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