Continuous tractive effort

Trains Magazine General Discussion
1

In the Wikipedia article about the F7 it states that the F7 had 20% greater tractive effort than an F3, with both engines geared the same way. How is that possible? With the same horsepower output and gearing, how was the F7 able to achieve such an improvement in continuous tractive effort?

http://en.wikipedia.org/wiki/EMD_F7

2

The short answer is that the F7 had better traction motors which could apply more horsepower continuously than could the traction motors in the F3. The F3 had a D12 main generator and four D17B traction motors. The F7 had a D12B main generator and four D27C traction motors.

3

Thanks DS… that’s quite an improvement in traction motors over such a short span of time.

4

Ulrich,

You may know but for those who do not, continuous tractive effort is what the unit can generate for an indefinite period of time. It is limited by the heat generated in the traction motor. Holding power input constant, as motor speed drops, heat in the motor increases.

CTE is usually expressed in pounds at the Minimum Continuous Speed. I suspect you would find the MCS of F3 units higher than F7, as delivered. I also suspect that roads with significant drag freight work put F7 traction motors under F3 units so mixed consists could be run together at TE and MCS of factory F7 units.

Mac

5

What none of us knows is: could the F7 motor handle more amps continuously because it didn’t get as hot for a given number of amps, or because it did get hotter but the heat didn’t damage it?

A 62:15 FT or F3 was rated 32500 lb continuous-- the F7 was 40000 lb. Alcos had a similar increase around that time, from 42500 to 52500 lb. No idea whether for the same reason.

6

Or a combination of both…

7

Actually the increased tractive effort started with the F5, an upgraded F3 with the D27B traction motors.An F5 demonstrator was built in March 1948 and sold to the KCS as their #59A that October. Regular F5 production started in August 1948. The “F5” was a term used by EMD’s engineering department, as EMD’s marketing department still called them F3s. Data from “The F5” by Larry G. Russell Extra 2200 South Issue #96. Units with 65:12 gearing used the D27E traction motor.