Tractive Effort, Horsepower and Slugs

[Anonymous]

Oh my...

Power sets the rate of which work is performed. 2000 hp loco will need 1/2 of time to do the same job that 1000 hp loco would do.

Also - if we disregard adhesion limit - 2000 hp loco willl pull twice the tonnage of 1000 hp loco.

The problem lies in the fact that adhesion is limited - mostly by locomotive mass and efficency of wheel slip control systems.

So - gp38 (2000 hp) with 55000 lb of TE is at its adhesion limit. gp40 (3000 lb) could theoreticlly do 82500 lb TE, but adhesion will limit it to about 55000 lb... So pretty much 1000 hp is not used.

hence the idea of slugs - the "extra" 1000 hp at slow speed is sent to the slug thus increasing TE to 110000 lb.

[ericsp]

QUOTE: Originally posted by uzurpator

Oh my...

Power sets the rate of which work is performed. 2000 hp loco will need 1/2 of time to do the same job that 1000 hp loco would do.

Also - if we disregard adhesion limit - 2000 hp loco willl pull twice the tonnage of 1000 hp loco.

The problem lies in the fact that adhesion is limited - mostly by locomotive mass and efficency of wheel slip control systems.

So - gp38 (2000 hp) with 55000 lb of TE is at its adhesion limit. gp40 (3000 lb) could theoreticlly do 82500 lb TE, but adhesion will limit it to about 55000 lb... So pretty much 1000 hp is not used.

hence the idea of slugs - the "extra" 1000 hp at slow speed is sent to the slug thus increasing TE to 110000 lb.

The maximum force a locomotive can pull with is its wieght multiplied by the coefficient of kenetic friction between the wheels and the rail. So if the 1000 HP locomotive and the 2000 HP locomotive have wheels made from the same materials, unless the 2000 HP locomotive weights twice as much as the 1000 HP locomotive, it will never be able to pull twice the tonnage.

Forget about the theoretical 82500 lb TE, that is based on horsepower and it should not be. If you take electric motors running at relatively slow speeds out, then there is no relation between power and tractive effort. Maybe the formula TE (lbf) x speed (mph)/308 = HP is confusing. This is Force * Velocity = Power. However, the limiting factor for the force is not the power, in this case. It is the weight of the locomotive multiplied by the coefficient of kenetic friction (which can be increased by wheel slip control program). However, to get the maximum tractive effort, the electric motor (traction motor) must supply enough torque to the axles so the the torque divided by wheel radius is equal to the weight of the locomotive on those two wheels multplied by the coefficient of kenetic friction (actually, static friction until the locomotive starts rolling). Multiply the torque by the angular velocity of the wheel gives the power that the traction motor must deliver. If this heats up the motor and the wirings beyond what they can safely handle, then this is the limiting factor.