PA vs E-Units

In short, the GE 752 traction motor with higher electrical ratings than the

EMD motors of the time . OF course GM was very aware of this and set about

to compensate with better motors. With the "nine" series in 1954, this was

largely accomplished, altho GE continued to upgrade their motors at the same

time.

Another thing which the PA's had which the E units (prior to the E-8) did not have was dynamic brakes.  For Mountain railroads, like SP, SF, and Rio Grande this was an important feature.  When EMD designed the E-8 an important improvement was the addition of dynmaic brakes.  On the earlier E units there was no location for the D.B. equipment.

Stuart

PA's had larger diameter drive wheels, which allowed the traction motors to turn faster at any given speed than did the 36" diameter wheels of the E-units,  That gave the Alco-GE units an advantage that, when combined with the tougher GE traction motor, allowed the PA to pull more tonnage on ascending grades.

Three Santa Fe PA-1's actually out-pulled four F3's in a test conducted in 1947.

A LARGER diameter wheel would make the traction motor run SLOWER for the same speed, if equipped with idententical gearing. Changing gearing ratios would also allow compensating for different size wheels.

Doug

?????  Think about it -- the teeth on the ring gear inside the wheel are the same size, but there are more of them, because the wheel is larger.  Therefore, the motor, using the same pinion gear, turns faster.

Sorry Number 90, but wheel size annd gear tooth count are NOT directly linked, you can get different gear ratios for the same size wheel. All 36 inch wheels do not have the same number of teeth on the bull gear. Different numbers of teeth are/were available for both the bull gear and the pinion gears for any given wheel diameter. 

 A larger wheel will travel farther per revolution than a smaller wheel will, hence  a larger wheel will have a SLOWER traction motor RPM at the same train speed as a smaller wheel will have at the same given speed(assuming identical gearing). Ring gear/Bull gear combinations are available in different ratios to compensate  for different wheel sizes, and/or different intended uses, such as Freight or Passenger use.

Don't equate a specific wheel diameter with having a certain specific bull gear tooth  count, to my understanding the gears and wheels themselves were/are seperate individual pieces, so that the  same wheel/axle assembly could have the gears changed for  a change of intended use, such as reassinging older passenger power to freight service when newer power was purchased for passenger service.

it was/is  not a small task to pull an axle assembly to change out a gear set, and was/is not done on a whim for a temporary change in duty, but was done for anticapated permanent or long term changes in use.

Doug

Number 90, please go to  http://www.pennlocomotivegear.com/gears_pinions.htm  this gives a couple of pics of some traction motor gears. The pinion gears run outside oof not inside the bull gears. That site will allso  show the various different tooth counts available for whatever service is intended for the particular locomotive being spec'd new or regeared.

Doug

Doug,

Changing the gear ratio on a single unit is not a big job for the engine house.  It would be done when the unit needs wheels. Each combo would be removed and replaced with a TM & wheel set with the new gear ratio.  The wheel shop would then dismount the wheels and the bull gear. The worn out wheels and the bull gear would be deposited in the scrap bin.  The TM would go to the motor shop, where  the pinion would be removed and placed in the scrap bin (preferably both pinion & bull gear should be cut with a torch.) 

Did that with a fleet of GP9's

Doug, I looked at your website and saw a gear catalog.  Not sure what else I should have seen.

We could argue this 'til the cows come home, but, without a table showing traction motor speed, torque, and current at a series of given speeds, there's no sense.

You can start up the 2% mountain with your 3 E6's (6,000 horsepower, A1A-A1A wheel arrangement, 36" wheels, 55/22 gear ratio) and 16 passenger cars, and I'll run around you about 10 miles up the hill with my 3 PA/PB units (6,000 horsepower, A1A-A1A wheel arrangement, 42" wheels, 61/22 gear ratio) and 16 identical cars.  You'll be stopped, cooling your traction motors.  I won't gloat -- I'll just whistle 2 short as I sail past.

Tom

For the same gear ratio larger wheels travel farther per revolution.  Electric motors have more torque the slower they go.  The reason the modern SD70s and AC44s have 44" wheels is to keep the motor turning slower thus keeping torque high (and internal centrifugal forces lower).

Richard

Richard,

You are trying to compare apples and oranges by trying to equate the AC traction SD70MAC units to older DC series wound traction motors on EMD E units up through SD40-2 units.

The main reason SD70MAC and newer units have 42" to 44" diameter wheels is for increased wheel life, and nothing really to do with rotor speed in the AC  three phase induction traction motors. Both the 42" and newer 44" "thick rim" wheels offer far improved wheel life over the "standard" 40 inch diameter wheels used from the FT days right up thru the 60 series units.

Generally "low rotor" RPM does NOT necessarily equate to "higher" adhesion/traction with a three phase, induction AC traction motor.

I never used the words traction or adhesion because there are many other factors involved. I simply tried to state that lower RPMs in any electric motor equates to higher torque.   DC motors are not effective as AC induction motors at creating that torque at lower speeds thus resulting in more heating vs creating torque.