[ausev] about trains

[John Pearce]

Hi All,

Just to clarify: all of the original diesel locomotives used series-wound DC motors (rotor and stator connected in series).  This is because series-wound DC has the highest "locked-rotor" (i.e. starting) torque.  There is an even number of traction motors in a locomotive.  Each traction motor is geared to the axle and the (fixed) gear reduction is something like 12:1 or 15:1 depending on the traction system design.  There is sort of the equivalent of a transmission (see below).  Most of the new modern diesels are AC traction motors (60 MAC, for
example) with pretty sophisticated controllers.  I'm not familiar with
their inner workings.

When starting a train  two of the DC traction motors are connected in series because the required voltage is low (pairs of traction motors are still in parallel, of course, so the name is "series-parallel").  As you pick up speed (usually somewhere in the 15 to 17 mph range ... i.e. main generator output voltage in the range of 960 volts in General Motors EMD engines, perhaps a bit lower in the Alco that we operate) you encounter "first transition" in which a parallel bypass current pathway around the stator (i.e. "field") winding is switched in (more current to the rotor at the same overall voltage) .  The two traction motors are still connected in series (it's called "series-parallel shunt").  At higher speeds (usually around 25 - 30 mph, more or less ... it is actually determined by the main generator output voltage) "second transition" puts all of the traction motors in parallel (called simply "parallel") and drops out the shunting current pathway
 around the field windings at the same time.  The final transition connection is "parallel shunt" in which the shunting pathway around the field windings is again connected in.  I don't know at what speed range that happens ... it's above any speed that we operate.

So, no, there is no transmission in a locomotive per se, but its equivalent, I think.  I know that several members of the club say that they never get out of 3rd gear in their EV.  I guess that it seems to me that if you want to go faster, then a higher gear (4th or 5th) ought to give you more speed at a lower voltage (i.e. a lower DC motor shaft rpm).  Is that a silly notion?


John Pearce
Diesel Locomotive Engineer
Austin & Texas Central RR

P.S. Not to put too fine a point on it or pick too many nits, but I believe that "shitty" conventionally has two "t"s.    :-)







________________________________
From: "ausev-request at austinev.org" <ausev-request at austinev.org>
To: ausev at austinev.org
Sent: Friday, December 19, 2008 6:00:41 AM
Subject: AusEV Digest, Vol 35, Issue 15




-----Inline Message Follows-----


I understand that electric motors have full power the second the shaft turns and all the way up to max. RPMs (10,000 rpms). I know that gas engines only have power between 1000 - 3,500 rpms. It is obvious that gas motors need a transmission. But right now I am under the impression that electric motors don't need a transmission. I think trains have a motor built right on to the axle. If an electric motor can break that kind of static friction to move a freaken train with out a trani then why would you need a trani for a car. The diesel motor on the train powers a generator,.... I have heard that the trani used in the EV1 & RAV4-ev was a "single speed reduction integrated with the motor and differential, to help with torque. I have also read that the Owen Magnetic made a car, before the depression, moved by an electric motor that did not have a trani. It was sold to rich people. Some electric motor companies will sell gear down boxes with their motors. To
 not have a trani in the car would mean WAY less friction losses and less maintenance,....

My question is do you need a trani for an electric car or not. Are my facts straight,... I am ready to be educated. Thanks for your time folks.

Jim



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