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Eastern Railroad Discussion > transition


Date: 05/04/07 00:08
transition
Author: PennCentral00

Even though I should know this I don't!! Locomotive school was 20 years ago and I've forgotten all that small stuff.Why does a locomotive need to make transition and how does it do it? I remember some of the old heads saying that that had transition levers on the covered wagons and it had to be done manually. I also remember seeing these levers on the GP7's and 9's during Penn Central but never saw an engineer using them. Do GP38's and 40's or any of the newer units need to "make transition"?
Thanks in advance..
~~SS~~



Date: 05/04/07 06:44
Re: transition
Author: run8

PennCentral00 Wrote:
-------------------------------------------------------
> Why does a locomotive need to make transition and how does it do it?

The purpose of transition is to keep the voltage and amperage in the working range for the traction motors and generator. If the amperage gets too high, it can overheat the motors, damaging them. If the voltage gets too high, the generator will not be able to put out enough voltage, and the horsepower will drop off, as I'll explain below. High voltages can also damage electrical components like rectifiers, or you can get a flashover, which also damages things.

The reason voltage and amperage changes on a DC motor is that as the motor turns, it creates a voltage called back EMF. Essentially, the motor is acting like a generator. The faster the motor turns, the higher the back EMF. To get it to act like a motor, the voltage from the generator simply has to be higher than the back EMF of the motor at whatever speed it is turning.

The resistance a motor provides to current flow also changes as the RPM of the motor changes. It is very low when the motor is barely turning, but increases as the motor RPM increases. That means the current passing through the electrical system can be high at low speeds, but drops off as the speed increases. This is the opposite of the voltage change.

Therefore, the designers of locomotives have to worry about high amperage at low speed, and high voltage at higher speeds. There are limits to both. Note that none of this affects the pulling power of the locomotive, other than for short time ratings. It is only necessary to avoid damaging the components.

One way of increasing the resistance of the motors is to put them in series. Two motors in series will have 1/2 the current passing through them as the same two motors in parallel. The down side of this is that as the RPM increases, the voltage of the motors in series will rise at twice the rate of the voltage of the same motors in parallel.

So consider a six axle locomotive, like and SD-40. At start, the motors are set up in three strings. Each string has two motors in series, and the three strings are wired in parallel. This reduces the currents to 1/2 of what they would have been had all motors been wired in parallel. The current with this series/parallel arrangement is within the acceptable range for normal service at lower speeds.

Now, as the locomotive accelerates, the voltage climbs in proportion to the RPM of the motors, while the current will drop off. It will reach a point where the voltage becomes too high for the generator, which will not be able to put out enough voltage to overpower the back EMF of the motors. At this point, the locomotive will have to transition to keep accelerating. Transition means that the motors will be reconnected from series/parallel into all motors in parallel. The effect is that the voltage will drop to 1/2 of what it had been, and the current will double. Everything is again moved to within the working range of the locomotive, and there is room to continue accelerating.

As previously stated, none of this has any appreciable effect on the pulling power of the locomotive. The tractive effort before transition is practically the same as the tractive effort after transition.

There was another type of transition that was sometimes used called field shunting. With this technique, the electrical field of the traction motors was weakened through the use of shunting relays. Locomotives like the C-425 and GP-35 used both types of transition. The problem with field shunting is that as the field is progressively weakened, the commutation of the motors becomes progressively unstable, with heavy arcing. If the voltage points that drive the automatic transition weren't set up properly, the motors could flash. It was a common problem with locomotives prior to the use of alternators.

> Do GP38's and 40's or any of the newer units need to "make transition"?

The original GP-40s used field shunting. GP-40-2s had large enough alternators that they could stay within the acceptable voltage/amperage range throughout the working speeds of the locomotive without the need for transition. The motors on both types of locomotives are wired permanently in parallel.

This was a bit of a compromise with the GP-40-2, because it would start losing power as it reached about 60 mph, because it was reaching its voltage limit. At 65 mph, it might be down a few hundred horsepower from its rated 3000 hp.

Newer six axle locomotives from GE have large enough alternators that the motors are premanently wired in parallel, and there is no transition. EMD uses transition on their six axle locomotives, but it has shifted into the generator. They have a pair of windings in the alternator which are switched between series and parallel at the transition point. This simplifies the design of the locomotive, and makes it easier to control wheel slippage.

AC traction locomotives don't need transition, since the power is applied to the motors using a different approach, so the voltage does not change with motor RPM.



Date: 05/04/07 07:57
Re: transition
Author: rhotond

For the non electrical types, tansition as explained above is nothing more than an "effective electrical gear ratio change" The loco's have only one gear ratio so us electrical guys have come up with a 'two (series to series/ parallel) or a three (series to series parallel to series parallel shunt field) speed transmission.

Note on the comments on AC drives for EMD that I know and possibly GE). Close examination of the Tractive effort curve indicates that the at high speeds either the alternator runs out of voltage, or the inverter(s) is voltage limited and thus the loco cannot develop full HP above 60 mph. This was the reason that early AC loco's recieved the comment from engineers "the loco pulls great but above 45 or 50 mph it is worthless". That is the reason DC motor units are more popular with TTX and high speed manifest (eg BNSF uses DC exclusively on the transcon) and the reason that NS feels that DC units are more versitle (and cheaper).

r



Date: 05/04/07 08:54
Re: transition
Author: run8

rhotond Wrote:
-------------------------------------------------------
> For the non electrical types, transition as
> explained above is nothing more than an "effective
> electrical gear ratio change"

I avoid that description, since many people have the impression it is like a mechanical transmission, and that locomotives without transition, like the GP-40-2 don't have good haulage capacity because they are trying to start trains in second gear. In reality, transition is not necessary on all locomotives, and the lack of transition by design doesn't significantly affect a locomotive's performance. Of course if the transition on a locomotive equipped with the feature fails, it does bad things to performance.

> The loco's have only one gear ratio so us electrical guys have come up with a
> 'two (series to series/ parallel) or a three (series to series parallel to
> series parallel shunt field) speed transmission.

It's been decades since anything started in full series.

Only an electrical type could have wet dreams about the 16 step transition on a GP-35:
     - series parallel - full field
     - series parallel - 20% shunt
     - series parallel - 35% shunt
     - series parallel - 44% shunt
     - series parallel - 51% shunt
     - series parallel - 57% shunt
     - series parallel - 61% shunt
     - series parallel - 65% shunt (three steps)
     - parallel - Full field
     - parallel - 20% shunt
     - parallel - 35% shunt
     - parallel - 44% shunt
     - parallel - 51% shunt
     - parallel - 57% shunt
What a nightmare to set up and troubleshoot.



Date: 05/04/07 09:03
Re: transition
Author: run8

rhotond Wrote:
-------------------------------------------------------
> Note on the comments on AC drives for EMD that I
> know and possibly GE). Close examination of the
> Tractive effort curve indicates that at high
> speeds either the alternator runs out of voltage,
> or the inverter(s) is voltage limited and thus the
> loco cannot develop full HP above 60 mph. This
> was the reason that early AC loco's received the
> comment from engineers "the loco pulls great but
> above 45 or 50 mph it is worthless". That is the
> reason DC motor units are more popular with TTX
> and high speed manifest (eg BNSF uses DC
> exclusively on the transcon) and the reason that
> NS feels that DC units are more versatile (and
> cheaper).

The price difference is the primary reason, since the railroads get so little benefit from the extra cost AC locomotive in higher speed services. However, a couple of railroads (UP and CP in particular) don't seem to care, and buy more expensive AC traction locomotives anyway. They seem to use them interchangeably in both unit train and intermodal services. It's nice to have extra money to burn, aince it makes operating management so much simpler.



Date: 05/05/07 17:17
Re: transition
Author: timz

> I remember some of the old heads saying
> that that had transition levers on the covered
> wagons and it had to be done manually.

As I recall, after around 1950 most/all diesels had automatic transition, but they still had the lever so the engineer could shift the older units that the F7 or whatever might be MUd with.



Date: 05/05/07 17:21
Re: transition
Author: timz

> It's been decades since anything started in full
> series.

Did any road locomotive ever start in full series?

And switchers that started in full series probably stayed there-- no transition?



Date: 05/06/07 09:51
Re: transition
Author: run8

timz Wrote:
-------------------------------------------------------
> > It's been decades since anything started in full series.
>
> Did any road locomotive ever start in full
> series?
>
> And switchers that started in full series probably
> stayed there-- no transition?

Alco S1 and RS1 locomotives both stated in series and had transitions to series-parallel and field shunting, as an example. Others of that era might have as well.



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