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Re: Locomotive electrical systems
Author: Dr Zarkoff
Date: 08-01-2013 - 21:24
>The consideration in EMD's case has nothing to do with the voltage rating of the traction motors. The apparent issue is the (lack of) capacity of the main generator. The typical low horsepower rating of switch engines did not justify the cost/investment in the largest main generators which would rarely even approach their maximum ratings. To connect a pair of EMD traction motor strings across a main generator requires around 1500-2600 amps to work at full capacity (depending on model/era of traction motor design). By connecting the four motors in series instead of series-parallel, a smaller (and cheaper) main generator of 800-1200 amps is more than sufficient. Since the typical switch engine is primarily operated at low speeds, this is not really a drawback under most conditions.
Sorry, I don't buy this for a number of reasons. If a system has motors connected to a generator and they exceed the capacity of the generator, the output of the motors can never exceed the generator's capacity for the simple reason that the generator can't meet the demand. The limiting factor has become the generator, not the motors. It's no different than screwing a 220v lamp into 120v. You get light, just not the amount rated for the lamp on 220v.
Another reason is why invest capital in inventory and engineering costs to design a number of different motors when you don't really need to do so? If you need to produce a switch engine which has less HP than a road engine, cut your costs buy using the same traction motors with a smaller generator. This way all you have in the way of development and manufacturing costs are those for the smaller generator, which you need to do -- you already have designed usable traction motors.
>The tractive effort formula for diesel-electric locomotives is traction hp x 375 x efficiency divided by speed, understanding that at low speeds the theoretical tractive effort will exceed available adhesion. In other words, the approximate tractive effort (drawbar pull) halves as speed doubles, and the ammeter will reflect that.
The beauty of a series wound motor is that it will draw precisely the current it needs to do the job demanded of it (accelerate the train). If you exceed that value, then there will be wheel slip.
>I run and work on the electrical systems of various GE and EMD switchers, and GP units built/rebuilt 1950s-1980s; and can attest that there is a wide variation in the electrical arrangements. I believe all the EMD switchers from at least the SW1/NW2 use the full series to series-parallel arrangement. The SW1000/SW1001/SW1500/MP15DC use a permanent series-parallel arrangement with a D32 main generator, though there was an option of the older/smaller D25 main generator and the older transition arrangement on the SW1000/SW1001. Even though the MP15AC uses an AR10 which will support all four motors in parallel at 4200 amps, the motors are in series-parallel with one step of field shunting with the AR10 limited to 2500 amps(per Service Manual). The AR6 would have been a good option for the MP15AC/GP15AC/GP15T but apparently parts standardization on the AR10 won out. IIRC the SW900 manual shows that in switching (instead of road), it operates in series-parallel instead of series. All these variations were apparently done by EMD to improve the operating characteristics of switch engines.
OK fine. Nice set of statistics too, but I've never heard of nor seen a diesel which had the necessary switch gear for series, series-parallel, and parallel (barring of course those slug units you mention, which are very special cases). There simply is no reason to have all three in a diesel. In a constant-voltage DC electric, yes (NYC and MILW), but not a diesel.
When you put it this way: "The SW1000/SW1001/SW1500/MP15DC use a permanent series-parallel", it means that these engines had no transition at all, which I don't think you meant to say, and definitely isn't the case.
In switching, I would more expect full field start than minimum field start. SP's SW-1500s had a switch position which would prevent forward transition ("Forestalling").
>The SD45 manual shows it has up to ten steps of forward transition: series-parallel with 5 steps of field shunting, and parallel with 3 steps of field shunting. Field shunting does not typically cause the loading to "stutter" compared to motor connection changes. Not sure when an SD45 would normally make series-parallel to parallel transition, but it may actually be fairly high (15-25 mph?) due to the number of field shunt positions and depending on gearing.
Any locomotive with series-parallel and parallel control has only ONE step of transition: between series-parallel and parallel. Field shunting ISN'T transition, no matter how many steps of it there are. You will see and hear "series transition" and "parallel transition" used, but they are misnomers -- same for calling field shunting positions "transitions".
>While temperature does have an effect on resistance, the primary cemf resistance change in traction motors is caused by the magnetic fields cutting through each other.
Nope. It's caused by the conductors in the armature cutting the magnetic field lines created by the field coils' magnetism. What the interactions of the two magnetic fields do is cause the armature to spin and the motor's [magnetic] neutral point to shift around the commutator as loading changes, unless the motor has compensating windings (a.k.a. "interpoles" or "commutating poles").
>GE, on its trolley, smaller and electric locomotive traction motors did have a wide variety of insulation valves and voltage ratings, with some of the same models having more than one voltage rating. (On GE part numbers, the suffix can be nothing more than an improved model or an entirely different gear ratio, voltage rating, interlock arrangement, etc.)
In the literature of the day, you will frequently see traction motors listed as "600v/1200v" produced by both GE and WH, but all this means is that they were designed to run at full speed on 600v and insulated for 1200v. This cut down on the high costs of traction motor design, manufacture, and inventory.
The only GE motor of the same number that comes to mind which was wound for operation at full speed on different voltages is the 205. It was produced in one or three versions wound for 1200v (differing wheel diameters and gear ratios). Other versions were wound for 600v, some of which were insulated for 1200v.
> On the Amtrak version, the 16-645 at 900 rpm in Run 8
When producing HEP, the engine doesn't quite attain "normal" Run 8 rpms but comes very close to it. It's similar with direct-connected GEs.
>GP9s and EMDs of the era use a load regulator to control excitation.
Along with a [DC] generator specially designed to match the motors' power curves more closely, this is the control system developed by GE's Hermann Lemp for EMC c1922. It was used by EMC/EMD, Alco, and GE, which is why they have always been able to be MUed together. EFI systems (both GE and EMDEC) emulate this.
Sorry, I don't buy this for a number of reasons. If a system has motors connected to a generator and they exceed the capacity of the generator, the output of the motors can never exceed the generator's capacity for the simple reason that the generator can't meet the demand. The limiting factor has become the generator, not the motors. It's no different than screwing a 220v lamp into 120v. You get light, just not the amount rated for the lamp on 220v.
Another reason is why invest capital in inventory and engineering costs to design a number of different motors when you don't really need to do so? If you need to produce a switch engine which has less HP than a road engine, cut your costs buy using the same traction motors with a smaller generator. This way all you have in the way of development and manufacturing costs are those for the smaller generator, which you need to do -- you already have designed usable traction motors.
>The tractive effort formula for diesel-electric locomotives is traction hp x 375 x efficiency divided by speed, understanding that at low speeds the theoretical tractive effort will exceed available adhesion. In other words, the approximate tractive effort (drawbar pull) halves as speed doubles, and the ammeter will reflect that.
The beauty of a series wound motor is that it will draw precisely the current it needs to do the job demanded of it (accelerate the train). If you exceed that value, then there will be wheel slip.
>I run and work on the electrical systems of various GE and EMD switchers, and GP units built/rebuilt 1950s-1980s; and can attest that there is a wide variation in the electrical arrangements. I believe all the EMD switchers from at least the SW1/NW2 use the full series to series-parallel arrangement. The SW1000/SW1001/SW1500/MP15DC use a permanent series-parallel arrangement with a D32 main generator, though there was an option of the older/smaller D25 main generator and the older transition arrangement on the SW1000/SW1001. Even though the MP15AC uses an AR10 which will support all four motors in parallel at 4200 amps, the motors are in series-parallel with one step of field shunting with the AR10 limited to 2500 amps(per Service Manual). The AR6 would have been a good option for the MP15AC/GP15AC/GP15T but apparently parts standardization on the AR10 won out. IIRC the SW900 manual shows that in switching (instead of road), it operates in series-parallel instead of series. All these variations were apparently done by EMD to improve the operating characteristics of switch engines.
OK fine. Nice set of statistics too, but I've never heard of nor seen a diesel which had the necessary switch gear for series, series-parallel, and parallel (barring of course those slug units you mention, which are very special cases). There simply is no reason to have all three in a diesel. In a constant-voltage DC electric, yes (NYC and MILW), but not a diesel.
When you put it this way: "The SW1000/SW1001/SW1500/MP15DC use a permanent series-parallel", it means that these engines had no transition at all, which I don't think you meant to say, and definitely isn't the case.
In switching, I would more expect full field start than minimum field start. SP's SW-1500s had a switch position which would prevent forward transition ("Forestalling").
>The SD45 manual shows it has up to ten steps of forward transition: series-parallel with 5 steps of field shunting, and parallel with 3 steps of field shunting. Field shunting does not typically cause the loading to "stutter" compared to motor connection changes. Not sure when an SD45 would normally make series-parallel to parallel transition, but it may actually be fairly high (15-25 mph?) due to the number of field shunt positions and depending on gearing.
Any locomotive with series-parallel and parallel control has only ONE step of transition: between series-parallel and parallel. Field shunting ISN'T transition, no matter how many steps of it there are. You will see and hear "series transition" and "parallel transition" used, but they are misnomers -- same for calling field shunting positions "transitions".
>While temperature does have an effect on resistance, the primary cemf resistance change in traction motors is caused by the magnetic fields cutting through each other.
Nope. It's caused by the conductors in the armature cutting the magnetic field lines created by the field coils' magnetism. What the interactions of the two magnetic fields do is cause the armature to spin and the motor's [magnetic] neutral point to shift around the commutator as loading changes, unless the motor has compensating windings (a.k.a. "interpoles" or "commutating poles").
>GE, on its trolley, smaller and electric locomotive traction motors did have a wide variety of insulation valves and voltage ratings, with some of the same models having more than one voltage rating. (On GE part numbers, the suffix can be nothing more than an improved model or an entirely different gear ratio, voltage rating, interlock arrangement, etc.)
In the literature of the day, you will frequently see traction motors listed as "600v/1200v" produced by both GE and WH, but all this means is that they were designed to run at full speed on 600v and insulated for 1200v. This cut down on the high costs of traction motor design, manufacture, and inventory.
The only GE motor of the same number that comes to mind which was wound for operation at full speed on different voltages is the 205. It was produced in one or three versions wound for 1200v (differing wheel diameters and gear ratios). Other versions were wound for 600v, some of which were insulated for 1200v.
> On the Amtrak version, the 16-645 at 900 rpm in Run 8
When producing HEP, the engine doesn't quite attain "normal" Run 8 rpms but comes very close to it. It's similar with direct-connected GEs.
>GP9s and EMDs of the era use a load regulator to control excitation.
Along with a [DC] generator specially designed to match the motors' power curves more closely, this is the control system developed by GE's Hermann Lemp for EMC c1922. It was used by EMC/EMD, Alco, and GE, which is why they have always been able to be MUed together. EFI systems (both GE and EMDEC) emulate this.
