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Discussion relating to the PRR, up to 1968. Visit the PRR Technical & Historical Society for more information.
 #517509  by urrengr2003
 
Unlike a diesel locomotive with DC Traction Motors that start at full field strength, the GG-1 starts with "weak fields." The bottom left warning light on the enginemans control panel was a yellow lense with the letters "Field Changeover." This was illuminated when the throttle was advanced to #1 position, when starting, and indicated resistance was placed in the fields of the traction motors. What caused this light to burn? How was transition controlled with a voltage or current relay? There was a time period allowed for this weak field operation that if exceeded caused the pantograph relay to operate effectivelly killing the locomotive. What was this time period & how was it controlled? Did advancing the throttle to above #3 position cause a shunt around the resistance to be established
and allow transition to occur?

Any PRR MofE man who recalls this would appreciate your recollections...would like to get this post into the hands of WD Volkmer.

Thanks for any help, realize this is rather a specific subject but trust someone can pick up on it that worked on these locomotives.
 #520702  by Nasadowsk
 
urrengr2003 wrote:Unlike a diesel locomotive with DC Traction Motors that start at full field strength, the GG-1 starts with "weak fields."
Actually, I think some EMDs did weak fielding, under some conditions...
The bottom left warning light on the enginemans control panel was a yellow lense with the letters "Field Changeover." This was illuminated when the throttle was advanced to #1 position, when starting, and indicated resistance was placed in the fields of the traction motors.
Actually, it wasn't a resistance, but a reactance. There were quite a few inductors in the thing besides the main transformer. Most or all PCB cooled.
What caused this light to burn?
Electricity :)

Actually, the light was on whenever FS2 was 'in'
How was transition controlled with a voltage or current relay? There was a time period allowed for this weak field operation that if exceeded caused the pantograph relay to operate effectivelly killing the locomotive.
It was controlled by a relay, in notches 2 - 14. I don't think it ran in the other notches, at least the schematics don't suggest that. Interpole shunting looks like it was always able to go on, though. A timer operating the PL relay? I don't see why, typically, the PL relay was to protect the transformer in a fault.
Thanks for any help, realize this is rather a specific subject but trust someone can pick up on it that worked on these locomotives.
I never worked on them, but I did have the schematics laying on my machine, and took a look. I think the actual answers are out there, but realize the last GG-1 ran in the early 80's, so the number of people who knew them is dwindling :(

 #521797  by urrengr2003
 
Thanks for your reply, you are correct in that Diesel Locomotives with DC Traction Motors did incorporate weak field (motor shunt) operation at the highest speed in their respective transition sequence. It was taken to the extreme with the GP-30 that had several stages of motor shunt operation controlled by a huge rotary switch (Program Switch) that was almost as large as a reverser. All this was necessary to reduce Back EMF in the DC motor that occur at higher speeds.

What I don't understand is the DC motor shunted at higher speeds while the AC motor (read GG-1) shunted when starting. At high speed the AC motor was operated at full field strength.

 #521938  by Nasadowsk
 
Might have to do with the way AC motors of the time were. remember, the reason the GG-1 had 12 motors in it was they couldn't build them powerful enough.

Today, with AC inverter drives and 3 phase induction motors, that level of power can be had in 4 axles (easily - even the DC AEM-7 can outpower a GG-1). In 1930, less was understood, the technology wasn't as good, etc.

I don't think more recent DC motors have all the interpoles and all that the GG-1 motors had.

I'll have to check, but didn't the GP-7 or 9 also do weak field starts? I seem to recall seeing that somewhere...

 #522677  by urrengr2003
 
GP-7 & GP-9 locomotives had all four motors in series (at full field strength) with the Main Generator when starting. The fields were shunted with resistance in both Series & Series-Parallel operation at the high speed ends of their respective transitions. Simply four stages of transition: 1- Series, 2- Series Shunt, 3- Series-Parallel, 4- Series-Parallel Shunt.

The difference between the AC motor & the DC motor being one starts with "weak fields" & the other uses "weak fields" at high end armature speeds.

 #522721  by Gerry6309
 
I am not much of an expert on single phase AC motors, but I do have reasonable knowledge of how the DC motors work. There are two ways to cushion the start of a DC series wound motor. One is to reduce the current through the motor either with a resistance, or a "chopper" in series with the motor or by reducing the output of the main generator in a diesel. The other is to weaken the fields either by tapping or shunting them. In a normal accelleration cycle the fields are quickly restored to full strength, and as the motor gains speed the current is allowed to increase until the motor reaches balancing speed, where the motor sees full voltage and given a constant load maintains a constant speed. At this point, shunting again comes into play as a means of increasing speed. At this point the speed and current increase as the power output drops.

Cheap AC motors have no means of controlling speed as there is only a closed loop in the rotor and limited control ability in the fields. The motors in the GG-1s probably had a current path through the rotors, enabling weak fields to be used to cushion starting, but I am totally unaware of how they were controlled.

 #523051  by Nasadowsk
 
Gerry - the GG-1's motors were close, but not quite, like a conventional DC motor. Within reason, this will work ok at low frequencies, and was the preferred method until rectifiers came out, though Hungry used commercial frequency electrification and MG sets feeding DC motors.

The most apparent difference is the need for more elaborate interpoles and all, when running AC.

Anyway, the GG-1 used a tap changer of conventional design, and just gave various voltages. The first tap was 352 volts, into a series string of 4 motors, this gave 88 volts per motor. FWIW, the (much later) DB class 103 has a first notch voltage of 18 volts across the motors, and it isn't until notch 5 (out of 39) or so this gets above 100 volts. Max voltage is 680 accross T00 and M01.

I bet the weak fielding was because the first tap sequence was such a high voltage, that a light motor, or a short train, would take off in 1st with a bit more omph than desired. Also, notch 1 -4 brought in the preventive coils, and their effects might have done interesting things, requiring some (semi) active field control. Might have done some fun thing to the power factor, too - the reason for the Silverliner's weird three tier rectifier arrangement is to keep the PF within bounds.

 #524154  by timz
 
urrengr2003 wrote:GP-7 & GP-9 locomotives had all four motors in series (at full field strength) with the Main Generator when starting....

Simply four stages of transition: 1- Series, 2- Series Shunt, 3- Series-Parallel, 4- Series-Parallel Shunt.
Or maybe series-parallel, series-parallel shunt, parallel, parallel shunt.

 #524222  by Gerry6309
 
nasadowsk wrote:Gerry - the GG-1's motors were close, but not quite, like a conventional DC motor...
The equipment was (as you indicate) wedded to 25 Hz, which eventually caused the demise of those classic & classy units. They could start a train as smoothly as a diesel or a steam engine. Riding Amtrak back in the day, you could tell by the start whether you had an E-60 or a GG-1 up front.

 #528367  by Allen Hazen
 
Was there any noteworthy difference between GG-1 with W'house and with GE electrical equipment? Would the engineer operating a GG-1 have to know which kind he had?
 #560380  by dvincenzo
 
Nasadowsk wrote:
urrengr2003 wrote:Unlike a diesel locomotive with DC Traction Motors that start at full field strength, the GG-1 starts with "weak fields."
Actually, I think some EMDs did weak fielding, under some conditions...
The bottom left warning light on the enginemans control panel was a yellow lense with the letters "Field Changeover." This was illuminated when the throttle was advanced to #1 position, when starting, and indicated resistance was placed in the fields of the traction motors.
Actually, it wasn't a resistance, but a reactance. There were quite a few inductors in the thing besides the main transformer. Most or all PCB cooled.
What caused this light to burn?
Electricity :)

Actually, the light was on whenever FS2 was 'in'
How was transition controlled with a voltage or current relay? There was a time period allowed for this weak field operation that if exceeded caused the pantograph relay to operate effectivelly killing the locomotive.
It was controlled by a relay, in notches 2 - 14. I don't think it ran in the other notches, at least the schematics don't suggest that. Interpole shunting looks like it was always able to go on, though. A timer operating the PL relay? I don't see why, typically, the PL relay was to protect the transformer in a fault.
Thanks for any help, realize this is rather a specific subject but trust someone can pick up on it that worked on these locomotives.
I never worked on them, but I did have the schematics laying on my machine, and took a look. I think the actual answers are out there, but realize the last GG-1 ran in the early 80's, so the number of people who knew them is dwindling :(
Are the schematics still available?
 #719431  by EDM5970
 
Not really germain to the G-motor discussion, but GP-7s did not have field shunting. They started in series-parallel, then went to full parallel. (GP-9s and all of the other early EMD "road" power went S-P, S-P Shunt, full P and P Shunt).

What the GP-7 did have, and I believe it was unique to that model, was the teaser excitation in "switch" mode, which bypassed the load regulator to some extent. There was a set battery field current for each throttle notch. In "road" mode, battery field was controlled by the load regulator. As EMD's first true road switcher, they may have put a bit more emphasis on the "switcher" part of road switcher.

The demise of the GG-1 was not caused by the 25 cycle power; Amtrak still uses it at least between NY and Washington, and out to Harrisburg. There were other factors.

Conrail stopped using them because many freights originated or terminated at terminals that didn't have wire, so diesels would have to be used for part of the run. Engine changes cost money. I believe Amtrak was charging Conrail a pretty hefty rate for electricity on the NEC. And the units were getting old; the frames were requiring a lot of heavy expensive maintenance. The frame issues also spelled the end of their use by NJ Transit.