This posting is a response to the error message I get when I attempt to use the required address for a private Forum message. My correspondant wanted information on load regulator controls, and with particular reference to a museum GP-7 operation.
The load regulator on a GP-7, and I think most other types are similar, sets the speed of the governor, which in turn determines the amount of fuel and air going into the injectors. On a GP-7 specific engine speeds are associated with specific horsepower output. On a GP-7 the voltages across the armetures and field coils, and currents, and the throttle settings provide analogue electrical inputs and incorperate an early form of slip control that reduces engine speed if there are major differences among the four motors analyzed. indicating one or more motors are slipping. (For all four motors to regester wheel slip at the same rate is a statistical improbability.)
This is all from memory, and anyone with additional information, on vintage or recent locomotives, can add or correct.
The engine governor steps up the RPM with each throttle notch increase. If you increase the RPM of an electrical generator the output voltage will increase as more lines of magnet field are cut per second. (Maxwell's equation says that Voltage is proportional to the rate of change of the magnetic field. By visualiizing magnetism as lines of force we can count the number of field lines crossing a wire as the rate of change.) Anyway the output voltage rises but internal resistances and losses as brushes shift from contact to contact cause the voltage not to go up proportional to RPM. To compensate, a "load regulator" increases the excitation on the main generator and its voltage goes up even more. This regulator is a rheostat that turns as RPM increases. This increases voltage to the motors and thus power to the motors.
You could increase the voltage (without increasing RPM) to send a higher voltage to the motors and thus speed up by just increasing generator excitation. The problem there is that to get much more power out of the generator its field and rotor windings would have to be very large wires to overcome resistive losses at high currents.
And for another way to look at this: the load regulator adjusts the voltage to current ratio at any prime mover RPM to match the impedance (voltage to current ratio) of the motors. When the impedance of the generator matches the impedance of the motors the most power is transferred from the generator to the motors.
The governor is a fly-ball governor designed to regulate engine speed. Each notch sets a certain engine RPM to be maintained. The governor is designed so that there is also a set fuel rack for each notch. The governor first wants the engine RPM constant, then it wants to hit the rack balance point for fuel. It will vary the load regulator position to do this.
The governor is tuned to do all these things almost at once. If RPM drops, the governor will drop load and increas fuel. IF the RPM increases, the governor will increase the load and decrease the fuel.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
In my initial reply, I made the error of neglecting the extremely important control of the field coil current of the generator. I did not have the opportunity to make this correction later.
More to the point: The GP-7 initially was available with two load-regulator setups. One was "Switcher" mode. When advancing the throttle from idle, the full field current was applied for the appropriate notch. This gave an initial rapid acceleration, which was useful in kicking cars during switch operations. The second setup was a "Road" acceleration, where full approrpiate field current was not applied untill the engine reached the appropriate speed for the particular throttle setting. This made for a smoother start, and actually reached speeds of say 20mph or more faster. At least some GP-7's were delivered with a switch permitting either starting arrangement. On GP-7's 1567 and 1568 for the Boston and Maine, delivered in the autumn of 1952, a compromise arrangement was installed that was simpler and used fewer parts, the 1567 being the simpler of the two and the one that was closer to being the prototype for the GP-9. What was determined was that if available power on startup from idle was divided in half, half going to accelerate the prime mover itself up to operating speed, and the other half for the short time acceleration of the train, the power would both be sufficient to kick cars and also to provide a smooth start for a road train. Both 1567 and 1568 were boiler equpped and used both in passenger and frieght service. They also were equipped for head-end electric lighting. On the Boston-Portsmouth passenger run leaving North Station at 4PM, neither was any kind of slouch of accelerating from the suburban stations to track speed, with an eight-car train, doing at least as well as the RDC's that succeeded them. The frieght on the return trip usually ran about 50 cars and involved some setout and pick-up.
The wheel slip system and the governer load regulator are entirely seprate and for the most part unrelated. The single purpose of the locomotive governer is to control engine RPMs . The load regulator is one of the tools the governer uses to maintain engine RPM. Imagine an EMD engine with a sudden load bogging down and losing rpm, the governer will use the pilot valve to back off the load until the set RPM for the throttle position is achieved only then will the governer move the load regulator back towards maximum field and allow more generator field exitation.
RSS
You are correct with regard to current practice. However, on the GP-7, the wheel-slip detection was done by measuring voltages at the motors rather than actual counting wheel revolutons, with marked differences between motors indicating slip. I don't remember whether this was voltage across the armature or across the field coils (on each motor they were in series on GP-7's, GP-9's and all EMD power up to that time and through at least the E-9's and FL-9's), and the same voltage measurements were used along with current measurements by the load regulator sensing circuits. The revolution-counter wheel-slip circuits came to EMD locomotives, later, and I do not remember which model inaugurated this more advanced technology. Perhaps you know. It may have been tried experimentally on some GP-9's. The sensing circuits for load regulation also had to be callibrated for each of the transition periods, series, series with field shunt, parallel, and parallel with field shunt, as well as sensing the points where automatic transition occured. And the sensing circuits won't allow the prime mover to be really bogged down because a sudden increase in required tractive effort to maintain speed will be reflected in a sudden increase in motor current, and the load regulator circuits can react to reduce generator field current quickly enough so that the diesel looses only a few RPM and recovers quickly. If the current increase is great enough, a transition to a lower speed position, such as removal of field shunts or shift from parallel to series with shunt can also occur. All this is GP-7 - GP-9 era DC technology. Today with AC generators and even with dc motors and AC generators, there are probably some major differences.
Even with the older thru cable relays the wheel slip system and the load regulator were not related. With the thru cable relays an armature lead from 2 different traction motors was passed through the center of the relay in opposing directions, If a current (not voltage) imbalance occurred the resulting induction would magnetize the relay (WSR) and it would pick up, , placing a resistance in the generator field circuit and worse case dropping out the shunt field contactor (SF) until the current imbalance went away.
Transition does have an effect on minimum field start locomotives, In most cases a parallel relay would pick up and energize the ORS (overriding solenoid) and drive the load regulator to minimum field. As soon as transition is complete the ORS de-energizes and control of the LR is returned to the governor pilot valve. On some locomotives there are limit switches inside the LR to operate the transition, the LR would swing around and trip a limit switch and pick up the PR relay and the switchgear.
On the experimental Boston and Maine 1567 and 1568, if my memory is correct, the voltages of all four traction motors were compared, not just two. You may be correct for the production GP-7's and GP-9's, however. (My memory doesn not extend to their innards.) There were other shared components between wheel slip protection and load regulation, in addition to the armature leads that you mention, and those are the relays and controlled contactors that reduce field excitation or increase it and remove and add field shunting. And again, my experience definitely does not include the later power, particularly those with ac generators.
From the GP-7 Operating Manual, August 1951 edition, for a GP-7 with two step transition:
131 Load Regulator The load regulator is located under the engine hood, adjacent to the air compressor. The operation of the load regulator is controlled by a pilot valve and a dump valve (ORS) in the engine governor. The function of the load regulator is to vary the battery field current in the main generator. Two tumble switches LRS and FTS have been added to the, load regulator, which are actuated by the movement of a three fingered plate that is bolted onto the load regulator arm shaft.
When the load regulator arm is in minimum field (4 o'clock), position, one finger of the platemoves LRS switch down. An open interlock on LRS prevents the shunt field cpntactor from closing. At the same time, a closed interlock on LRS energizes LRC, which partially establishes the circuit for the throttle controlled "Teaser" circuit. With the switch in this position, the amount of battery field current is varied by each throttle position, Fig. 1-4. When load regulator arm reaches mid-position (12 o'clock) a second finger moves LRS to opposite position. This allows the shunt field contactor to close, making main generator excitation normal, and cuts out the throttle controlled" Teaser" circuit. LRS switch will stay in this position until load regulator arm returns to full minimum field position, Fig. 1-5. When load regulator arm reaches maximum field (8 o'clock) position, the third finger closes the other tumble switch FTS, changing the motor connections from series-parallel to parallel, Fig. 1-6. FTS will open as the load regulator arm moves back away from maximum field, but the motor connections will stay in parallel until the backward transition relay is energized by approximately 2500 amperes main generator current. When the throttle is closed to idle, all P contactors open, and the S contactors close. As the throttle is reopened, the motor connections will be series-parallel, remaining so until the load regulator arm reaches maximum field, when the transition cycle is repeated.
More info may be founf at:
http://gelwood.railfan.net/manual/gp7opman.html
There was only one shunt-field contactor, and any operation of either the load regulator or wheel slip to activate it activated the same shunt field contactor. Again, my memory is only of the two particular GP-7's B&M 1567 and 1568, but this statement appears to apply to production models as well.
One of my assignments as a junior engineer was to try to simplify designs and remove more parts from locomotives than add in making any improvements. So I may have found other ways to share components that were not applied to production models, but were possibly to the GP-9 that followed. Possibly a comparison of the GP-9 and the GP-7 may bring some to light.
I designed the circuits for convering the B&O FT's from manual to automatic transmission. While the detials of the design have not stayed with me, I do remember removing more parts that adding, and still coming up with a successful design. It was applied to other FT's as well, but I don't know which railroads.
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