While looking through some old papers, I came across a sheet with calculations I made several years ago, trying to figure out what actual resistances were used in the control system for the wood cars. It's rather cryptic, so I reformatted it and added some explanations. This should apply to all CA&E cars with GE-66 motors.
We start with the data for the grid elements used on these cars, taken from the GE data sheet:
- Grid TypeResistance (ohms)Current Rating (amps)40.0237560.0386070.0475580.05950
Then here is the resistance part of the motor circuit schematic. R1 through R7 are labels both for the contactors, and for the resistances between them. It should generally be evident from context which is which. All of these contactors are connected to the 600V supply when the trolley contactors are closed. RX is just my name for a junction point in the circuit.
The next table lists the arrangement of grid elements in each box, the total resistance of each group, and the effective resistance for the resistors shown in the schematic.
|
Box
|
Grid
Arrangement
|
Connections
|
Resistance
(ohms)
|
Resistor
#
|
Total
Resistance
|
|
1
|
8A10
|
R1-R3
|
0.590
|
R1
|
0.590
|
|
6A8
|
R3-R4
|
0.304
|
R3
|
0.456
|
|
|
2
|
6A4
|
0.152
|
|||
|
4A14
|
R4-R4A
|
0.322
|
R4
|
|
|
|
3
|
6A18
|
R2-RX
|
0.684
|
R2
|
0.682
|
|
4
|
4A10
|
R4A-RX
|
0.230
|
R4
|
0.552
|
|
7B8
|
RX-R7A
|
0.094
|
R7
|
|
|
|
5
|
8A7
|
R5-R6
|
0.413
|
R5
|
0.413
|
|
6A5
|
R6-R7
|
0.190
|
R6
|
0.190
|
|
|
7B6
|
R7A-R7
|
0.071
|
R7
|
0.165
|
Note that some of the resistors are split between two boxes: R3, R4, and R7.
Now, starting with the control sequence, we can calculate the total resistance on each point as the controller is advanced. I also calculated the percentage by which the resistance is decreased on each step. Points 1 through 5 are series, 6 through 10 are parallel.
|
Point
|
Contactors
Closed
|
Resistors
in Circuit
|
Total
Resistance (ohms)
|
%
of Previous Point
|
|
1
|
R1
|
R1+R3+R4+R7
|
1.76
|
--------
|
|
2
|
R1,R3
|
R3+R4+R7
|
1.17
|
66%
|
|
3
|
R1,R3,R4
|
R4+R7
|
0.72
|
61%
|
|
4
|
R1,
R3 to R5
|
(R5+R6)║(R4+R7)
|
0.33
|
46%
|
|
5
|
R1,R3
to R7
|
None
|
0
|
0
|
|
6
|
R2
|
R2+R7
|
0.85
|
--------
|
|
7
|
R2
to R4
|
(R2║R4)
+ R7
|
0.47
|
55%
|
|
8
|
R2
to R5
|
((R2║R4)
+ R7) ║ (R5+R6)
|
0.27
|
57%
|
|
9
|
R2
to R6
|
((R2║R4)
+ R7) ║ R6
|
0.135
|
50%
|
|
10
|
R2
to R7
|
None
|
0
|
0
|
+ means “in series
with” ║ means “in parallel with”
So starting with the question "Why in the world did this have to be so complicated?" we have a least a partial answer. This was really an interesting problem to work out.
This type of resistor scheme is actually very common even in industrial applications such as overhead cranes. It more efficiently uses all the resistors and hence somewhat smaller lighter banks are needed.
ReplyDeleteI spent several years in the late 1970s replacing cast iron resistors on about 25 overhead cranes. As they rust, the lose cross section and hence the resistance goes up but also the current capacity goes down. Also the thinner sections break much easier. Basically, a death spiral. Too much down time. And they they have the "temporary" repairs that throw of the smooth steps and that makes other mechanical/electrical issues.
Dennis Bockus
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ReplyDeleteHi
ReplyDeleteI guess it does seem complicated. The resistor values are carefully worked out so as to keep the accelerating current within pre defined limits over the acceleration cycle. This applies to both series and parallel notches. The current taken by the motors is determined by the resistor values which limit the acceleration rate to a value that will not result in overheated motors or wheel slip. The resistors have to be designed to create an almost seemless transition from series to parallel and also ensure the current throughout each notch is within certain limits. The accelerating current is set so as to make best use of the output capacity of the motors. It would be simpler, I guess to design the circuit to step through the resistors one by one. However, it was found that this can waste a lot of energy, so the series parallel scheme was invented. This has a profound effect on the energy consumption and also lowers the heat dissipation requirements of the resistors. Series parallel also gives a half speed notch that can be use without burning out the grids as the motors can be connected in series across the line with no resistors in circuit. So the circuit and the resistors have been designed for a system that has evolved from a simple series circuit to a quite sophisticated and efficient way of accelerating the train with minimum waste heat being generated in the grids. Colin in the UK
I noticed that some of the newer steel cars have more than four traction motor leads. Are they field tapped or field shunted ?
ReplyDeleteGenerally, all of the later standard cars have a field tap. I'm not sure when field shunting was first used, but I believe it didn't come into vogue until the pre-PCC cars. Richard must know.
ReplyDeleteRandy- That's about right. If a "traditional" car had a short-field capability, it was generally tapped. PCCs had inductive shunts, as do our later 'L cars.
ReplyDeleteR. W. Schauer