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| Re: What is the mechanical condition of the Low V? (662533) | |||
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Re: What is the mechanical condition of the Low V? |
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Posted by Jeff H. on Wed Aug 6 21:56:51 2008, in response to Re: What is the mechanical condition of the Low V?, posted by Gerry6309 on Wed Aug 6 07:22:37 2008. Maybe someone forgot to swap the forward and reverse wires on one end, or swapped them where they shouldn'thave? Nope, because that would have caused a consistent condition, whereas this was intermitten. If the reverser doesn't throw. the interlocks will prevent the Line Switch from closing. True, but what did Holmes say about the impossible vs the improbable? I'll explain in agonizing technical detail what happened. I know you are very familiar with PC-5, but I don't know how well you know PC-10. The line switch pickup circuit starts with the forward or reverse trainline energized (they are inexplicably numbered 4/5 on IRT equipment rather than the more conventional 1/2). Let's say we want to go forward. One branch energizes the FWD magnet valve, through an interlock which is made when the reverser is NOT in forward. On the negative side of that magnet valve circuit is an interlock proving the line switch in the OUT position, then to ground. The other branch continues through an interlock on the reverser which is only made when it is fully forward. So far, so good. Now that latter branch, as you know, continues through various interlocks, e.g. the overload relay is not tripped, the PC-10 drum is in the home position, and eventually winds up at the coil of the line switch, where the circuit completes through the '0' trainline back to the master controller to ground. I'm sure you know how GE relays are constructed in general, and perhaps you've seen a GE potential relay (PTR). With a GE relay, the moving armature and the coil core and support frame are all connected, as is the moving contact. The stationary contact is held on an insulated block. In this particular case, the moving contact is part of that line switch pickup circuit (32V). The coil of the potential relay is connected to 600V, through some dropping resistors. The coil insulation had developed a puncture, such that in certain positions of the car, the coil winding touched the support frame, creating in effect a high-resistance short between 600V and the line switch circuit. All of this might have not caused a noticeable problem, but the reverser was also a little bit sluggish. So, if power was requested in the forward direction, but the reverser was in reverse, this 600-32 V short would effectively bypass the reverser interlock, allowing the line switch to pick up. Once that happens, the reverser magnet valve interlock is broken, and the reverser, being stiff, stays in the reverse position. At any rate this problem has long since been corrected. Once the PTR was removed, the bad spot on the coil was quite obvious (but while it was on the car it was pretty much impossible to see). The relay was rewound. |
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