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| Solenoids in Electromechanical Interlocks |
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Diesel-Electric locomotives are so named because large diesel engines are used to turn huge on-board generators. This energy runs the electrical systems and powers the motors that provide the pulling force.
As can be imagined, managing this power safely is of primary concern. In the engineer's cabin area is an array of controls, gauges, and distribution boxes. One such box is the main power switch enclosure. to protect rail employees from dangerous voltages, one supplier desired to incorporate an electromechanical interlock system on the access door.
The purpose was to prevent the door from opening when the internal circuits were live. Power would be supplied to the interlock solenoid as long as the main power switch was on. Upon removal of power by an external switch lever, the interlock solenoid deactivates, allowing the door to be opened.
This method offered more safety than a strictly manual system which could erroneously allow access after the lever was thrown if any linkages failed or contactors stuck. The lever could also be moved to the ON position after the doors had been opened, a common practice when electricians needed to take electrical readings inside the switch box with hot circuits.
The solenoid interlock scheme requires than an internal service override device be tripped to allow the circuits to be energized with the door open. It was also necessary that the door could be closed even with power on the circuit. The electrician could close the door and walk away without remembering to disable the override and power down the circuits. Access then would only be possible by a complete locomotive power down to allow removal of the override trip.
The interlock solenoid required three key elements. The device needed to be rated for continuous duty, have a substantial spring, and provide at least a 0.300" stroke in a push configuration. The Soft Shiftâ solenoid offered all of these features and a few others, such as a more robust shaft than the tubular line, a bearing system with very little radial play to keep the shaft properly aligned, and the high starting force needed at continuous duty power to overcome a strong built-in spring. Since the armature is captured inside the Soft Shift solenoid, no additional plunger retainer was needed. Locomotive engines are expensive, thus the added cost of the Soft Shift unit is not a deterrent since the device represents an off-the-shelf, simple and self-contained solution.
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