Big Switches

On Monday, the first task was to buy a load of nice clear poplar and transport it to the Museum. This will be used for making a complete set of tack molding strips. I am hoping to be able to replace the roof on the 319 this winter, and will get as much done in advance as possible. I also picked up some pieces for use on the 1797 by Tim.

Then there was a big switch move to put the 308 over the pit, so I could begin inspection. Since I was busy doing this by myself, I didn't take any pictures. Inspection has gone well so far.

So let's take a break and look at some contactors. These, of course, are just big switches for making the appropriate connections to regulate motor current. Here's a DB-15, the first successful contactor design for large current applications. This was designed about 1901 or so. It's sitting on the bench upside down, so these pictures have been inverted to show what it would look like when mounted inside the box.

Here, the door is closed.

When we open the door, we see the arc chutes and the contactor tips (white arrows)
which in this case are very worn. The yellow arrow points to the blow-out coil. Motor current flows through this coil to produce a magnetic field; when the contactor opens, the field produces a force pushing the arc into the arc chute on the left.

Here's a view of the bottom. The white arrow points to a hinge pin; the movable arm of the contactor is in two parts. The black arrow points to the arm which supports the lower contact tip. When the contactor closes, the arm moves slightly, producing a wiping motion at the contact surface. This should prevent the two tips from welding shut.

This particular contactor has no interlocks.

Finally, when the main coil is energized, the lower tip is raised and makes contact as seen here. When the coil is turned off, the lower tip falls by gravity, and the arc is blown into the arc chute. And this happens millions of times over the life of a 100+ year old device.

On the 308 we have a slightly more modern design, the DB-131. The box is open, but we did not disassemble the individual contactors, of course. You can see the simpler style contactor tips, I think. In this design the arc is blown out towards us, into the box.


Here's a view from underneath, showing the interlock design. These are auxillary connections to protect the control circuits. The white arrow points to a flat plate which makes the contact. When the contactor is energized, the (rusty) rod it's mounted on moves back. The left and middle interlocks are normal closed, the one on the right is normal open.