Engine shutdown

[Tadman]

I recently read about an engineer shutting off the throttle when he hits the diamonds at a crossover in an E8 to prevent a flashover. What would cause this flashover? I've never heard of such a practice.

[CN_Hogger]

Traction motors have no suspension, they ride on the axle. Hitting a diamond while pulling a high amount of amperage will cause a flashover. If I remember correctly, a flashover is when the brushes actually touch the comutator(sp?) bars and acts similar to a short circuit(someone please correct me if I'm wrong!).

As far as I know, most modern train handling rules require you to be in Notch 4 or less when crossing a diamond if you're above 23-25mph, below that speed all you need to do is drop the throttle one notch. Of course, this rule doesn't apply to AC traction units.

[nickleinonen]

Traction motors have no suspension, they ride on the axle. Hitting a diamond while pulling a high amount of amperage will cause a flashover. If I remember correctly, a flashover is when the brushes actually touch the comutator(sp?) bars and acts similar to a short circuit(someone please correct me if I'm wrong!).

As far as I know, most modern train handling rules require you to be in Notch 4 or less when crossing a diamond if you're above 23-25mph, below that speed all you need to do is drop the throttle one notch. Of course, this rule doesn't apply to AC traction units.

the brushes are always in contact with comm... when you get flash, the electricity arc's between the brushes & brush holders to the motor internals [either bell housing end covers or other parts] burning through the insulating paint

i've never heard of that [shutting down at crossings], but i've seen many motors that have been flashed real good...

[FDL4ever]

Traction motors have no suspension, they ride on the axle. Hitting a diamond while pulling a high amount of amperage will cause a flashover. If I remember correctly, a flashover is when the brushes actually touch the comutator(sp?) bars and acts similar to a short circuit(someone please correct me if I'm wrong!).

Well, the brushes are SUPPOSED to always ride on the communtator... I think what happens when a traction motor under high current gets a really hard bounce is that th brushes flutter *off* the commutator and then an arc can form either between the brush holder and the motor housing, or perhaps bar-to-bar around the commutator leading to an eventual brush-to-brush arc.

[mxdata]

This is always a fascinating topic, you have hit the major points here, I would like to add a couple of details to this which may provide a bit more insight into the reasons it happens.

Because the motor is suspended on the axle on one side (by support bearings with a relatively tight clearance) and on the nose support on the other side of its case, it is following the rail profile and gets hit with impact loading almost as high as the wheel and axle. The truck spring suspension does not help cushion the motor at all, as it is between the journal box and the truck frame. Consequently it is not unusual for the traction motor to see impact loadings in the 17G to 20G range. This not only effects the brushes and brush holders, it also shakes the armature within the bearings that support it inside the motor housing, which also causes the brushes to move in the brush holders. If any of the brushes draw an arc during this beating, it can ionize the air around the brush, and once the air becomes conductive you can create a path to directly short the electrical potential across the motor, either brush holder to brush holder (usually last stage of a really bad flashover as FDL4ever noted) or by going through the adjacent motor structure (which is a much shorter path), just as Nick pointed out.

Years ago EMD did a high speed camera film of traction motor flashovers, and they used to show it in their training center. It explained the reasons it happened, and pointed out the effects of various contributing factors. It was extremely spectacular, it is amazing how much damage can be done in an instant.