DC Crane Coils

An IR sensitive video will detect problems....

http://www.flir.com/thermography/americas/us/

Place two contactors in series with a safety relay monitoring the contacts. Incorporate basic alarms for fault identification and your good to go.

paulengr. your comments are right on the money. I have looked at all of the suggestion you have made. These cranes are massive as you can imagine. The room or spare room for relays is at a min, most of the crane space is used up in resistor banks etc., as there are 2 hoist on each crane. But, it is fun to see what others may think of the condition and getting input does sometimes open other avenues and possibilities.

Thanks

Fixitorelse

Hi!

paulengr is your man..

Cheers,

Stu.

What was the condition of the contacts, were they well worn but serviceable? Has the Dynamic Braking circuit been checked because it seems that an increase in load has been introduced during a specific function or the quality of the contact tips has made this possible from poor quality causing overheat or the ground is not properly passing power through circuit causing contact tips to overheat to melting point. I not sure what a watch dog system would do unless your going to run another contact in tandem and cancel out first circuit in failure episode. This system you have sounds like it has been around as long as my mother in laws hairdo and I don't think the answer is necessarily the additions you want to make , but it cant hurt.

See PaulEngr comment. Well done.

I have seen a number of contactors fail on the mechanical life cycle. Everything from pivot points failing to bus bar pieces developing stress cracks and eventually falling apart.

I have recently been involved with several dockside cranes. On one crane we kept the DC machines and put in new digital DC regen drives (lowest cost). On two others cranes we replaced the DC drives with regen AC drives (ore loading and unloading has a lot of grit in the air and "grinds the commutators down" so AC TEFC gives superior life).

The power savings with the regenerative drives (versus burning up the lowering and stopping energy in DB resistor grids) paid for the drives in about 2 years. These are special AC drives - cheap ones still burn up the lowering energy and stopping energy in DB grids). Some manufacturers will supply a special front end with low harmonic content and power factor correction -- again more $.

Many power authorities or government agencies also have incentive programs in place to help finance or partially pay for the efficiency upgrades.

You say it is a DC "controlled" crane.

I'm assuming the drive is DC. If it's a Ward Leonard system have a look at the suicide and reverse shunts. It's the shunt that should stop the load, the brake is your back up. OK once the load is at rest the brake holds it.

The systems I've worked on had two brakes, a working brake and an emergency brake.

Here's a picture of the little babies (only 170 ton) I worked on. You can see the motor generator set on the platform overhang.

All good responses to my question. Thanks to all. We currently inspect these units 2-3 times per month. Could we have missed something possibly, but I don't think so given the failure node we found. The coil shorted through the center lug mount to the flat washer it is held in position with. Seems it had some vibrations that caused the issue. The contact tips take a beating very hour. We use the best materials out there for these units for that reason. But, Like I said, it is an old crane and with it's sister unit runs 24-7 365 days a week. Can't ask for more out of them then we do.

As far as the two brakes, all hot metal handling crane in the USA are required to have duel brake systems. But if you fail hot, the coils are going to hold the brake open. Operator did the best he could do in holding it in reverse up 1st point. Burnt the resistors bank out by the time it stop. But, did not spill a drop.

Have a good day and if you think of something else jot it down.