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Joined: Dec 2000
Posts: 218
spkjpr Offline OP
I am hoping some of you on here can help me. I am having a problem with a overhead crane motor. the speed is controlled by progressively shorting out banks of resistors. I need to know how the resistors are connected internally to the motor windings. I have speed control going up but not down. Also any troubleshooting hints. This motor is 480 V 3 Ph. I tried another site but no info. Any help would be appreciated.

Joined: Nov 2000
Posts: 123
doc Offline
Do not quote me on this but I sure think that u need some sort of certification to work on cranes,You are accepting a lot of liability by working on it and not being trained,I have worked in several plants over the years and 2 things that the company never allowed me to work on was any cranes and adjusting the tension on overhead doors,could check fusses or adjust rollers but anything other than that they called the pros. Talk this over with your bosses and call someone out

Joined: Jan 2001
Posts: 1,044
Tom Offline
I've worked on several of these, but I'll be dipped if I can remember for sure, but I believe that the resistors will be in series with the windings.

Maybe this would be a good time to contact the manufacturer & obtain a wiring diagram, which would be handy to have in any case.

Maybe it is time for a VFD. I wired a double bridge system with 4 hoists last year that had VFD's on the bridges, the trolley & the hoists. Talk about sweet. No jerking, both bridges & all 4 hoists moved at exactly the same speed.

Few things are harder to put up with than the annoyance of a good example.
Joined: Dec 2000
Posts: 218
spkjpr Offline OP
Tom, I would love to see a VFD put in, but management won't turn loose of the money. Seems business is slow and money is tight. Thanks for the reply. A vfd would also end the axle snapping we encounter from the operators braking the bridge with the reverse lever!! Yep, gotta love the production people.

Joined: Jul 2001
Posts: 599
JBD Offline
You did not mention if this was for hoisting or traveling, but you are describing a "Wound Rotor" motor which uses resistance in the rotor windings to control the speed of the motor. The resistor banks are connected in a "wye" configuration to the M1, M2 and M3 motor leads. The more resistance the slower the speed. There is not much unique in troubleshooting a wound rotor motor, most of the problems can usually be found in the control circuits.

Most crane system use a single set of shorting contactors for controlling the speed whether moving forward or reverse. In some cases there is a timing relay between several speeds, such as a delay between 3-4 when hoisting and 4-5 when lowering.

If you have speed control in one direction but not in the other then you have problems with the control circuit in your Master Switch. Do any of the shorting contactors work in the "bad" direction?

Joined: Aug 2001
Posts: 31
I agree with the last post (JBD)100%.

Depending on the age of the crane and the mfg. A lot of round rotor motor hoists have eddy current brakes. If they loose the current sensing relay in the control ckt or the actual power to the brake, it will affect the hoist in a similar manner.

Check the low points on the control. They get dirty and you have to clean them. PM (I know impossible when you are under-staffed....) goes a long way.

Glad to help out.

Carl Lee Tolbert
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Joined: Dec 2000
Posts: 218
spkjpr Offline OP
Thanks Tom and ctolbert. I posted this question on a motor specific site and have received no answers there. But my bud's here came through for me. I did smoke the festoon wire the other night though, the magnetic brake contactor failed and didn't release the brake. #12 wire fused at 60 A,humm! Thanks again guys, all here are very helpful.

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