How to repair crack rotor bar for induction motor

Get a ready new Rotor from maker.

Or Recast the Rotor after removing old Bars.

TIG weld repair might have failed due to following:

#1 Did you use the AC TiG?

#2 Did weld reach the stamping bar-slots. How do you know?

#3 Even if Weld reached slot bottom - steel would quench it fast - local shrink Crank-Hot spot- Crack again

get a new rotor with a 50% larger cross section at the failure prone end. ideally should match mainshaft diameter.

machine other parts to fit it, find a way also to reduce jogging under load. if these parts are breaking, the manuvering procedure is improper for the equipment. a shock absorbing coupler is not out of the question. in any case this machinery may be loaded beyond its safe working load [SWL]

Guest #2 excellent answer

" if these parts are breaking, the manuvering procedure is improper for the equipment. a shock absorbing coupler is not out of the question. in any case this machinery may be loaded beyond its safe working load [SWL] "

Read the chapter on "Induction Machines", Electrical Machines and their Applications (John Hindmarsh) ... or similar resource. A conical-shaped rotor...?

Sure, it can be done. (So can a hi-fi speaker be made to reproduce a 39kHz tone... but why bother?!) Remember that starting current can be 6x or MORE than rated-load running current. So what currents are induced in the rotor bars each time you "jog" it repeatedly? With an "imbalanced" rotor circuit, your heat dissipation will not be uniform. Under such operating conditions you are merely begging for repeated failures. And, trying to weld-repair a cast-in-place high current bar? Yes, I'll admit that someone, once-upon-a-time succeeded at same... on the right motor, that was being run beneath its design limits. But even if you did so in a lab under a microscope, you'd wind up with a high resistance point in that bar, subject to another premature failure with THIS design.

By the same token, at the risk of sounding bombastic... regarding:

"get a new rotor with a 50% larger cross section...machine other parts to fit it..."

This isn't the "Light Speed Scooter" discussion, here...! The owner of this poorly designed hoist isn't going to find such a rotor, machine-out the stator to accomodate it with appropriate air gap; then add appropriate back-iron to make up for what got machined away, and then rewind it appropriately...! C'mon, be reasonable! For that investment he could attend G.E.'s "5-Star Motor School", learn how to specify the right motor for the job, then investigate and buy a more suitable hoist.

I *WILL* agree with #1's initial statement; "Get a new O.E.M. rotor"... then, upon properly reassembling the unit, perform proper load-testing, WITH an ammeter in-line. AND, concurrent with those tests, acquire the appropriate type thermometer to do a temperature-rise check of the system, paying strict attention to staying within safe limits. Verify that it is CAPABLE of doing what you want it to do (knowing that the potential for failure might still exist).

You MAY find that the original failure was a 'fluke' (you had a rotor with a cracked bar to begin with, and your weld wasn't "heavenly-perfect"!). The new one MAY do the job for you. But the appropriate system checks should be made anyways. You might find that selling the repaired, and still functional unit ... then purchasing a more suited-to-the-task unit is the better option.

Wishing you success ~

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When I worked in a large motor repair shop (20 years ago) we had a 100 HP motor that cracked at the short circuiting bar ends. The material was copper and we were able to braze it with a torch. This will not work in your case because you have aluminum. The point that I need to make was that we had to do it again at yearly intervals until I think we finally replaced the motor.

This is an old and common problem, I have not heard of a good fix, but I also have not been in the business in 20 years so I am not up on the latest technology. It sounds like your rotor is a special design and not easily replaceable.

Snakers

The motor you are using is a Demag conical Rotor Brake motor and is probably the most effective and reliable motor for any hoist application as the brake does not rely on solenoid operation. However on any electric motor you CANNOT weld the rotor bars and expect it to last. Get a new rotor or motor from your local Demag agent They are represented in every country in the world. If you get a new rotor only the get a full set of shims and set the air gap according to the manual. An incorrectly set air gap can cause overheating and other problems.

There is a lot of old equiptment out there on the market you should be able to find a replacement very quickly with a lot less expence then buying directly from your area Demag associate. Locate a motor equiptment specialist who has the contacts one I have dealt with is a Maynard at a business called Canara in Hamilton, Ontario, Canada but there are a lot of businesses that specialize in old motor parts all over the world . Find some on the Internet give them the model # and save your selves the aggrevations of temporary repairs.

Your are right, it is a Demag rotor, looks like we are using the same type of motor. I can buy a new rotor from the local agent, but the price is too expensive now, US$ 3000+ for a KBH 160 rotor ! to us this is consider very expensive.