Load Slipping In Hoist
Posted 27 November 2006 - 10:22 AM
If there is no run command & Till load is slipping how to control the load ?
Posted 27 November 2006 - 03:23 PM
Use a MECHANICAL BRAKE to keep drum from turning when motor is de-energized / not controlled.
To sense slipping, you could apply a mini-PLC to monitor hook position by comparing Encoder output against a known reference position.
Canned program for mini-PLC is not likely to exist, but doing a GOOGLE search for same is advised. If none found, you will either have to write program yourself, or pay someone to write it for you.
There are VFD manufacturers that offer special software option for hoist applications. ABB is one (software was originally developed for Harnishfeger P & W Crane - now mfg'd under license by Morris Material Handling) Magnetek's Electromotive Group has a series of VFD's designed specifically for overhead material handling (cranes. lifts, etc) applications. click on link below.
NB! the above references are informational only and are neither an endorsement or promotion for the companies and their products.
Posted 27 November 2006 - 06:06 PM
What these special hoisting programs do is what is called a "torque proving" loop to be used WITH a mechanical brake. When the Run command is given to the VFD, the torque proving algorithm begins applying full motor torque to the shaft BEFORE the mechanical brake is released, so that the load does not begin dropping as the VFD tries to "find" it. Even dropping for a fraction of a second can create enough momentum that the motor may not be able to overcome it and gain control. That's why the VFD "proves" that the motor is going to be able to hold the load before the mechanical brake releases it.
If you were relying solely on the VFD to prevent the load from slipping, you need a mechanical brake. If your load is slipping with a mechanical brake, you need a better brake. If you have a mechanical brake but your load is slipping when you release it, you need a better VFD.
There may be more, but the 4 I know of that have good torque proving functions are the Electromotive and ABB that jOmega mentioned, plus the Siemens Master 440 series and the Schneider ATV71. Be careful though, only those specific versions from Siemens and Schneider have that capability, the others may not. (The new Siemens Simatics may, I don't know yet).
Posted 29 November 2006 - 06:59 PM
Great info. I'll give some detail about Schneider's ATV71 and hope to get some feedback about other brands. ATV71's way of doing this is combination of fluxing and applying some torque to the motor and releasing the brake at in the right moment; all of these parameters are for user to test and apply during the start-up. Works great but it it something you need to really play with. What is your experience? btw: i wouldn't call this "torque proving" - ATV71 calls it "brake logic control" and it is used with or without encoder feedback.
jraef: i find it interesting that it might take only "a fraction of a second to create enough momentum that the motor may not be able to overcome it and gain control". I don't have experience to comment, but I ask: isn't it neccessary to make sure you have strong enough motor before making this sort of application?
Posted 30 November 2006 - 04:20 AM
Absolutely. Don't misinterpret what I said, of course it is necessary to properly size the motor, but sizing it to lift is one thing, sizing it to overcome the momentum of a falling (and therefore accelerating) load is another thing entirely. I was just pointing out that this is why the specialized logic is so important.
For example, I once did a project for Boeing on the hoists used to move 777 aircraft as they are being assembled. During the hoist proving phase, one of the Boeing crane mtc. people (who thought he knew everything), decided he was ready to test a 40 ton hoist with the test weight, which was only 10 tons. He did not know that the special torque proving circuit of the Electromotive drive had to be enabled in setup, he ASS-U-MEd that it just came that way from the factory. So when he released the brake to have the VFD take control of that test weight, it dropped 20ft to the brand new concrete floor. Big mess! That hoist was rated to lift and hold 40 tons, but once the test weight (1/4th of the capacity) had started falling, the motor could not overcome that momentum and stop it, even though it was in closed loop vector mode.
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