VFD driving two different HP and slip motors

[electromike]

What sort of problems should I expect if I have a 10 HP 1695 rpm motor driving one side of a 10 ton bridge crane and a 15 HP 1720 rpm motor driving the othe side of the same bridge? Both of these motors are powered by the same VFD that uses an open loop V/F control method.

 

[davidbeach]

How far can the bridge skew before it jams? If you want to use those two motors you will probably need encoders and separate drives.

 

[DickDV]

Simply put, you will have nothing but trouble. First, due to the differences in slip, the motors will not load share properly. The motor with the least slip will do most or all of the work.

Second, if the bridge is not absolutely foursquare on the rails and mechanically held into alignment, it will tend to skew and eventually lead to a nasty mess with the bridge jammed between the ways.

Bottom line, don't do it. Now, if you procure two motors with identical hp and slip, you will likely have a working system although the load sharing will not be precise.

 

[jraef]

Been there, done that, got the T-shirt.
Even with identical motors, avoiding skew is a daunting task, one that gets exponentially worse with bridge length. Minor diameter changes and/or wear on the bridge drive wheels, torsional stresses, debris on the guide rails etc. etc. Encoder feedback into closed loop vector control on separate drives is a must, and even then it takes a good programmer to monitor skew and adjust the drives on the fly through a PID loop in a PLC. I can't even imagine trying to make that work with mis-matched motors and one VFD.

Start over.