fuse in VFD output?

i am having a trouble in crane hoist application. the mechanical shaft of gear after coupling to motor shaft is braking after some operations. the shaft is also called penion shaft. motor hp is 41 (to get larger starting torque manufacturer is said to have given a 60 hp motor with 41 hp namepalte) starting torq is 375% and rated current is 60.9 amps speed is 990 rpm . there is said to be under rated design of mechanical side. period.

my problem is at all times the crane was not loaded to its capacity and has broken on very low values of loads. also there is a coordinated opening and closing operation for mechanical brakes. also had there been no drive and the mechanical jamming is taking place sometimes during operations, the fuses would have tripped the power to all (motor and brakes) and indicated the locking. in case of vfd the fuses are not there in output and i can install an additional overload relay to have reliable operation etc still there is no protection against locking as such. the overload trip comes into picture if calculated motor thermal state violates some preset value. Since the current to motor can also be controlled below certain limit value with drive (max value of drive's transient rating or as per motor's heating observations) in case of jamming the thermal trip in case of conventional mode(without drive) will be faster (as current is unlimited).

in this case isnt the observation true that the mechanical design has to use the pull out torque of motor (2XX% Tn) as the max torque seem by mechancial system? and also shall consider the time for which it is seen (may be order of mins) in case of vfd?

i mean if there is no vfd the mechancial part of crane is not subjected to continuous over torque from motor as the fuses will cut the motor power. if it is partial jam still the thermal trip is faster than with drive as current is unlimited.

my worry is if the crane motor is controlled with vfd, is it also providing the protection to mechanical system and to motor in case of mechanical lock? also is this protection up to a level of that offered by conventional thermal trip device and fuse combination?

what is the solution? shall i put fuse in vfd output to cut motor power in case of mechanical jam since the motor current is limited (and so over current or short circuit trip is not taking place)? also i need some method to coordinate brake supply with the inclusion of fuse at vfd output

thank you dhananjay

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Where I live cranes must be worked on by "authorized and trained" professionals. Or go to jail. This has to do with OSHA. Just like elevators, man lifts, and truck cranes.

You really feel confident to abort/modify the design of the manufacture? Every VFD I have ever worked on has settings that you can use for protection and performance. I do not believe by the description in your post that you really understand the controls and design of the equipment. I am not trying to be nasty. I get myself into that situation frequently.

Call the manufacture of the crane, talk to them about your concerns. They may already have a fix for you. You really should talk to the Safety officer, Insurance people in your plant. They may have something to say about your "modifications".]

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I wouldn't recommend doing it. Breaking one phase of a VFD output could quite easily break the VFD.

What make is the VFD? I'm sure it has an overload setting.

If you have to do this, put in an overload relay in the output that knocks out a contactor supplying the VFD.

As the other poster said, with all due respects, you need to get someone in to look at it who knows what they are talking about.


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No you CANNOT put a fuse on the output of the VFD. The PWM output is very rich in harmonics and the fuse will heat disproportionately to the current up and blow. When fuses blow, they essentially arc inside of the fuse case, albeit in sand. That arcing will damage the transistors of your VFD.

As others have said, most VFDs have a torque limit or current limit setting, or a feature called an Electronic Shear Pin that will take care of your problem. If you can't figure out how to engage that and set it, find an experienced professional who can.

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Bob Ferapples

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