Dear Engineers,

Please , assist me control a grab crane.

A grab crane is using two frequency converters to run motors for close/open and hoist(hold)/lower

The operations of the sugarcane grabber crane seems simple. With the grabber lowered to the sugarcane heap, Operator turns Joystick switch to close position.

Closing drive is turned on and accelerates to 50Hz in order to close the grabber.

Hoist (hold) motor is off at this point.

With grabber fully closed, operator turns joystick to CLOSE AND HOIST position.

Closing motor continues running while Hoisting(hold) motor accelerates to 50HZ from rest to reach the speed of closing motor. This will ensure that the grabber remains closed as it is hoisted to the position that the operator may require.

The difficulty I am facing is that the hoisting rope remains very slack as the grabber is being hoisted. This makes the crane use only two ropes tied to the closing drum instead of four ropes. We have put the hoist drive acceleration time to 0.1 secs and no matter how long we set the acceleration of closing drive, the hoist rope remains slack during hoisting.

Would you assist us correct this slackness issue.

Regards and best wishes ,
Bob.
Email:fas [at] iconnect.co.ke

 

The way you are describing, the hoist cable will never take up load, as, given the end speed of the two motors is the same, and the grab motor is tightening its rope earlier, the hoist motor will never catch up.

I would recommend that the hoist motor is accelerated to a slightly higher speed (eg. 53Hz) until the motor takes up load at which point its speed is lowered to 50Hz to continue following the grab motor.

Obviously the crucial point would be when the hoist motor starts taking up load, as at this point it should not re-open the grab.

I believe that as long as the hoist motor can be operated for a short time at frequencies higher than 50Hz, the above suggestion should work, but it would need proper tuning up to work properly.

 

Hello Jojo,

To detect when hoist motor starts taking up load to hoist the grab and ensure it should not re-open the grab is the terribly difficult part for me.

I could use a plc to measure the currents being drawn by the hoist and grab frequency converters and then compare the two to pick when hoist is above the grab as this will be when hoist motor will be hoisting, but this seems like a complex programming.

Would someone with experience in this work suggest a simpler way.

Regards
Bob

 

I don't have experience with this sort of crane, but I think that what you need to do is to operate the "grab" drive in torque mode (with a velocity limit) and the "hoist" drive in velocity mode.

When you close the grab, the drive would apply a limited torque which will not be enough to raise the load. When you activate the hoist drive, it will raise the load while the grab drive will automatically maintain the same torque, keeping the grab closed. This will keep the same tension on the grab cable, preventing it from going slack. This also allows you to set how hard the grab closes on the load. The proportion of load taken up by either drive will depend on the torque limit in the grab drive. The grab drive needs a velocity limit as well as a torque set point so that it won't run away under low load and close too quickly.

 

Is your original question driven by the fact that the design of your grab system dictates that the hoist cable should be used to lift when the grab is closed?

Having said the above, my previous comment was in fact intended to point to the use of a PLC to achieve the required control function.

By the way, have the VFDs been always installed, or were they a retrofit, which, despite all good intentions, have created this problem? If they were a retrofit, what was the originl setup of the lifting system?

 

We tried setting the grab VSD to torque mode but failed. The response was very sluggish even at maximum setting so We returned to frequency control.

This crane has been on slip ring motor control but there was need to upgrade to modern control using VSDs. Slip ring motors do well but difficulties with maintenance and parts obsolescence has necessitated the upgrade.

Regards
Bob

 

Not sure what the problem was with that but you don't necessarily have to be in torque "mode" on the grab motor. You could be nominally in speed mode, with a higher speed setpoint than the hoist, and a torque limit, which ends up amounting to the same thing.

 

The frequency converters are well able to control the load in frequency mode.

My problem is only to synchronize the grab close motor to hoist motor so that once the close motor has closed grab, hoist accelerates to closing motor speed and at this point both share the hoisting load. This way the hoisting rope will not slacken.

Bob

 

"The frequency converters are well able to control the load in frequency mode."

Apparently not or you wouldn't be asking the question. Using torque mode on one motor is a way of making them share the load at matched speed.

 

Dear Steve,

Still can not figure it out. Would you kindly elaborate on how this would be done. Each of these motors are controlled by a reference from a joystick potentiometer which gives a 0-10vdc reference.

Which one do you set torque mode.
Bob

 

You will need to put at least the grab drive in torque mode or perhaps still velocity mode but with a torque limited to less than full motor torque, because the grab motor has enough torque to lift the load, which you don't want.

I'm not sure why when you put the grab drive in torque mode it was "sluggish" unless there were some other parameters that needed adjustment.

You want to share some of the load with the hoist motor, but you don't want to overrun the grab motor, or the grab may loosen. You can try to synchronize the operation of the two motors with speed settings (the grab motor should have a slightly higher top limit of speed to make sure the hoist motor doesn't make its cables go slack), or you could also limit the torque of the hoist motor so it cannot lift the load by itself either. Then if it tries to overrun the grab motor, it cannot accelerate to a speed fast enough to take the load entirely.

I suggest the first mode, but that is the one you said you already attempted and it didn't work. I suspect it can be made to work well if you get the parameters correct. Have a representative from the drive manufacturer or distributor come to the site and help you with the parameter configuration.