Crane Jerk Problem during starting and stopping

Hi All,

What would be the cause of heavy jerking of our crane during long travel operation? Here is the scenario;

• The time I issued a command to start and the crane started accelerating and ramping up the speed a heavy jerk is observed, once it’s picking up the speed till it reached to maximum speed it runs very smoothly.
• During stopping it did the same thing and vice versa.

I am using Power Flex 700 VC VFD that drive the 2 long travel motors (I know I suppose to use V/Hz but this set up (sensor less vector) is working very well with our other cranes without a problem since commissioning.

What really puzzled me is that, we have two identical cranes, the same type of VFD, the same parameter settings, the same motors but the other crane is working smoothly and precisely, no jerking, no shaking, no vibration, no nothing.

Could this be a problem of mechanical backlash or the crane requires stiffening on mechanical parts. In need of your help
Thank you,
Don

Could be anything mechanical. bearings, gears etc

If they have an "S" curve ACC and DEC, use it.

I try the S-curve at 15% but it didn't help, it creates additional problem during stopping as it tends to coast rather than ramp to stop.

dondon_alba said:

I try the S-curve at 15% but it didn't help, it creates additional problem during stopping as it tends to coast rather than ramp to stop.

Click to expand...

That isn't the problem with s-curves. s-curves do require that the drive be able to handle decelerations about 50% higher than the average deceleration and that could trip the motors current limit.

Static friction ?

dondon_alba said:

Hi All,


What would be the cause of heavy jerking of our crane during long travel operation? Here is the scenario;

• The time I issued a command to start and the crane started accelerating and ramping up the speed a heavy jerk is observed, once it’s picking up the speed till it reached to maximum speed it runs very smoothly.
• During stopping it did the same thing and vice versa.

I am using Power Flex 700 VC VFD that drive the 2 long travel motors (I know I suppose to use V/Hz but this set up (sensor less vector) is working very well with our other cranes without a problem since commissioning.

What really puzzled me is that, we have two identical cranes, the same type of VFD, the same parameter settings, the same motors but the other crane is working smoothly and precisely, no jerking, no shaking, no vibration, no nothing.

Could this be a problem of mechanical backlash or the crane requires stiffening on mechanical parts. In need of your help
Thank you, Don

Click to expand...

By travel I assume you have a bridge crane AND your problem is moving the bridge not the hook not the trolley.

Hmmm two motors drive the bridge are they hard coupled or independent? Are you positive they both run at same speed. If not the bridge can crab and bind on the rails or jump a rail - had to put one or two back on the rails. Does one motor have more friction ie load than other -- check em with a tach ??

Think I would give the track a check over - does problem occur in same area?

Are motor leads from VFD equal length?

Dan Bentler

Hi d_a!

Did you log and compare the current/power/torque curves?
What about phase currents? Motors D-Y checked? Brakes disengaged properly and not via motor power(clack-clack-clack)?

Kalle

leitmotif said:

By travel I assume you have a bridge crane AND your problem is moving the bridge not the hook not the trolley.

Hmmm two motors drive the bridge are they hard coupled or independent? Are you positive they both run at same speed. If not the bridge can crab and bind on the rails or jump a rail - had to put one or two back on the rails. Does one motor have more friction ie load than other -- check em with a tach ??

Think I would give the track a check over - does problem occur in same area?

Are motor leads from VFD equal length?

Dan Bentler

Click to expand...

Hi Dan,

You are right this is a double bridge crane with a girder width of 40 meters and a trolley travel moving end to end with the girder and where the cabin operator is located. This is a huge crane (Furnace Tending Assembly Crane) used for charging and discharging anode in our anode baking furnace. Yes, my problem is in long travel operation or in the double bridge it is jerking during starting and stopping.

The two motors are driven by 1 VFD Power Flex 700VC, they are independent in a way parallel with each other. It running smoothly, jerking is present during starting and stopping, so I would say it runs at same speed. I’m going to check it with tachometer, some kind of friction is evident but I don’t know where, need to do a lot of inspection with mechanical.
Tracks or rails are ok. Motor leads are almost same.

Thank you,
Dondon

Originally posted by Peter Nachtwey:

s-curves do require that the drive be able to handle decelerations about 50% higher than the average deceleration and that could trip the motors current limit.

Click to expand...

In the normal world this would be correct. In th ePF700VC world, however, this doesn't apply. The accel rate will never be more agressive than the accel rate defined by ( max frequency / accel time ). The s-curve percent increases the accel time by the percentage of the setting and adds that time on both ends as s-ramp time. That's why is seems like it is coasting. It takes longer to stop. But it isn't really coasting. Take a look at the S-curve description here.

I'm still amazed that the thing works at all in sensorless vector with two motors connected to the drive. I would think the drive wuld have a REALLY hard time figuring out what is going on at the motors in a system like this. Regardless of how your other systems work, have you tried running this one in scalar (Volts/Hz) mode? At the very least that may help you determine whether it is drive/motor related or something with your physical system.

Keith

Hi,

In the previous PLUS Version, there used to be a DROOP function. Try this if present in PowerFlex. Use it on any one drive only.

Ron

The same is true during deceleration when using S-curve. I tried using V/Hz control but it is a bit unstable especially during slow speeds, this maybe because of the size of the crane.

We are still on the process of determining where the problem is, mechanical is suspecting it is the holding brake problem that does not release properly which we are also going to look at. I am suspecting one of the gearbox of the motor causes the problem. We scheduled the shutdown of the crane next week for total inspection. I’m a bit upset as people is pointing that it is VFD and electrical problem, or maybe they just don’t know how our control works and its limitations.

Don

did you check both motors winding?

Kamenges said
QUOTE I'm still amazed that the thing works at all in sensorless vector with two motors connected to the drive. I would think the drive wuld have a REALLY hard time figuring out what is going on at the motors in a system like this. Regardless of how your other systems work, have you tried running this one in scalar (Volts/Hz) mode? At the very least that may help you determine whether it is drive/motor related or something with your physical system. UNQUOTE

OK both motors are on one drive setup for sensorless. THe question then becomes how does the VFD know "which sensorless signal" (very loosely phrased) to believe?

I think I would give some thought to driving each motor with it's own VFD, add encoder to one motor and slave one VFD to the other.

Dan Bentler

leitmotif said:

Kamenges said
QUOTE I'm still amazed that the thing works at all in sensorless vector with two motors connected to the drive. I would think the drive wuld have a REALLY hard time figuring out what is going on at the motors in a system like this. Regardless of how your other systems work, have you tried running this one in scalar (Volts/Hz) mode? At the very least that may help you determine whether it is drive/motor related or something with your physical system. UNQUOTE

OK both motors are on one drive setup for sensorless. THe question then becomes how does the VFD know "which sensorless signal" (very loosely phrased) to believe?

I think I would give some thought to driving each motor with it's own VFD, add encoder to one motor and slave one VFD to the other.

Dan Bentler

Click to expand...

Even me I was amazed with the setup. We have two stacker cranes working on the same setup (2 motors, 1 VFD & sensorless vector) working smoothly. There is an encoder on one motor. Another thing the (Parameter 80 Feedback Select) is also “ENCODER” not OPEN LOOP.

How does the VFD know "which sensorless signal" (very loosely phrased) to believe? I really don’t know.

We sum up the two motors (total power, amps,) and entered & do the parameterization.

I think I would give some thought to driving each motor with it's own VFD, add encoder to one motor and slave one VFD to the other. This is a good suggestion with a slim chance to be done since this setup is working for seven years, until the problem mentioned came.

Thanks,
Don