PF525-2 vfd sync

I have 2 VFDs. They are both controlling 1 single belt. Current setup Micro-1500 and 2 PF700's. They have a program that uses an offset value that uses main motor Spd_Fdbk to adjust the slack motor speed. I have been tasked with making the system work with L18 and 2 PF525 without encoder. How do I start without having anything else except current setup. The system has 1 motor that only runs the slack belt section motor. The other motor controls the main drive motor.

From the post I assume you are load sharing
First you can't do load sharing with a speed follower setup the drives will always fight each other.
you need to do a torque follower setup
do a google search on Torque Follower
the master drive is setup to control the speed the slave follows the current of the master

Doesnt Suppot

The does not support torque follow though.

I see what you mean from what I see the 525 dos not support torque control
The you have to replace at least one of the VFD's with one that will support torque control
if you try to control with a master slave in speed mode the 2 drive will fight each other and could fault out.
I have seen where other have tried to share a load with speed control both drive show high current and over 6 hz difference between them
you don't need a plc to do this it all can be done with just the VFD's a alone
again goggle it "Torque Follower" or you can search UTube as well
there are many articles and video's on this available to learn from.
A Common Load on a Common Shaft with multiple drives
the Master sets the speed the other slaves supply the additional torque necessary to maintain the set speed
It's a simple setup that works

Correct, you need a torque follower setup but the 525 does not do torque control mode, you cannot use it for this with any reasonable degree of success unless maybe you have a dancer loop in the belt, which will not actually share the load between the drives, it will just control the amount of slack.

The 525 is smaller and cheaper than the 700 or 750, this is one reason why. So just because someone wanted to do it with a cheaper drive does not mean it is possible.

Dancer Loop

I do have a loop of 10' of slack at beginning of belt. The drives are already purchased. The Logix500 program used an offset that updated the speed of the slack motor. It used Ft/m and to compute speed. from hz feedback from main motor. Then multiplied offset and current speed of SP of slack motor then sent this value to slack motor VFD commanded speed. I am not sure how they calculated Feet Per Minute in old program but it looks like it was messaged directly from old VFD. Not sure the 525 has this value. They also want to add torque limiting to 1 of the drives as to not snap the belt. Is there any easier way with what they have?
The 525 does have a 4-20mA output and torque can be selected. It will also use a 4-20mA input for a speed reference. Is this not the same as what torque following would be doing?

You can do current limiting in the drive via a fixed parameter. It's enabled by default to some very high value. Not very precise, but might be good enough for what you're working with.

I don't remember the parameter offhand but it's in the user manual.

Current Limiting

I thought about using current limiting only by was not sure how fast the response time would be. I would have to do this to the main drive vfd only and not sure if this would react until all slack is gone or if it would work the second the slack was starting to get taken up. Would I not be better using the torque current as a speed value to the slack motor?

This looks like this is a example of a system that was poorly designed by somebody that didn’t know what they were doing. They like most thought they could use speed follower to control tension and share the load and found out that it didn’t work. Now they dumped the whole thing on you in hopes that you can fix it for them. And to add to the mess you are to use the bad materials that they already have. Been there done that more times then I want to cone. At a minimum you need to change the vfd’s to ones that can run in torque control there may be other things that need to change as well.
It’s funny they always they always think you can compensate for bad design or bad hardware with programming. Maybe you should consider bringing in somebody from the outside to have a look and evacuate this in person. With what I see here you don’t need a PLC to control it just 2 vfd’s the are torque control capable.

You would be better off keeping the PF700's and just update the ML1500 to CompactLogix and reuse any of the existing IO modules if there are any.


Alan.

700 blew up

One of the 700s blew up and they already replaced with 525 and 1400 that is communicating between it and the 1500. Is there no way to make this work even with having the extra 10' of slack?

CNTRLFRQ said:

One of the 700s blew up and they already replaced with 525 and 1400 that is communicating between it and the 1500. Is there no way to make this work even with having the extra 10' of slack?

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Well, technically there IS, but not with the hardware you have now... What someone was doing (I'm guessing) was to somehow provide an educated guess at what was needed on the two drives to try to share the load, but it likely was not actually working. The fact that you ahve a slack loop is the indicator of that.

The 525 does have a 4-20mA output and torque can be selected. It will also use a 4-20mA input for a speed reference. Is this not the same as what torque following would be doing?

Click to expand...

No, not the same. the output of the PF525 will tell you an estimate of what the torque is on the motor shaft, but it is not CONTROLLING that torque. The torque output of the drive will vary depending on what it needs to do to maintain SPEED. That is fine for the MAIN drive motor in your application, that is what you want. The problem is on the 2nd motor. It needs to match the torque of the main motor, not the speed, otherwise it is not going to share the load. So it is the 2nd motor that needs to be a "torque follower" to the first motor, which means it needs to operate in torque control mode. That is what the PF525 cannot do. The PF525 is a Vector Drive, but it is specifically a VELOCITY VECTOR drive; Rockwell states this over and over, but many people don't pay attention or don't understand what that means.

When you get into the world of Vector Control, it can generally be thought of as a more accurate way to maintain speed control, but there is more to it than that. You can have VELOCITY Vector Control and you can have TORQUE Vector Control, the difference being what takes priority in the response algorithm.

In Velocity Vector Control, it is the speed (velocity) that is considered the most important factor, torque is USED to maintain accurate speed and provide for a faster "step change response" to a change in the load, AS IT RELATES TO SPEED. Since this is what MOST people use VFDs for, the basic Vector Control drives (like the PF520 and most other smaller/cheaper "component class" drives) come with a Velocity Vector Regulator loop in the microprocessor.

Torque Vector Control makes torque the prime point of control, even at the EXPENSE of speed. You want the torque to be the thing that the motor maintains as constant, regardless of what that means to speed. But BECAUSE it ALSO involves speed, the microprocessor in the VFD must then have a Velocity Vector PLUS a Torque Vector Regulator loops in the drive, meaning the microprocessor has to do more math simultaneous processing. That then means a more powerful mProcessor or adding a math co-processor inside of the VFD. More powerful or multiple processors means more power and heat, which means larger components and heat sinks etc. on the control boards. Smaller/cheaper drives like the PF520 series sacrifice that capability to be able to keep the size (and cost) down, compared to more capable drives like the PF700 (or now the PF750) class drives.

There is no free lunch...

Just because it might be possible to get it working with enough band-aids and duct tape, doesn't mean its the right thing to do...

I've done many such projects that worked for years, but I was not ever really satisfied with how it would look to the poor guy who had to troubleshoot it later.

If the boss won't let you do the job with the right tools, then make certain you document in the programming or drawings the patches and hole plugs you had to use for the bugs that infest the solution.