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Old January 18th, 2020, 05:41 PM   #1
geofftheelectrician
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525 VFD overspeeding in weight lowering application

I have an issue with an elevator being driven by a 525 where when the second shelf (of 8 tiers) gets loaded on to the conveyor the drive over speeds and then the decel command to reduce speed (commanded by prox on cam switch) seems to have no effect. Monitoring B001 on the drive as the machine is cycling the command freq of 50 hz over speeds to 62+ hz and never seems to get to the slow speed of 10 hz for positioning and overshoots. Iíve advanced the cam timing for the slow, lengthened and shortened the decel time and nothing seems to successfully stop it in the right position.

The only thing that has worked is reducing my speed to 30 hz and I finally get the carriers to stop in the right position when the second shelf is being carried by the chain. 2 concerns remain, I might not be able to make rate Monday morning when we start loading product in to the machine and I am not sure how it will react with 2000 +/- kg of product on the shelf unit (x2) when fully loaded.

Thoughts? Thanks in advance.
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Old January 18th, 2020, 06:27 PM   #2
rpoet
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Does your drive have a brake resistor installed? If yes, is it the right value (resistance) and large enough (wattage) for the duty cycle you're operating at?

I'm not directly familiar with the 525; does it need parameter(s) adjusted to work with an external brake resistor?

Did this setup ever work correctly, or is this a new project? If it worked at one time, what changed?

If the drive can't regen into a resistor to dissipate the energy generated by lowering a load, the drive won't slow down and you will have trouble controlling the load.



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Old January 18th, 2020, 08:09 PM   #3
Ken Roach
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This sounds very much like a dynamic brake application or configuration problem.

First, the PowerFlex 525 might not be the right drive for a vertical load application, but I don't know your machinery. Do you have similar systems running with PowerFlex 525 drives ?

The behavior you describe sounds exactly like the DC Bus Regulation mode is enabled. When the DC bus voltage goes high because of an overhauling load, the drive will increase the output frequency and voltage, to keep the DC Bus voltage from getting too high.

When DC Bus Regulation mode is disabled, the drive lets the DC bus rise and keeps its control over the Output Frequency, and trips if the bus gets too high. You'll also have to set the duty cycle and stop mode; see the user manuals or RA Knowledgebase 591021.

Obviously the second big question is if you have an external braking resistor connected to the drive, to dissipate the energy from the lowering load.
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Old January 19th, 2020, 06:02 AM   #4
geofftheelectrician
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Thanks for the reply..

Yes there are brake resistors installed and they should be sized properly for the drive.

The project is an upgrade on an old panel with old SLC controller corporate said had to be replaced (went with 1756-L73 processor). The VFDs in the old panel were no longer available so it was thought best to replace them as well. I am working with an engineering/integration company and we are scratching our heads but i see Ken Roach has also replied with a knowledge base article and some ideas I think we can look at in the morning.
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Old January 19th, 2020, 06:05 AM   #5
geofftheelectrician
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Thanks Ken, To get it out of the way first, yes a DC braking resistor is installed and correctly sized for the drive.

We never thought of DC bus regulation being a possible culprit. Thanks for the pointer and reference to the knowledge base. I’m off to read that with my morning coffee and we will look at that first thing in the morning Monday as we have a few hours of production backed out of the schedule because the production lines were overhauled as well over the last few weeks.

Cheers.
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Old January 19th, 2020, 10:38 AM   #6
GaryS
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I hate to be the bearer of bad news but the system should have been designed to use an elevator or hoist drive from the beginning. A standard vfd will have difficulty handling the demands of a hoisting application.
Just adding a DB resister to the drive will not do it. Most internal DB resisters and driver are only rated for 10% duty cycle thatís not near enough for your application. If the vfd faults out while it controlling the load, the load could drop causing major damage or injury to personal. Remember when you are lowering the load all the energy to slow the load must be absorbed through the vfd and dissipated as heat on the resister.
Depending on the size of you drive ( you did not tell us the size). You may be able to add a DBU (Dynamic Breaking Unit ) and resister to your vfd depending on the options available for your vfd.
You should be able to completely stop and hold the full load without the mechanical brake.
You would also want to have proof of torque before the load is moved
Hoisting applications are different and require more things to be considered when working with them.
You may want to look at Yaskawa Matrix drive they are full line regeneration. No resister necessary in fact no DC Buss at all. Or look at Electromotive for the correct drive configuration for your application
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Old January 19th, 2020, 03:03 PM   #7
geofftheelectrician
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Thanks Gary for your input. I will discuss tomorrow with my integrator and design team and see what is the best way to proceed.
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Old January 20th, 2020, 07:30 AM   #8
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Do you have only resistors or a properly sized chopper? Every crane I've seen running Powerflex drives required a braking chopper also.
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Old January 20th, 2020, 07:49 AM   #9
geofftheelectrician
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This isn’t a crane, it’s an ice cream hardener. The fresh product gets loaded onto shelves which travel through the hardener and 8 hours later head out to palletizing to be stacked on skids. The tray/shelf it is loaded onto/unloaded from is being lowered, gets to the bottom and then transfers to the back. Some 40 pushes later it is raised up to make the trek back to the front. We do have a properly sized braking resistor in the system and now based on previous comments the parameters set right, along with a nice slow decel. Waiting for product this morning to confirm everything works with weight on the shelves.
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Old January 20th, 2020, 02:11 PM   #10
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"Properly sized...". How, by virtue of the manual? Because as Gary correctly pointed out, you may have missed the part that says even if you properly size the brake resistors, the PF525, like all "component class" drives, is only capable of 10% duty cycle, they are NOT capable of continuous braking. There is a "brake chopper" transistor built into the PF525, but that is the limiting factor on how much braking you can accomplish. Think about it; when motoring, all of the kinetic energy required for the load is going into the motor as electricity but is shared among 6 transistors. When braking all of the kinetic energy if going back through those same transistors, but then has to go through ONE transistor into the resistors to be burned off as heat. So to keep that transistor from frying itself, it is pulsed on an off for short periods, aka the "duty cycle". So once you reach the max of what that can do, the drive can no longer help slow it down.


If you had an older drive that was working for years, it likely either had an external brake chopper, sized for the duty cycle the task requires, or the entire drive was selected for that, resulting in it appearing to be "over sized". I've seen many times where someone decides that the initial application was a mistake and tries to save money by using a "correctly sized" drive, not understanding that the perceived over sizing actually was based on knowledge of the demands on the unit.


One possible solution for you now may be to go ahead and add an external brake chopper to that drive, something like a Bonitron unit, sized for the actual load inertia that you are experiencing.
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Old January 20th, 2020, 03:01 PM   #11
GaryS
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keep in mind if you are going to add a brake chopper (DBU) to existing vfd you must use the one for the vfd you are using they work off the buss voltage if you get it wrong the vfd may shut down on a buss overvoltage or it may drop the buss voltage to low and make the vfd unusable
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Old January 21st, 2020, 11:33 PM   #12
Jasondelane
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Quote:
Originally Posted by geofftheelectrician View Post
This isnít a crane, itís an ice cream hardener. The fresh product gets loaded onto shelves which travel through the hardener and 8 hours later head out to palletizing to be stacked on skids. The tray/shelf it is loaded onto/unloaded from is being lowered, gets to the bottom and then transfers to the back. Some 40 pushes later it is raised up to make the trek back to the front. We do have a properly sized braking resistor in the system and now based on previous comments the parameters set right, along with a nice slow decel. Waiting for product this morning to confirm everything works with weight on the shelves.
I understand you've properly sized the resistor. I understand it's not a crane but operates on some of the same principles when it comes to the VFD.

A chopper will move the work being done by the drive to a separate module that can better handle the duty cycle needed to stop and hold a suspended load.
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