# VFD's not syncing up.



## mitch65 (Mar 26, 2015)

http://www.controldesign.com/articles/2002/235/


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## just the cowboy (Sep 4, 2013)

*Follower mode*

Yes the drives usually are hooked together electronically. Some call it follower mode some call it master slave or some in motion control call it ELS (electronic line shaft) .


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## JRaef (Mar 23, 2009)

Small cheap drives do not have that functionality, so whomever told you that this would work was basically "uninformed".

You cannot really do an ELS function unless you use shaft encoders on each motor with AC drives and the ability to do true torque control in the VFDs. The PF40 was never capable of this; the PF525 is capable of accepting shaft encoders for closed loop VELOCITY Vector Control only, but not true Torque Vector Control, which is what you need to accomplish true load sharing. These are what are called "Component Class" drives and are meant to be less expensive, so the Vector Control Algorithm lacks a torque regulator loop (because that needs more hardware).

If you have a VFD with true _FLUX VECTOR CONTROL_ (FVC) for TORQUE, then the way it works is that one drive is a Master, the other is a Follower. You feed your speed control signal into the Master drive which is set up in FVC for Velocity control, then the Master puts out an analog signal proportional to its output TORQUE. The Follower Drive is then set up in Flux Vector Torque Control mode as a Torque Follower using that analog signal from the Master as it's command. That means it puts out the same TORQUE as the Master, which means it will share the load equally (within the tolerances of the analog signals, which is usually +-1% in my experience).

More powerful VFDs like the PF700 are capable of this with an encoder feedback, the newer PF750s can now do FVC without an encoder (using the internal sensors). You might be able to get SOME better sharing performance with a PF525 using SVC (Sensorless Vector Control), but the SVC is not tailored to provide true torque control as I mentioned; they are intended for Velocity (speed) control only. You can try doing Master-Slave _speed _follower using SVC in both drives and see if it helps, but that will not work for accurately share the load because there will be differences in friction, belt slippage, stretching, etc. If you TRULY need to accurately control the load sharing, you would need to upgrade to a PF755 (or a PF700 + encoders).


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## garfield (Jul 30, 2009)

Are you checking amperage on the drive itself? If not, is your meter rated for checking amperage on a vfd? Could you set the maximum drive output amps on each one and just keep reducing it until the have to share the load? Could you just adjust maximum speed on one or the other until they shared the load? What about a ct then scale the output to whatever would control the other drive? How about putting a plc in the mix that would control both motors and just control the speed of each based on the amp draw. How about a pid loop controller hooked to one drive that controls the other? It can't be critical that they are exactly evenly loaded or it wouldn't have run that long without a problem. FYI, These are questions, not advice.


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## Sparky Mcgregor (Sep 7, 2015)

JRaef said:


> Small cheap drives do not have that functionality, so whomever told you that this would work was basically "uninformed".
> 
> You cannot really do an ELS function unless you use shaft encoders on each motor with AC drives and the ability to do true torque control in the VFDs. The PF40 was never capable of this; the PF525 is capable of accepting shaft encoders for closed loop VELOCITY Vector Control only, but not true Torque Vector Control, which is what you need to accomplish true load sharing. These are what are called "Component Class" drives and are meant to be less expensive, so the Vector Control Algorithm lacks a torque regulator loop (because that needs more hardware).
> 
> ...


Thanks, that's really helpful. The shafts are currently fitted with encoders but I suspect they're used to for timing purposes in order to let the system know when to actuate the tray drops. We're currently being steered towards the 525 but I mentioned their limitations to my employer so he's aware. The product being conveyed right now is very light and driven a low speeds so imagine the 525 will be all we need, especially since prior to that one incident the system has been running fine for a while, even with zero communication between the drives and only one of the two drives carrying the brunt of the load.


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## Sparky Mcgregor (Sep 7, 2015)

garfield said:


> Are you checking amperage on the drive itself? If not, is your meter rated for checking amperage on a vfd? Could you set the maximum drive output amps on each one and just keep reducing it until the have to share the load? Could you just adjust maximum speed on one or the other until they shared the load? What about a ct then scale the output to whatever would control the other drive? How about putting a plc in the mix that would control both motors and just control the speed of each based on the amp draw. How about a pid loop controller hooked to one drive that controls the other? It can't be critical that they are exactly evenly loaded or it wouldn't have run that long without a problem. FYI, These are questions, not advice.


Thanks, all great questions. I'm taking my amperage reading just past the contactor prior to the drive. I don't think this is a critical issue as you mentioned. I'm mostly just trying to make sense of my readings to get a better handle of the situation. So far it seems like my interpretation of what's going on in terms of load sharing (or lack there of) is sound. I'll suggest tweaking some of the parameters as you mentioned to my employer. Perhaps he'll want to explore some of these options before purchasing new drives. In terms of using the plc to control both motors, I'm not sure I'd be able to implement that since our programmers are steering us toward upgrading our VFD's. My understanding so far is that upgrading to a PF 525 would be the minimum requirement for a PID loop.


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## John Valdes (May 17, 2007)

Like said above, running two motors in master/slave is the best option even if the ratio is 1:1.
With a closed loop system or with a torque drive, fluctuations in load do not interfere with motor speed.
The issue regarding the chain might be explained by one motor operating at one speed vs the opposite motor operating at another speed. Even if just for a second or two?

Personally I prefer closed loop for this application. But I am not up on all the newer drives and how well they function open loop. But I do understand they are quite good at maintaining speed regardless of load.


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## telsa (May 22, 2015)

I have to wonder if you even need the second drive
.
I suspect that this was so engineered because of a brain fart.

Why ever have two motors when a single puppy might do ? ( Like 5 hp )

We are talking about essentially horizontal conveyance, are we not ?

3 hp is a LOT for such a scheme. 

I must assume that your target loads are absolute beasts.

You make me think that the revs on these babies are cranked WAY down.


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## Sparky Mcgregor (Sep 7, 2015)

telsa said:


> I have to wonder if you even need the second drive
> .
> I suspect that this was so engineered because of a brain fart.
> 
> ...


It's actually the VFD's that are 3hp. The motors are rated at 7.5 Hp and yes they're pretty big motor's. The product is not that heavy but the actuating tray's are made of solid steels and attached all along the drive chain. I'm not sure what the reason for using two motors is but I suspect there is one. This is a pretty standard configuration as near as I can tell. They do the same thing for all the horizontal carousels I've seen which are pretty similar in design. Perhaps it's just easier on the motors sharing the load and it's a way to make them last?


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## Sparky Mcgregor (Sep 7, 2015)

A quick update for those who are interested. The same programmers who were telling us to upgrade to the 525's are now saying we can't do that since AB runs on ladder logic and our sorter is programmed using steeplechase. This doesn't jive with my understanding of how powerflex VFD's work. I thought the whole reason that they wanted to upgrade was that PID communication was built right into the 525's and was controlled using parameter settings and feedback from the encoders. 

Also, even if this isn't the case couldn't the plc just monitor the encoders which are already inputting into steeplechase and monitor them trough some sort of compare instruction? All we want is the system to know if the vfd's aren't syncing up so that they don't keep running and cause damage if they go out of sync. We don't care how it's achieved. Any thoughts?


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## JRaef (Mar 23, 2009)

They are BSing you... the VFDs are not "programmed in ladder", they have their own internal programming parameters. How you command them is via hard wiring to I/O or communicate to them is via Ethernet I/P or Modbus RTU protocol with command lines. What kind of programming is used in the overall control system ahead of it is totally irrelevant. All they need to do is be able to put out a message with a command in a format the drive can read, and Etthernet I/P or Modbus are the two most common protocols for industrial control now.

The more likely real problem here is that Steeplechase is gone; bought first by Phoenix Contact in the 90s then again later by Schneider maybe 15 years ago, who eventually killed it and merged it into other software platforms (assuming that part). So most likely it's not very well supported any more and they can't find anyone who can handle it. But in it's day, Steeplechase was just a PC based software program that used flow-chart programming instead of something simpler like ladder programming. The software + the PC was called a "VLC" for Visual Logic Controller. It never really took off. 

If someone knew how to program it still, they could make it work. But given that it's been dead for 15 years or more, they probably can't find anyone that knows how to do it.


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## Sparky Mcgregor (Sep 7, 2015)

JRaef said:


> They are BSing you... the VFDs are not "programmed in ladder", they have their own internal programming parameters. How you command them is via hard wiring to I/O or communicate to them is via Ethernet I/P or Modbus RTU protocol with command lines. What kind of programming is used in the overall control system ahead of it is totally irrelevant. All they need to do is be able to put out a message with a command in a format the drive can read, and Etthernet I/P or Modbus are the two most common protocols for industrial control now.
> 
> The more likely real problem here is that Steeplechase is gone; bought first by Phoenix Contact in the 90s then again later by Schneider maybe 15 years ago, who eventually killed it and merged it into other software platforms (assuming that part). So most likely it's not very well supported any more and they can't find anyone who can handle it. But in it's day, Steeplechase was just a PC based software program that used flow-chart programming instead of something simpler like ladder programming. The software + the PC was called a "VLC" for Visual Logic Controller. It never really took off.
> 
> If someone knew how to program it still, they could make it work. But given that it's been dead for 15 years or more, they probably can't find anyone that knows how to do it.


Yeah, that's inline with my understanding of how VFD's and PID control works. Thanks for confirming that for me. That's interesting what you were saying about steeplechase. I know I'd never heard of it prior to working at my current post. I always assumed they used this language for proprietary reasons. Making us require their services for support and limiting competition.


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## JRaef (Mar 23, 2009)

Sparky Mcgregor said:


> Yeah, that's inline with my understanding of how VFD's and PID control works. Thanks for confirming that for me. That's interesting what you were saying about steeplechase. I know I'd never heard of it prior to working at my current post. I always assumed they used this language for proprietary reasons. Making us require their services for support and limiting competition.


Probably true. Using an off brand of products is a strategy for limiting your customers from getting help outside of your own services group.

Steeplechase was just one company's way of trying to enter the controller field without having to spend the money to develop controller hardware, by using existing commercial off-the-shelf PCs to do the number crunching. What happened though is that the traditional PLC mfrs all came up with "soft PLC" versions of their own shortly thereafter, which used the same programming as their "hard" PLCs, so it was more useful. You can buy Rockwell Studio 5000 to program all of their PLCs, then if you have a machine in which you want to run your controls on a PC, you can just buy an add-on that emulates a Logix PLC on the PC hardware. Siemens offers the same thing. That's kind of why Steeplechase went away. When Schneider bought it from Phoenix, they just rolled it's functionality into their Quantum PLC line as their "Soft Quantum" product.


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## telsa (May 22, 2015)

Steeplechase ran into a hurdle, you say ?

Oh, my.


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## JRaef (Mar 23, 2009)

telsa said:


> Steeplechase ran into a hurdle, you say ?
> 
> Oh, my.


LOL.
Yeah, someone raised the bar on them...


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