# VFD feedback



## Sparky Mcgregor (Sep 7, 2015)

This thread might be a bit confusing as I'm a little confused myself but I'll do my best. I'm trying to educate myself more on AB VFD's, more specifically powerflex 40 and I've reviewed the manual in a fair bit of detail but still haven't grasped a full understanding. I'm trying to come up with a solution involving two VFD's running in sync and would like our system to shut down if one goes out of sync. My understanding is that that this could be done by and incremental encoder mounted to the shafts that run in unison, which feed back into the VFD/PLC? In the manual it talks about "reading the drive via the network by sending Function code 3 reads to address 8452". I only have a vague idea of what that means. If there's someone on this forum who could dumb that down for me a bit (or a lot) I appreciate it.

I'm PLC certified from George Brown and have a good understanding on how to write ladder logic. I don't feel like that course near prepared me for real world applications of the program as it relates to hardware though. Also the VFD's have digital and analog output terminals. How could these be used in a practical a application? I'm assuming those are for digital and analog signals for PLC input terminals? Could these be utilized to monitor the drive frequency and perhaps use some sort of compare instruction to shut down the system if one VFD goes out of sync? I really want to get a better understanding of VFD's but so far this has proven to be a difficult task given all the VFD's I have access to are in use. I'm thinking about buying a 120v power flex 40 for myself to play with but it might get a little costly trying to mimic control applications using encoders ect.


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## John Valdes (May 17, 2007)

I'm not sure how much help I can be, but I will try.

Most controls (VFD) either are designed for encoder feedback or a expansion card to accept the encoder digital signal may be used. (closed loop)
The ones I used only employed a twisted pair between each drive and an encoder signal from each motor.
Then all you have to do is set the ratio. Yours will be 1:0 - 1:0. One drive will be the master and one the slave.
I'm certain there is provision for fault should either drive see a difference in speed.

Jareaf works for AB and should show up here sooner than later. He will know exactly what to do.


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## JRaef (Mar 23, 2009)

You cannot do encoder feedback with a PF40, you would need a PF40P version and they are a lot more expensive. In theory you COULD put the two encoder signals into a PLC if you knew how to do that, then tell the VFDs what speed to run at, but you do not have high precision control to accomplish perfect synchronizing of two motors with that kind of VFD, you need one with a lot more brain power like a PowerFlex 7 class. Synchronizing two separate motors is a lot more difficult than you might think. You might bet within 3-4% of each other (each VFD will be +-1 to 2%) with PF40s, but I don't know what you are trying to attain here.

So lets back up and start off with a detailed description of what you are trying to accomplish.


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## just the cowboy (Sep 4, 2013)

*more info needed*




Sparky Mcgregor said:


> This thread might be a bit confusing as I'm a little confused myself but I'll do my best. I'm trying to educate myself more on AB VFD's, more specifically powerflex 40 and I've reviewed the manual in a fair bit of detail but still haven't grasped a full understanding. I'm trying to come up with a solution involving two VFD's running in sync





Sparky Mcgregor said:


> Are they sharing the load as in torque sharing or are they two separate shafts trying to match speed? And would like our system to shut down if one goes out of sync If they are running master/slave you should be able to get a error alarm bit. My understanding is that that this could be done by and incremental encoder mounted to the shafts that run in unison you say shafts in plural, which feed back into the VFD/PLC? In the manual it talks about "reading the drive via the network by sending Function code 3 reads to address 8452" Sounds like a Modbus address. I only have a vague idea of what that means. If there's someone on this forum who could dumb that down for me a bit (or a lot) I appreciate it.
> I'm PLC certified from George Brown and have a good understanding on how to write ladder logic. I don't feel like that course near prepared me for real world applications of the program as it relates to hardware though. Also the VFD's have digital and analog output terminals. How could these be used in a practical a application? I'm assuming those are for digital and analog signals for PLC input terminals? Could these be utilized to monitor the drive frequency and perhaps use some sort of compare instruction to shut down the system if one VFD goes out of sync? I really want to get a better understanding of VFD's but so far this has proven to be a difficult task given all the VFD's I have access to are in use. I'm thinking about buying a 120v power flex 40 for myself to play with but it might get a little costly trying to mimic control applications using encoders etc.




First off if you are having trouble understanding this, don't worry it is advanced control system. 
Next you need to find out are you, matching Speed, position or sharing load, all three us a different approach.
Speed matching is when you want to have two or more machines run at the same speed and it just needs to be close, sometimes an extra feedback device is used such as a strain gage or dancer arm.
Position matching goes a step farther and uses an encoder or other feedback device that know where it is at as well as how fast it is going. This type of control will make "part a" of a machine meet "part B" of a machine at the same time every time.
Load sharing is when the motors are trying to drive the same load, you don't want one driving and the other working against it, so the drive work as a master and a slave.


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## Sparky Mcgregor (Sep 7, 2015)

JRaef said:


> You cannot do encoder feedback with a PF40, you would need a PF40P version and they are a lot more expensive. In theory you COULD put the two encoder signals into a PLC if you knew how to do that, then tell the VFDs what speed to run at, but you do not have high precision control to accomplish perfect synchronizing of two motors with that kind of VFD, you need one with a lot more brain power like a PowerFlex 7 class. Synchronizing two separate motors is a lot more difficult than you might think. You might bet within 3-4% of each other (each VFD will be +-1 to 2%) with PF40s, but I don't know what you are trying to attain here.
> 
> So lets back up and start off with a detailed description of what you are trying to accomplish.


Right... I had another thread talking about this but I decided to start a new one because my objective has changed slightly. Basically we have two motors running at 47hz each with their own PF40 VFD running the same drive chain. It's a drop tray sortation system that function somewhat like a horizontal carousel. Currently we have no reason to think that the VFD's communicate with each other.

The drop trays are all attached to the drive chain and are actuated by solenoids at each discharge chute. We had an incident in which the drive chain pulled off one of the sprockets while the other one continued to function and cause a whole lot of damage. Were trying to avoid a repeat occurrence. There are encoders mounted on each drive shaft but they didn't prevent the incident so I suspect they're used mostly for timing in order to actuate the drop tray's. 

There's some guess work but were pretty sure one of the VFD's faulted and the other one just kept on trucking. One theory is that a bad connection at the switch prevented the signal from getting back to the PLC when it faulted because I was told a blinking LED was found on the switch indicating a bad connection. Unfortunately this can't be confirmed since I wasn't there for the incident nor do I have access to the program logic.

I'm still nor sure what the best way to create a safety net for this occurrence is but I was leaning towards having a separate AB controller monitor the VFD HZ if possible and send a signal to stop the sorter if they start going out of sync. 

Another approach would be to borrow the output of the encoders currently feeding into the PLC and feed that into an AB controller to achieve the same thing. The situation is that I can program using RS logix (kind of) but not steeplechase which currently runs the sorter, nor would I have access to that program if I did know the language. Pulling this off would be above and beyond since programming isn't really expected of me but It's a good opportunity to further my programming skills. If I could at least come up with a solid way to pull this off we could hire the right people to implement it.


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## Sparky Mcgregor (Sep 7, 2015)

just the cowboy said:


> First off if you are having trouble understanding this, don't worry it is advanced control system.
> Next you need to find out are you, matching Speed, position or sharing load, all three us a different approach.
> Speed matching is when you want to have two or more machines run at the same speed and it just needs to be close, sometimes an extra feedback device is used such as a strain gage or dancer arm.
> Position matching goes a step farther and uses an encoder or other feedback device that know where it is at as well as how fast it is going. This type of control will make "part a" of a machine meet "part B" of a machine at the same time every time.
> Load sharing is when the motors are trying to drive the same load, you don't want one driving and the other working against it, so the drive work as a master and a slave.


 
Thanks Cowboy, 

I suspect speed and position matching would both work for our application. Our loads aren't very heavy and are distributed all along the sorter chain. I did a google search on Modbus last night. That does seem to be what protocol AB is using here. I'm not sure how much further along this gets me though. I'm still not sure what is exactly is achieved by "reading the drive via the network by sending Function code 3 reads to address 8452".

 I find a lot of the language used in automation is often vary cryptic and worse sometimes open to interpretation. I've studied network stuff in my PLC course but I'm still having a hard time matching all this language to real world applications. Maybe this stuff is covered in more depth in a IT course?

 I'd really like to get a more comprehensive knowledge on AB VFD's since they're so widely used, but so far it's proven to be a sluggishly slow process although my understanding has greatly improved. I purchased some software that is supposed to help you learn VFD's but all it did was transfer information directly from the manual with no further exploration of the content. I basically spent 200$ on a copy of the manual I already had. What a waste of money that was. 

All the courses AB has for learning VFD's are geared towards businesses and cost thousands of dollars. I understand this stuff is advanced but wish there were a clearer path to achieving an advanced understanding. I love learning I just don't know what approach to take on this one.


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## oliquir (Jan 13, 2011)

modbus is a pretty universal protocol, most plc have function blocks to read modbus data easily.


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## John Valdes (May 17, 2007)

JRaef said:


> You cannot do encoder feedback with a PF40, you would need a PF40P version and they are a lot more expensive. In theory you COULD put the two encoder signals into a PLC if you knew how to do that, then tell the VFDs what speed to run at, but you do not have high precision control to accomplish perfect synchronizing of two motors with that kind of VFD, you need one with a lot more brain power like a PowerFlex 7 class. Synchronizing two separate motors is a lot more difficult than you might think. You might bet within 3-4% of each other (each VFD will be +-1 to 2%) with PF40s, but I don't know what you are trying to attain here.
> 
> So lets back up and start off with a detailed description of what you are trying to accomplish.


Your post reminded me of a sales pitch regarding motors following one another precisely.
It was with some guys from ABB.
We were trying to get a big order/contract so ABB came to town with a simulator with two motors and two drives set up to follow.

We played around with it in the shop the day before and one of the ABB guys pulled out a $100 bill. Folded it and taped it firmly to each shaft. Each shaft was facing each other in perfect alignment.
Then he ran it up to full speed and then hit stop with maximum braking time enabled. We worked this thing in forward and reverse and could not tear the bill. 
The next day, while showing this to our prospective customer, I decided to be the big shot and pulled out my own $100 bill. After all they were MY customer.
When the ABB guy ran it up and hit stop, it tore my $100 dollar bill to shreds. :blink: :no: :whistling2: Oh well.

It turns out when I walked away for a few minutes, the ABB guy replaced my bill with a $1 bill and set the ratio different.
He had my $100 bill in his pocket.

Joke was on me. We did get the order and the customer rode me for years about it. I'm fairly certain that joke was what sealed the deal.


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## JRaef (Mar 23, 2009)

That "$100 bill coupling" demo unit has been floating around for years, I used to use it when I worked for ABB in the early 90s. Pretty ballsy to pocket your $100 bill though... :laughing:


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## Sparky Mcgregor (Sep 7, 2015)

I went back over the manual again. Some of this VFD stuff is starting to gel now. I was looking into the digital and analog inputs and how they can be programmed for specific functions. It occurred to me that there might be some sort of workaround. You can set one of the digital inputs to give a Auxiliary F2 fault when the input is low (which I think means it will shut down the VFD). Also if I understand correctly you can use (relay out sel) to output a high signal as long as the VFD is at the reference frequency (I think). I thought maybe you could use the these together to get the VFD to shut down when the other one goes out of frequency and vice-versa but I'm not 100% sure I'm understanding these features correctly. At any rate I told my employer the best fix would be to upgrade VFD's and use their master-slave function. Not sure if he'll want to move forward or just cross our fingers and hope the same issue doesn't reoccur.


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## John Valdes (May 17, 2007)

JRaef said:


> That "$100 bill coupling" demo unit has been floating around for years, I used to use it when I worked for ABB in the early 90s. Pretty ballsy to pocket your $100 bill though... :laughing:


He only pocketed it for the duration of the joke. Once his bill was destroyed he gave me mine back.
It was worth every minute as we/I got the order.
The plant manager once told me that was the reason they went with us. That the interaction with us was something he never forgot.
Got all their motor work too. They called us for everything. They had me sourcing stuff we did not even sell.


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## JRaef (Mar 23, 2009)

Sparky Mcgregor said:


> I went back over the manual again. Some of this VFD stuff is starting to gel now. I was looking into the digital and analog inputs and how they can be programmed for specific functions. It occurred to me that there might be some sort of workaround. You can set one of the digital inputs to give a Auxiliary F2 fault when the input is low (which I think means it will shut down the VFD). Also if I understand correctly you can use (relay out sel) to output a high signal as long as the VFD is at the reference frequency (I think). I thought maybe you could use the these together to get the VFD to shut down when the other one goes out of frequency and vice-versa but I'm not 100% sure I'm understanding these features correctly. At any rate I told my employer the best fix would be to upgrade VFD's and use their master-slave function. Not sure if he'll want to move forward or just cross our fingers and hope the same issue doesn't reoccur.


You can try it, but I'm not sure you will get the desired effect. The reference frequency issue will not change just because the chain slipped off, the drive is still GETTING the same frequency command and will respond accordingly, whether the chain is connected or not. Unfortunately that drive does not have a good way to detect an "under load" condition.

The only thing that would be close is to use the "Above PF Ang” threshold on the Output Relay (setting 18 of A055) and then set the level in A056 at a level that you determine by running the motor with the chain disconnected. Because with no or low load, the Power Factor of the motor will drop, so if you set the threshold to detect a HIGHER value than the unloaded PF, if the chain slips off, the PF drops and the output changes state. The tricky part to that is going to get the system to start up in the first place. There is a Timer function for the Opto outputs, so you could use one of them as the timer, then the Relay as the Above PF and put them in series, setting the Logic as Normally Closed for the Opto at a time value long enough to let the conveyor start moving, then use the NO contact of the relay, set to "Above PF Ang” so that it closes once the PF is higher, but before the Opto times out and opens. So put this circuit of Drive A into the Run Command circuit of Drive B and vice versa.


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## buffalonymann (Dec 25, 2016)

Sounds like there are several fail-safe mods needed to be made - I just upgraded one of our machines to AB master slave configuration. You need to step back and look at the whole picture


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## DesignerMan (Jun 13, 2008)

While I certainly am not an expert when it comes to this type of control- I have seen 2 VFDs setup for load sharing applications. I was just the electrician helping on the install- the engineer I was working with did all the design work. In any case that involved 2 drives on one chain loop and worked out quite well.
I'm sure JRaef would know how this is accomplished and if it's even applicable in this situation.


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## Sparky Mcgregor (Sep 7, 2015)

JRaef said:


> You can try it, but I'm not sure you will get the desired effect. The reference frequency issue will not change just because the chain slipped off, the drive is still GETTING the same frequency command and will respond accordingly, whether the chain is connected or not. Unfortunately that drive does not have a good way to detect an "under load" condition.
> 
> The only thing that would be close is to use the "Above PF Ang” threshold on the Output Relay (setting 18 of A055) and then set the level in A056 at a level that you determine by running the motor with the chain disconnected. Because with no or low load, the Power Factor of the motor will drop, so if you set the threshold to detect a HIGHER value than the unloaded PF, if the chain slips off, the PF drops and the output changes state. The tricky part to that is going to get the system to start up in the first place. There is a Timer function for the Opto outputs, so you could use one of them as the timer, then the Relay as the Above PF and put them in series, setting the Logic as Normally Closed for the Opto at a time value long enough to let the conveyor start moving, then use the NO contact of the relay, set to "Above PF Ang” so that it closes once the PF is higher, but before the Opto times out and opens. So put this circuit of Drive A into the Run Command circuit of Drive B and vice versa.


Thanks, I was thinking more in terms of avoiding the chain coming off in the first place. Were just concerned that the frequency not matching is what caused the chain to slip off. Technically the jam eye reading the carriers should catch it fairly quickly if the chain actually does come off. But by then were into downtime and it's too late. Your suggestion sounds doable but a bit too advanced for me still, although I think I understand the principal of what your suggesting. I've relayed all the information over to my employer. Realistically upgrading our VFD's sounds like it's really the way to go in this instance although it's been interesting trying to figure out a work around. Right now this project seems to be in limbo. I'll provide an update if this ever gets resolved.


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## Sparky Mcgregor (Sep 7, 2015)

DesignerMan said:


> While I certainly am not an expert when it comes to this type of control- I have seen 2 VFDs setup for load sharing applications. I was just the electrician helping on the install- the engineer I was working with did all the design work. In any case that involved 2 drives on one chain loop and worked out quite well.
> I'm sure JRaef would know how this is accomplished and if it's even applicable in this situation.


The truth is it sounds like there's lots of way's to achieve this through plc programming, upgrading VFD's to a master/slave function ect. Our main problem right now is getting it implemented and seeing it to fruition. There's a lot of resistance to altering the system since so far this issue was a one off event. It might have to happen again before we get some traction on this.


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