# Vector control verse encoder feedback



## just the cowboy (Sep 4, 2013)

What are the results of using a drive with an encoder on the motor shaft or a drive with vector control? I know what the differences are but I would like to know the end results. We got a replacement drive in that doesn’t have the encoder option that the original did. It is being used in a speed follower application on a paper converting machine.


----------



## JRaef (Mar 23, 2009)

.1% speed regulation in Sensorless Vector Control (SVC) or Direct Torque Control (DTC), vs .001% speed regulation in Field Oriented Control (FOC) or Flux Vector Control (FVC) using an encoder feedback. That can make a big difference in something like a paper machine where drives are coordinated. But there are a few advanced drives now that can do FVC or FOC without an encoder feedback. The A-B PowerFlex 755 is one of them. You can't do that with a PF700 or even a 753, only the 755. So if you had a PF700 with an encoder feedback in there before, and someone sent you a 755 as the replacement and said you no longer need the encoder, that might be valid.

In addition using an encoder feedback, you can achieve the holy grail of motor performance; 100% torque at zero speed with approved safety for hoisting applications. That wouldn't be applicable to a paper machine application however, I'm only bringing it up as another differentiator.


----------



## just the cowboy (Sep 4, 2013)

*Perfect*



JRaef said:


> .1% speed regulation in Sensorless Vector Control (SVC) or Direct Torque Control (DTC), vs .001% speed regulation in Field Oriented Control (FOC) or Flux Vector Control (FVC) using an encoder feedback. quote]
> 
> Thanks they are the numbers I was looking for. It will work 99% of the time since we run this station with an open nip. The paper is slipping over a roller, if we close the top roller .1% may not work.
> 
> ...


----------



## KennyW (Aug 31, 2013)

JRaef said:


> .1% speed regulation in Sensorless Vector Control (SVC) or Direct Torque Control (DTC), vs .001% speed regulation in Field Oriented Control (FOC) or Flux Vector Control (FVC) using an encoder feedback. That can make a big difference in something like a paper machine where drives are coordinated. But there are a few advanced drives now that can do FVC or FOC without an encoder feedback. The A-B PowerFlex 755 is one of them. You can't do that with a PF700 or even a 753, only the 755. So if you had a PF700 with an encoder feedback in there before, and someone sent you a 755 as the replacement and said you no longer need the encoder, that might be valid.
> 
> In addition using an encoder feedback, you can achieve the holy grail of motor performance; 100% torque at zero speed with approved safety for hoisting applications. That wouldn't be applicable to a paper machine application however, I'm only bringing it up as another differentiator.


Keeping track of all of rockwell's acronyms is exhausting. Another thing I came across recently was a set of drives that someone had meant to set up to load share via droop control, but they were set to FVC, but had no encoders (there were 700VC's). As far as I know droop is not applied when the drive is set to FVC, which makes sense in a way since having droop+encoder feedback is a little odd. But what I don't know is how does a 700 act when set to FVC and open loop? Seems this is kindof an undefined combination of parameters. To me, the drive should almost fault and make you set it to sensorless vector in this scenario. 

Anyways my experience that that most of the time adding and encoder is not a terribly expensive ordeal, and it is piece of mind. Sensorless vector performance seems to depend on things like the motor tune - many cases I deal with, uncoupling the motor from the load is impractical, and in those cases when only a static tune can be performed, I find SVC to be mediocre. They should really add some fine print to the marketing literature, imo.


----------



## JRaef (Mar 23, 2009)

No argument from me on that. The problem is, the original SVC was developed for the locomotive industry to allow them (mainly GE) to replace big DC locomotive motors that needed constant maintenance on the brushes and commutators, with induction motors. The performance they needed was not as critical as what FVC was providing at the time, because they were moving freight trains, not paper machines. All they wanted was to not need the encoder, because that would never survive on that application. But once Toshiba (who paid for it in a JV with GE) got hold of the SVC algorithm, they immediately pushed it into their LV industrial drives and marketed it as being almost as good as FVC. It never really was, plus there was no "official" definition of what SVC was anyway. It's still like that to this day. Some are "velocity" vector control, meaning there is no torque regulation loop, some offer some sort of rudimentary torque control, I even see one company pawning off a V/Hz control drive as being SVC by adding Torque Boost at low speeds. It's cheap, but it's not really SVC, yet since there is no real definition enforced by anyone, they can get away with that. That's why when Rockwell developed a very high performance open loop control method, they decided to NOT call it SVC, because that term was already compromised by the bottom feeders.


----------



## KennyW (Aug 31, 2013)

Thanks J, makes sense, well, kinda.  So SVC and Sensorless FVC ("S-FVC"? lol) are both valid control modes. Seems a little redundant but the help mentions that FVC requires an autotune and seems to imply SVC does not... I guess the idea is if you cannot do a rotate/dynamic motor tune you are best to use SVC... 

Then again I have never found Rockwell's descriptions for when an autotune should or should not be performed to be terribly consistent either...


----------



## JRaef (Mar 23, 2009)

Both require an autotune procedure. 

One main (but over simplified) difference is that in FVC, you can control both the speed and torque independently of each other. SVC is usually just speed OR torque, but not both at the same time. In other words if you want torque control, SVC will do it by adjusting speed, overriding what your commanded speed is set to, which you may not want. In FVC you can limit torque without changing speeds from what you command it to run at. So a big example of that is in winding operations. If you are running the take-up roller on a VFD, the torque might be critical because you don't want to break or stretch the material. But as the material gets added to the roller, the diameter is increasing, so you have to be changing the rotor speed to keep the line speed the same. Simple SVC can do it, but not as perfectly as FVC. So if the material is something like plastic wrap or craft paper, SVC might be fine. But if it's steel or wire where the thickness might change if pulled too much, you need FVC. 

If you are going to lift a 747 with overhead cranes, the hoist motors must have a positive position indication, so that the FVC can know EXACTLY where the rotor is positioned when you want to release the mechanicsl brake, otherwise the load may start dropping before the motor can develop enough torque, and dropping a 747 is bad news. So that's one application when you need the original FVC using an absolute encoder, I wouldn't trust the open loop FVC for that.


----------

