# Allen Bradley VFD Motor Control modes question.



## JRaef (Mar 23, 2009)

V/Hz is good for a 4:1turn down ratio, maybe 6:1 at best. So below 15Hz, accuracy drifts. Response to a step change in load can be measured in seconds. 

SVC in velocity vector mode is good for 100:1, so speed accuracy is maintained down to less than 1Hz, but the motor must move. Response to a step change in load can Be measured in fractions of a rotation, called a radian. SVC response in the PowerFlex drives is roughly 128 radians for a 100% step change in load. That equates to about 20 revolutions of a motor. Most SVC drives do not have a torque regulation loop, but the fast response improves it so much that most people don't realize that. 

FVC or FOC ( Field Oriented Control) is good for 1000:1 and can do full torque at zero speed. The step change response is now roughly 10 radians, a little over 2 revolutions. This is also where you get true torque control with high accuracy.


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## KennyW (Aug 31, 2013)

One thing I would add is that there's a little bit of an "arms race" between manufacturers with respect to the performance of their respective flavors of sensorless modes and I feel like they almost put pressure on you to not use an encoder. If you have an application that needs accurate speed or torque control at speeds lower than 2x the slip speed of the motor, my advice is this: unless you are an OEM where the savings associated with eliminating an encoder is multiplied over a whole bunch of machines, and you are able to spend time getting the tune and everything just right, err on the side of caution and just install an encoder. Let the "experts" who aren't actually responsible for making sure it works right sit at there desks all proud of themselves telling you the encoder wasn't needed. Drives these days can do amazing things with or without encoders but man with an encoder things just get a lot easier.


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## Hybwolf (Jul 31, 2011)

Thank you for the replies, this is helpful information.


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## JRaef (Mar 23, 2009)

KennyW said:


> One thing I would add is that there's a little bit of an "arms race" between manufacturers with respect to the performance of their respective flavors of sensorless modes and I feel like they almost put pressure on you to not use an encoder. If you have an application that needs accurate speed or torque control at speeds lower than 2x the slip speed of the motor, my advice is this: unless you are an OEM where the savings associated with eliminating an encoder is multiplied over a whole bunch of machines, and you are able to spend time getting the tune and everything just right, err on the side of caution and just install an encoder. Let the "experts" who aren't actually responsible for making sure it works right sit at there desks all proud of themselves telling you the encoder wasn't needed. Drives these days can do amazing things with or without encoders but man with an encoder things just get a lot easier.


I agree. There are however some applications where an encoder is just impractical. Anything involving high vibration or dust for example. I had tried to do load sharing on rock crusher motors with sensorless vector drives in the past and it didn't work, yet there was no way an encoder would have survived. But the new Encoderless FVC in the PowerFlex 755s is now as good as the encoder version for torque control and it works flawlessly. AB claims it can do Torque Proving for hoists without encoders too, but I used to do hoists for Boeing, no way would I trust that when lifting a 747... I want that encoder.


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## KennyW (Aug 31, 2013)

Ironically enough I have worked on crushers with encoders, most of our work materials handling and mining. Lots of coal shiploaders and the like. Lots of dust, lots of vibration. 

The encoders we use typically look something like this and are magnetic, not optical, more like a resolver. You will not kill such a beast. 










They are not cheap but of course as you know not much is in the mining and oil & gas world.


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## JRaef (Mar 23, 2009)

KennyW said:


> Ironically enough I have worked on crushers with encoders, most of our work materials handling and mining. Lots of coal shiploaders and the like. Lots of dust, lots of vibration.
> 
> The encoders we use typically look something like this and are magnetic, not optical, more like a resolver. You will not kill such a beast.
> 
> ...


Who's product is that? I don't recognize it, it would be good to know.


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## John Valdes (May 17, 2007)

JRaef said:


> Who's product is that? I don't recognize it, it would be good to know.


The color says Weg? I know they use that color on most all their stuff, or at least they used to.
And its IEC as you can see the connector.


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## KennyW (Aug 31, 2013)

They are made by Avtron. 

http://www.nidec-avtron.com/encoders/family/heavy-mill-duty/xr45


The are cULus listed for Class 1 Division 1, so not just IEC (ignore the cable gland that's installed, it is a 1/2NPT conduit entry into that connector, so the installed gland is basically irrelevant). We use them in unclassified areas as well. We tend to find that, generally speaking, if something is C1D1, it'll be good in unclassified but nonetheless harsh environments as well.


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## JRaef (Mar 23, 2009)

Ahhh... Avtron. I forget about them. Must be a metal block, because they took a huge project away from me years ago for engine test stands at Boeing that had I won, would have let me retire at age 40...


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