What is the difference between Open or Closed Loop Flux Vector Control 3

[pirates]

I have a 2 VSD’s which are Leroy Somer UMV3301’s They are described in the literature as being ‘Open or Closed Loop Flux Vector Controller’ Simply what is the difference between Open and Closed? What are the implications of running the drives in V/f ratio mode? The drives have been set up by the factory engineer in V/f Ratio Mode.

I have a book, which says that flux vector controllers operate by monitoring the position of the rotor and adjusting this position in comparison to its orientation to the stator field. For this it will obviously need an encoder. In V/f Ratio mode is this still necessary?

Surely VSD s cannot induce their own amperage!! Why do we get a difference between AC amps going in to the VSD and the amps going into the motor? Amps into motor higher than amps into the VSD.

Both of my VSD s are fitted to two refrigeration compressors which are common in all respects (load conditions). Why does one of them draw between 40-90 amps more than the other depending on loading? Where should I start looking.

Although the VSD s are badged Leroy Somer I do not think they are made by them. Who makes them? Where in the UK can I find a serious company to come and check them out and assist us with the proper commissioning of the drives. Most of the drive people want me to send them to them. Considering that they are 250 kW drives this is very difficult and costly.

 

[Marke]

Hello pirates

For some more information on vector control, see faq237-1062
There are two main forms of vector control, open loop and closed loop.
The closed loop format requires the use of an encoder on the motor shaft and the open loop format does not. The open loop format relies on a mathematical model of the motor to provide the correct information and performance. The major difference in operation between the two is that closed loop will give good torque performance down to zero speed and open loop will give good torque performance down to about 3 - 5 Hz.
Vector control is mainly of benefit where you are looking for high torque performance at low speeds. For most applications where you are looking for speed regulation such as pumps and fans, V/Hz is all you need.

The Leroy Somer drives will almost definitley be made by Control Techniques. They are both part of the Emerson group.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[pirates]

Thanks Mark that is the feedback that I need. My application is a refrigeration-reciprocating compressor, which by it’s nature does not have a perfectly constant torque characteristic, on top of this we have a habit of unloading cylinders as need be. This leads to sudden increases/decreases in torque. The drive battles to maintain and the speed and is always rising and falling. I think an encoder is in order. I am not that convinced about the V/f ratio. Knowing that an encoder would have cost implications Maybe we should go for open loop! How do I check what the V/f ratio is actually set two and how do I change it?

 

[jraef]

pirates,
I doubt that adding an encoder will do anything more for you in this application, you just need tighter control on your loop tuning characteristics. No way to know how to do that in a generic sense, every VFD is different, so if you can, find someone with experience in that brand. In general you want to look for parameters that control the rotor speed PID loop. Play with turning up the Porportional gain and turning down the Derivative gain. Don't do it by much though, just tweak it and test it, tweak and test. Good luck.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[Marke]

Hello pirates

For a recip compressor, I do not believe that you will need an encoder.
I suspect that there are two things possibly going on.

It is possible that the drive has been sized for 110% torque and on torque peaks, it is curent limiting. This causes the frequency to drop. For a recip compressor, I would recommend a drive with 150% torque to cope with the torque peaks. The other thing that can happen is that on the compression stroke after tdc, the compressor can be overdriving the motor. This could intern cause the voltage to rise and the drive to go into voltage limit mode. (this will cause the frequency to rise)

My recommendation is to use either open loop vector or V/Hz mode and ensure that the drive is rated and set up for 150% torque limit. If the drive increases speed at times, you may need to look at some form of brakeig resistor.

As jeff sid, you may need to look at your control loops and possibly play with some settings their also.

Have a good day
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[torslum]

Pirates wrote: Surely VSD s cannot induce their own amperage!! Why do we get a difference between AC amps going in to the VSD and the amps going into the motor? Amps into motor higher than amps into the VSD.

-

Well, it can Just as you describied yourself, the V/f-ratio makes the voltage quite low at low frequencies. Meaning that full voltage is only put out at nominal speed (or at least depending on a parameter setting). The only thing in this world that cannot suddenly appear or disappear is energy, i.e. also power. try to calculate the power and I guess it will equal on the output and input. You will find a rather low current combined with the grid-voltage on the input, and you will have a rather high current and very low voltage on the ouput when running at low speeds. But the power should be approximately the same - don't forget that the powerfactor is different as well.

Why is this, you could ask. Check some other threads in this forum, discussing how to run a 60Hz motor on a 50Hz grid for example. Then one should reduce the voltage accordingly, i.e. keeping the V/Hz constant because this will keep the flux in the motor constant.

 

[Marke]

Hello pirates and torslum

Pirates wrote: "Surely VSD s cannot induce their own amperage!! Why do we get a difference between AC amps going in to the VSD and the amps going into the motor? Amps into motor higher than amps into the VSD".

It is really quite simple, KWin almost equals KWout. There is a small loss in the inverter. The output of the inverter has both resistive current and reactive current. The reactive current is supplied by the inverter. The input has resistive current only (no VARs)

At half speed, the output voltage is halved. Therefore the current on the output will be about twice the current on the input provided that the reactive current is small relative to the total motor current.

Best regards,

Mark Empson

http://www.lmphotonics.com