What is a Vector Drive? 18

[kingjoey]

I am very much confused on how a vector drive works. I cannot get a clear explaination from anyone on how a vector drive controls an AC induction motor. Can anyone help?
Thanks,
joey

 

[jbartos]

Comment: It is not clear on which level the explanation is sought. In the meantime, visit

http://www.findarticles.com/cf_dls/m0BFD/3_77/114856022/p1/article.jhtml

http://www.ecmweb.com/mag/electric_understanding_variable_speed_4/

etc. for more info

 

[kingjoey]

Thank you jbartos. The links help. I am curious as to what a flux vector drive does to the motor stator field, current and voltage in order to control torque and speed. I am familiar with the volts/hertz technique but I do not understand vectoring.
Thanks again,
joey

 

[jraef]

Here is how I teach it (condensed since you already understand PWM theory)

A standard VFD (lets call it a Scalar Drive) puts out a PWM pattern designed to maintain a constant V/Hz pattern to the motor under ideal conditions. How the motor reacts to that PWM pattern is very dependent upon the load conditions. The Scalar drive knows nothing about that, it only tells the motor what to do. If for example it provides 43Hz to the motor, and the motor spins at a speed equivalent to 40Hz, the Scalar Drive doesn't know. You can't do true torque control with a scalar drive because it has no way of knowing what the motor output torque is (beyond an educated guess).

A Vector Drive uses feedback of various real world information (more on that later) to further modify the PWM pattern to maintain more precise control of the desired operating parameter, be it speed or torque. Using a more powerful and faster microprocessor, it uses the feedback information to calculate the exact vector sum of voltage and frequency to attain the goal. In a true closed loop fashion, it goes on to constantly update that vector to maintain it. It tells the motor what to do, then checks to see if it did it, then changes its command to correct for any error. Vector drives come in 2 types, Open Loop and Closed Loop, based upon the way they get their feedback information.

A true Closed Loop Vector Drive uses a shaft encoder on the motor to give positive shaft position indication back to the microprocessor (mP). So when the mP says move x radians, the encoder says "it only moved x-2 radians". The mP then alters the PWM signature on the fly to make up for the error. For torque control, the feedback allows the mP to adjust the pattern so that a constant level of torque can be maintained regardless of speed, i.e. a winder application where diameters are constantly changing. If the shaft moves one way or the other too much, the torque requirement is wrong and the error is corrected. A true closed Loop Vector Drive can also make an AC motor develop continuous full torque at zero speed, something that previously only DC drives were capable of. That makes them suitable for crane and hoist applications where the motor must produce full torque before the brake is released or else the load begins dropping and it can't be stopped. Closed Loop is also so close to being a servo drive that some people use them as such. The shaft encoder can be used to provide precise travel feedback by counting pulses.

Open Loop is actually a misnomer becuase it is actually a closed loop system, but the feedback loop comes from within the VFD itself instead of an external encoder. For this reason there is a trend to refer to them as "Sensorless Vector" drives. The mP creates a mathematical "model" of the motor operating parameters and keeps it in memory. As the motor operates, the mP monitors the output current (mainly), compares it to the model and determines from experience what the different current effects mean in terms of the motor performance. Then the mP executes the necessary error corrections just as the closed Loop Vector Drive does. The only drawback is that as the motor gets slower, the ability of the mP to detect the subtle changes in magnetics becomes more difficult. At zero speed it is generally accepted that an Open loop Vector Drive is not reliable enough to use on cranes and hoists. For most other applications though it is just fine.

This is all done at very high speeds, that is why you did not see Vector Drives as available earlier on. The cost of the high speed mP technology has now come down to every day availability.





"Venditori de oleum-vipera non vigere excordis populi"

 

[Skogsgurra]

Hi,

jraef is absolutely correct in his "crash course" on VFD. One might add that the "vector" that pops up in the description and the name of this drive technology is the rotating space vector that describes the flux in the motor. Since flux and current are in phase, it also describes the current in the stator.

An induction motor is very similar to a DC motor. It needs a magnetizing current and a torque producing current. In a DC motor, these two currents are fed to two different windings; the field winding and the armature winding. In an induction motor, there is only one set of windings: the stator winding. So the vector drive has to separate the two components some other way.

It does this by keeping in mind that magnetizing current always lags (inductive) the voltage by 90 degrees and that the torque producing current is always in phase with the voltage. It controls the magnetizing current (usually named Id) in one control loop and the torque producing current (Iq) in another control loop. The two vectors Id and Iq, which are always 90 degrees apart, are then aded (vector sum) and sent to the modulator, which turns the vector information into a rotating PWM modulated three-phase system with the correct frequency and voltage.

As soon as a deviation from correct speed or torque or magnetizing current is detected by the control loops the corresponding variable will be changed by the controller to correct the variable.

If - for example - the speed is wrong, the output frequency will be corrected and also the voltage so that the correct magnetizing current is maintained. And correspondingly, if the stator winding heats up the magnetizing current would go down if the decrease wasn't detected and corrected by the controller. The action in this latter case is that the voltage goes up (PWM adjusted), but not the frequency (the speed was already correct.

Vector drives are among the most complex standard equipments that exist. But keeping in mind that there are always two control loops, one for magnetizing current and one for speed/torque will help thinking about them.

 

[jbartos]

Suggestion: Reference:
A.E. Fitzgerald, Charles Kingsley, Jr., Stephen D. Umans "Electric Machinery," 6th Edition, McGraw-Hill, 2003,
Section 11.3.2 Torque Control

 

[jraef]

Thanks skogsgurra, I may plagiarize your addition into my training if you don't mind. That is a good easy to understand synopsis of what is going on behind the scene in the vector part of the vector drive. I especially like the analogy to a DC motor.

"Venditori de oleum-vipera non vigere excordis populi"

 

[sreid]

The previous explanations are very good but slightly miss the fundamental problem that vector drives solve. As stated in previous posts, to have a high quality servo drive one needs to know the magnitude and position of the flux vector on the rotor. In, say, permanent magnet AC servos, the rotor flux magnitude is fixed and the flux vector is known. In an induction motor, the rotor currents (and the rotor flux vector due to the rotor currents) are created by the stator currents and the rotor slip.

A vector drive uses a mathematical model of the motor to calculate and control the rotor flux position and magnitude based on stator currents and (often) motor shaft position.

 

[Skogsgurra]

OK, jraef. I don't mind at all. And if sreid and still some add their views this could turn into a good basic lecture on vector and scalar drives.

 

[Danfossegypt]

Dear Kingjoey

please try this link

http://drives.danfoss.com/SW/DDdriv...lick=c_{6FAE27E6-D1FE-11D4-94FC-00508B8B37A1}

and then press on (Facts worth Knowing about frequency converters)

this pdf file will give u a complete survey and detailed defferences between technologies of generating variable frequency sine wave.

 

[jraef]

sreid,
So if I read you correctly, Vector drives actually out perform servos? Or are you just clarifying how they are capable of matching them? I am contemplating writing an FAQ for this forum on this (since I see it come up quite often) and I would like to include your comment as part of it. Maybe it deserves it's own FAQ though since servo applications are a horse of a different color compared to the majority of industrial Vector Drive applications.

"Venditori de oleum-vipera non vigere excordis populi"

 

[sreid]

I think at the practical limit, a vector drive would not have the performance of a permanent magnet AC sero because the flux vector for the induction motor is only an estimate. One of the biggest problems is that the rotor resistance increases with temperature which changes the L/R time constant for the rotor. This, of course, changes the motor model with temperature.

AC brushless servo motors become too expensive at higher horsepowers due to magnet costs; Rare Earth magnets cost more than iron and copper per pound and the weight goes up by the cube of the dimensions. The rotors of induction motors are, of course, iron and copper(or perhaps aluminum). Induction motors are also rugged, brushless and mass produced.

There has always been a desire to get better, say, speed control of induction motors beyond what VF control can provide; one way was slip compensation. Also, it's handy to be able to control down to zero speed with torque available. Vector Control allows these things to be done and be done now at affordable prices.

The cost today to build a vector drive is no more than any other drive. The primary additive cost is the Engineering software development cost. Vector Drives allow really good drive performance if you need it but things can be detuned in less demanding situations. Most drives will allow one to run VF open loop if that is all that is required.

 

[aolalde]

Thanks to jraef (Electrical), skogsgurra (Electrical) and sreid (Electrical). To me your explanation is excellent. I have voted each of you a star.

 

[Skogsgurra]

Re: DC servo AC servo PM servo

I have tested an ordinary 0,75 kW AC induction motor together with an 1,5 kW NFO Sinus inverter and got a quite good speed loop performance. A 20 % step in speed reference gave me 20 % speed increase in 6 milliseconds. That is about as good as a DC servo used to be back in 1990. Siemens Masterdrive and an AC induction motor is also pretty good as a low end servo. I think that most vector drives perform well in this respect. But they cannot beat a PM servo motor. If you need sub-ms torque and ms speed response then you need PM servo (where the drive system is very similar to an ordinary PWM vector inverter).

 

[jbartos]

Comment: It is actually specification on the motor output side, e.g. motion, dynamics, oscillations, etc. what determines the application of a servo or VFD with inverter duty motor (up to certain HP). There appears to be no record of 20000HP or so servo, although, some of the servo feature might be found very useful in that range, e.g. for precise navigation of battle ships.
Generally speaking, there is an enormous volume of literature covering servo applications, theory, etc.; especially, many books dealing with controls, e.g. Reference: Yasundo Takahashi, Michael J. Rabbins, David M. Auslander "Control and Dynamic Systems," Addison-Wesley Publishing Company, 1972.
Also, the theory and applications of servos and VFDs are very appealing to students and professors.

 

[sreid]

The machine tool industry uses Vector Drives for the spindle in Milling Machines because you can do "Ridgid Tapping." This is where the spindle is servoed in coordination with vertical motion of the part to be tapped. Before vector drives, tapping was done as a secondary operation or special tap holders were use that had vertical compliance.

 

[jbartos]

Comment on skogsgurra (Electrical) May 15, 2004 marked ///\\I have tested an ordinary 0,75 kW AC induction motor together with an 1,5 kW NFO Sinus inverter and got a quite good speed loop performance. A 20 % step in speed reference gave me 20 % speed increase in 6 milliseconds. That is about as good as a DC servo used to be back in 1990.
///Impressive. How many changes per time unit have been tested?\\ Siemens Masterdrive and an AC induction motor is also pretty good as a low end servo.
///For what kind of duty in time?\\ I think that most vector drives perform well in this respect. But they cannot beat a PM servo motor.
///Very true.\\ If you need sub-ms torque and ms speed response then you need PM servo (where the drive system is very similar to an ordinary PWM vector inverter).
///Not only for this but also for more demanding duty in terms of changes in time.\\\

 

[Skogsgurra]

jb,

I used a function generator set to 1 Hz square when I did the test. The motor ran without added inertia (load) and the speeds involved were 1000 and 1200 RPM. Very little heat in the motor.

I also ran 1000 RPM CW and 1000 RPM CCW with the same setup and 1 Hz. Motor slewed on torque limit and used 27 ms to go from minus 1000 to plus 1000 RPM. It got a lot hotter, but still tolerable after more than 15 minutes.

The 200 RPM test never got the drive into torque limit but the 2000 RPM test did. Both results include time needed to settle to within 5 percent of final speed. That's why there is no strict proportionality between the two results.

 

[jbartos]

Suggestion to the previous posting: It is good to know. However, my previous posting referred to still unaddressed aspects of a number of motor changes, e.g. in speed, direction of rotation, load, etc. per specific time unit, e.g. per hour. A process may require changing the direction of motor rotation 1 times per minute or 60 times per minute, e.g. fly cutter.

 

[Farkel]

Look at the Thor drive made by custom Power Technologies, which can be used as a servo, and can be positioned to .005 degrees of one rotation, plus memorize eight different positions, or other information such as speeds, etc. Since there is no legal definition of a vector drive.

http://www.thortechnology.com.