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WHAT IS A VECTOR CONTROL

Created by: Tambe at: 4/17/2008 5:35 AM (8 Replies)

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4/17/2008 5:35 AM	Rate (3)
Tambe Posts: 148 Rating: (11)	Dear All, I have a very good piece of information downloaded form NET on the VECTOR CONTROL I think it will be useful for the all the users of this forum. Your Comments and any more inputs to this Thread is highly welcomed. 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 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. Further to add this 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.
Tambe Posts: 148 Rating: (11)	Last edited by: tambe at: 4/21/2008 5:09 AM
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This contribution was helpful to 2 thankful Users yaylaci MIBASHA

4/17/2008 9:22 AM	Rate (0)
9InchNails Posts: 43 Rating: (1)	Hello, Thanks so much for this info, very clear and it helped me to understand some issues. Greetings!
9InchNails Posts: 43 Rating: (1)
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4/28/2008 11:26 AM	Rate (0)
RicoT Posts: 24 Rating: (0)	Thanks on the information, good food for the brain
RicoT Posts: 24 Rating: (0)
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4/28/2008 11:31 AM	Rate (0)
joji Posts: 1 Rating: (0)	Nice explanation!!! Got me going!!!
joji Posts: 1 Rating: (0)
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5/2/2008 12:28 PM	Rate (0)
diwa Silver Member Joined: 9/20/2006 Last visit: 12/18/2025 Posts: 307 Rating: (97)	Nice piece of information. Love reading such articles.

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5/30/2008 2:43 PM	Rate (1)
JensS Posts: 739 Rating: (55)	Dear all, pls. have also a look into this http://support.automation.siemens.com/WW/view/en/7494205FAQ
JensS Posts: 739 Rating: (55)
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6/1/2008 9:07 AM	Rate (0)
Ayman Elotaify Diamond Expert Joined: 5/28/2008 Last visit: 12/28/2025 Posts: 4522 Rating: (843)	Thank you. Very Nice Information With best regards,
	Ayman Elotaify
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6/25/2009 9:23 AM	Rate (0)
mohit Posts: 89 Rating: (1)	Thanks tambe very useful information
mohit Posts: 89 Rating: (1)
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6/26/2009 3:34 PM	Rate (0)
dzwilu Posts: 146 Rating: (4)	Short, clear, useful. As it should be. Good work !
dzwilu Posts: 146 Rating: (4)
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