What is Vector Control in a VFD

A standard VFD (let’s 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).

These problems associated with the scalar VFDs inability to alter it’s output with changes in the load gets worse as the speed reference goes down, so the “rule of thumb” in determining the need for which technology to use is that scalar drives work OK at speed ranges between 5:1 (50Hz applications) or 6:1 (60Hz applications). So if your application will need accurate control below 10Hz, scalar may not work for you.

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. (Note: See Addendum below for additional information)

Open Loop is actually a misnomer because 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.

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 added (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.

It might be worth noting that microprocessor technology is rapidly making scalar drives obsolete except in the smallest of sizes. The low cost for processing power has made the issue of having and maintaining separate designs untenable for many manufacturers.

An additional important issue on the importance of encoder feedback with Closed loop Vector drives:

When talking about speed control and error in a drive/motor system, it is important to understand that there are several different aspects to the issue.

First, speed error is generally due to changes in torque demand. In an induction motor, this error is mostly slip. So the question becomes, how well does the drive compensate for torque induced slip speed changes. With a good vector drive, this can get down in the range of one-tenth of motor slip without an encoder. If you need better than that, an encoder is required. Note here that the error is a result of torque changes. If your torque doesn’t change, you won’t have much speed error to start with.

Second, in some applications, especially those involving web products and tension control, cumulative error is just as important as actual error. For example, even if you are very accurate with actual error, if it is all negative or all positive, eventually you are going to have too much or too little tension. No encoderless system will assure non-cumulative error. For that you need an encoder.

Third, speed reference error is often overlooked. That is error either in the speed signal going into the drive or error in the drive translating the input command into an actual output speed. Usually, the majority of this error is due to the analog input terminal analog-to-digital conversion. A 10 bit resolution A/D input will not be nearly as accurate as a 14 bit resolution input. This is a matter of purchasing a drive with the input resolution adequate for the intended purpose.

Thank you to jraef and @Skogsgurra for the above tutorial.