Motion control of AC Induction Motors

Hi Cabe,

You are a busy fellow here on the element-14 forums. I once had a customer who wanted to design a controller for a permanent magnet synchrounous motor, the project never came to fruition however I did a lot of research on the subject.

For good applications info, look through some of the microcontroller manufacturer's app notes.

I found a lot of good info at microchip.com, ti.com, and freescale's site. They have dedicated motor control sections on their sites so that will get you going in the right direction.

Microchip

AN1162     Sensorless Control of AC induction Motors

AN887       AC Induction Motor fundamentals

Ti.com

http://focus.ti.com/docs/solution/folders/print/745.html

Freescale

AN1931      3-Phase synchrounous vector control...

I hope your sharp on your linear algebra, you'll be needing it to implement the Clark and Park transformations used for Vector motor control.

I'm sure that if you search through the above websites, you'll find all the info you'll need to do this project. If your handy with an MCU (doesn't matter the manufacturer, I'm a PIC kind of guy, however there are plenty of powerful MCUs that can handle these types of applications) and take a little time to review your Matrix Algebra you shouldn't have a problem.

Hope this helps,

Jorge Garcia

Cadsoft Computer

Hi Cabe,

You are a busy fellow here on the element-14 forums. I once had a customer who wanted to design a controller for a permanent magnet synchrounous motor, the project never came to fruition however I did a lot of research on the subject.

For good applications info, look through some of the microcontroller manufacturer's app notes.

I found a lot of good info at microchip.com, ti.com, and freescale's site. They have dedicated motor control sections on their sites so that will get you going in the right direction.

Microchip

AN1162     Sensorless Control of AC induction Motors

AN887       AC Induction Motor fundamentals

Ti.com

http://focus.ti.com/docs/solution/folders/print/745.html

Freescale

AN1931      3-Phase synchrounous vector control...

I hope your sharp on your linear algebra, you'll be needing it to implement the Clark and Park transformations used for Vector motor control.

I'm sure that if you search through the above websites, you'll find all the info you'll need to do this project. If your handy with an MCU (doesn't matter the manufacturer, I'm a PIC kind of guy, however there are plenty of powerful MCUs that can handle these types of applications) and take a little time to review your Matrix Algebra you shouldn't have a problem.

Hope this helps,

Jorge Garcia

Cadsoft Computer

Hi Cabe,

You might consider dsPIC. Microchip has application notes on AC induction motors and Vector control on their website.

Best regards,

Enrico Migchels

Jorge,

Great answer. I will take a look at these options for sure. And I feel my algebra is pretty strong too.

Cabe

A crash coarse in vector control (via research):

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).

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.