Confused about PID controller for current control with PMW

[mechamania]

Thanks for all the time spend in this thread.
I learned a lot and will learn more reading it over a few times more.

Thanks all.

 

[jim hardy]

thank you indeed, miles.

The motor has inductance so this changing stator current must produce an inductive voltage drop.

i'm a picture thinker not a math thinker.

Is this similar to "Armature Reaction" i learned fifty years ago? Which relates to Synchronous impedance ?

if so, it'll help me get fluent with this new(to me) FOC technique. I see it's been around since eighties at least...

curmudgeon that i am, i must admit that controlling a motor in this fashion is one of the most interesting things for a computer to do that I've ever seen .

just think what one could do with an H-bridge and an old car alternator...

thanks again

old jim

 

[milesyoung]

I only know of 'armature reaction' in the context of synchronous generators. It had something to do with how the armature and field flux interact when you hook up a load - the armature "reacts" with its own flux distribution in the air gap due to a load current.

Field oriented control (often also called vector control) is basically just an idea of how you can convert 3 phase machine quantities to meaningful vector representations and apply control to these vectors. Let me give an example of how to do torque control of a PMS machine using vector control:

Let's say you have a 2 pole machine so you can view the rotor as a bar magnet. The stator consists of three phase windings 120 deg apart, call these phase a, b and c.

I run current through phase a only. This produces a magnetic field with its magnetic axis aligned with the winding and the rotor will then align itself to this axis (if the field is strong enough to overcome friction). I could do the same for phase b and c and in both cases the rotor will align itself to whatever winding is energized.

Now, if I run current of equal magnitude through phase a and b, the rotor will align itself in between these windings. If I increase the current through phase a while keeping the phase b current constant, the rotor will begin to turn towards phase a. I'm in effect rotating the resulting magnetic field created by the phase currents and the rotor magnetic field is aligning itself with the stator field.

I could now define some vectors that are aligned with the phase windings that have magnitudes equal to the corresponding phase currents. You can show that the vector sum of these space vectors, call it I_sum, is aligned with the resulting magnetic field produced by the phase currents.

If the magnetic axis of the rotor is aligned with the magnetic axis of the stator, no torque is produced. If, on the other hand, the magnetic axis of the stator is at a right angle to the magnetic axis of the rotor, maximum torque is produced. The objective is then to always keep I_sum at a right angle to the magnetic axis of the rotor (for a non-salient rotor).

This is exactly the purpose of using the dq-transformation. The d-axis component tells you how much of I_sum is aligned with the magnetic axis of the rotor. The q-axis component tells you how much of I_sum is at a right angle to the magnetic axis of the rotor.

Anyway, you really need some diagrams to go with all this. I can recommend:
http://www.eal.ei.tum.de/fileadmin/tueieal/www/courses/PE/tutorial/space_vector_2011-12.pdf
Field Orientated Control of 3-Phase AC-Motors

for a quick introduction.

Edit:
There are plenty of other control topologies in field oriented control. Controlling the stator current space vector for maximum torque per amp of current is just one of these. Others include controlling the stator current space vector in such a way as to always draw current at unity power factor (you'd make some friends at your local utility company), or maybe you'd like to control the stator flux vector to make sure you're not damaging your permanent magnets. There are many variants.

 

[mechamania]

thank you indeed, miles.

i'm a picture thinker not a math thinker.

Is this similar to "Armature Reaction" i learned fifty years ago? Which relates to Synchronous impedance ?

if so, it'll help me get fluent with this new(to me) FOC technique. I see it's been around since eighties at least...

curmudgeon that i am, i must admit that controlling a motor in this fashion is one of the most interesting things for a computer to do that I've ever seen .

just think what one could do with an H-bridge and an old car alternator...

thanks again

old jim

Hi jim,

The compensation with the ω.Ld.Id, ω.Lq.Iq and ω.λm terms we talked about have nothing to do with the basics of FOC. They are improvements. Miles: please correct me if I'm wrong.

This compensation may be confusing when studying FOC.

 

[jim hardy]

Thank you for that whole post.

This is the pragraph that cleared it up.

If, on the other hand, the magnetic axis of the stator is at a right angle to the magnetic axis of the rotor, maximum torque is produced. The objective is then to always keep I_sum at a right angle to the magnetic axis of the rotor (for a non-salient rotor).

Please appreciate that in my power plant world, if you let a generator get to 90deg you are on the cusp of a pole slip which is violent on a big machine,

i had wondered how close to that point you could run with this FOC

now I've accepted you actually CAN do that it opens a path of thought i'd kept shut.

Thanks !


and yes, re armature reaction: reactive component of armature current produces MMF that directly aids or opposes field MMF because it's aligned. Real component of course is perpendicular, so total MMF (and flux) is vector sum of the three.


over and out !