Field oriented control

[mjsas]

I have read a few application notes about FOC but it is not clear what is really going on. FOC can be used on either 3 phase induction motors or 3 phase synchronous motors and gives better steady state and transient control. Also better efficiency. What I get out of it is that FOC lines up the voltage and current vectors.

The note also discussed SVPWM (space vector pulses width modulation) which makes more efficient use of battery voltage. The maximum voltage is 4/3 times the maximum given by voltage sinusoidal drive from the same battery.

Does anyone have a clear explanation of FOC and SVPWM?

[AndyKunz]

I didn't look to check it, but historically the best places for motor control theory and app notes (ie, explanations mere mortals can understand) is the TI site ti.com. If I were interested, they'd be my first stop.

Andy

[mjsas]

That's where I have looking.

Application notes tend to be for people who are developing products and tend to theory instead of a simple explanation.

Here is TI's explanation:
The Field Orientated Control (FOC) [1][3] consists of controlling the stator currents
represented by a vector. This control is based on projections which transform a threephase
time and speed dependent system into a two co-ordinate (d and q co-ordinates)
time invariant system. These projections lead to a structure similar to that of a DC
machine control. Field orientated controlled machines need two constants as input
references: the torque component (aligned with the q co-ordinate) and the flux
component (aligned with d co-ordinate). As Field Orientated Control is simply based on
projections the control structure handles instantaneous electrical quantities. This makes
the control accurate in every working operation (steady state and transient) and
independent of the limited bandwidth mathematical model

[Ron van Sommeren]

http://www.innovatia.com/Design_Center/Power_Electronics.htm

[AndyKunz]

That's where I have looking.

Application notes tend to be for people who are developing products and tend to theory instead of a simple explanation.

Well yes, they typically are for people developing projects, but they aren't especially theoretical (TI's are better places to find theory than other companies, true). What was wrong with what you quoted from the app note?

Andy

[mjsas]

This is what I understand FOC to be: The rotor has to lag the rotating magnetic field to produce torque. This lag is called the torque angle and maximum torque is produced when the torque angle is equal to arctan (xp/rp) where xp is the reactance and rp is the resistance. This maximum torque angle is close to 90 degrees (electrical).
FOC senses the rotor angle and keeps the current 90 degrees ahead of it. Also the effective voltage that causes the current to flow is the sum of the applied voltage and BEMF.
When the motor is run at maximum torque, does that result is maximum efficiency?

[Markz]

Om from what I understand FOC is more geared toward precise angular position, and can better compared to microstepping than to our typical BLDC controllers.
With microstepping, phase currents are modulated to drive to rotor to a certain position, but there's no feedback to control current (open loop). Efficiency is good only around a certain torque value
With FOC the rotor position is used to modulate phase current in order to keep efficiency high.
Our BLDC controllers seems to be doing a good job at managing the optimal phase angle within a certain range, and since nothing but the ESC requires to know exactly what the rotor position is, I don't see how this theory can be used to improve anything with our typical power systems

Marc

[clicky]

Is microstep similar/same to half step? If so - explanation is simple - only have of the windings are used for it!

[mjsas]

I get the impression FOC works best with sinusoidal drive and sinusoidal BEMF. Aircraft ESC's have square wave drive and weird BEMF. Not even sure that torque angle can be defined for these systems.

[AndyKunz]

Actually, it's called trapezoidal drive and it's done that way because it's cheap and easy, not because of anything special about the motors. Eventually it will end up being sinusoidal drive - in fact there are already a few (expensive) sinusoidal controllers out there.

Andy

[Markz]

Is microstep similar/same to half step? If so - explanation is simple - only have of the windings are used for it!

Have a look at the LMD18245 Datasheet (http://cache.national.com/ds/LM/LMD18245.pdf) for a nice explanation

Marc