Vector Control of Induction Motors

[shahvir]

Hello,

In Vector control of induction motor, stator currents iqs (torque component of stator current) and ids (field component of stator current) are controlled separately just as in the case of a separately excited DC motor by keeping either of the current components constant.

My question is, how is such independent control electrically possible in induction motors when the rotor current itself is dependent on the rotating magnetic field (ids) by transformer action. How does rotor current increase or decrease independently without varying field current ids?

Iqs is just the reflected rotor current and should have no direct influence whatsoever on the rotor current iqr and hence output torque, as iqr is a result of induction due to the rotating magnetic field (ids).

Thanks.

 

[jraef]

The simplest way to look at it is that the current must FIRST establish the magnetic field strength, THEN produce torque, and this happens in EACH cycle over and over. The vector control algorithm is therefore separating out and controlling the current waveform very very quickly. That’s why this technology could not exist until the processing power and speed existed.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

The field component is always kept fully excited (except for some clever modes not germane to this discussion) so only the torque component needs to actively change. With that said whatever they both are they're both vectors. The VFD sums those vectors them and sends the resulting single value to the motor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[shahvir]

I understand the control part, it's actually the electrical part I am confused about.

From what I understand, the torque component of stator current iqs cannot have direct influence over the rotor current iqr, as iqs is just the balancing Amp Turns of rotor current reflected into the stator. So, in my opinion rotor current iqr can influence stator current iqs but not vice-versa.

 

[Gr8blu]

shavir: Remember that an induction motor has no separate power source for the rotor - ALL the output work comes from the energy input to the stator. (For a generator, the input is mechanical work at the shaft and the output is electrical energy at the stator connection.) Also understand what the "d" and "q" represent: "d" refers to the DIRECT AXIS components - the ones that create the magnetic "poles" in both stator and rotor. The "q" refers to the QUADRATURE AXIS components - which translate into the vectorial offset required to change the angle between rotor and stator "poles". The control only adjusts the stator current (direct and quadrature components), to provide a result in the rotor and (ideally) maximize power transfer across the air gap.

Converting energy to motion for more than half a century

 

[shahvir]

I understand the control logic, but I have difficulty in understanding how changing the q axis current iqs (keeping d axis current ids constant) changes rotor current values.

The current in rotor is due to induction (transformer action), so only changes in ids will electrically influence changes in rotor current iqr. How does changes in q axis stator current iqs able to influence changes in the rotor current? Iqs is just the reflected rotor current for balancing the amp turns in the stator. This is my query.