The inspiration to write about life of an electrical engineer in dq axis came to me while trying to consolidate my understanding of the AC.


Have you seen the dq axis ?

To be honest, I never saw the dq axis. But I searched for it. The very first time I was introduced to this topic was when I was going through Synchronous Machines; to be specific; Synchronous Generators.

For a student from science background, maybe most of us were fascinated by the fact that magnestism and electricity are entangled. I always collected magnets of varying shapes when I was kid; maybe I do it now too, haha! But first time magnet experience was amazing. Current, did you experience it ? Not so amazing, right ? But, knowing these electrical and magnetic properties were intertwined from a book's prespective with all those formulae was not worth it for me. So had to make a motor, dc one simple (not so simple, did not work on several attempts).

So a motor rotates, actually the rotor inside the motor rotates. Till now they were just entangled but now they produce a rotating effect. Ok, even dc current carrying conductor in a magnetic field did the rotating dance. Yeah, we tilted the conductor and all those fleming's left hand rule. But where is this dq axis ? DC quantity, a simple tilt and a magnet produced the rotating effect; but all our works are mostly on AC domain. So, for AC quantity, a simple tilt and a magnet, will it work ? No that easy, AC is pulsating, and its average is zero. So in a cycle, the effect is canceled. So just like tilt, a phase difference is required. And then it works for AC, wait (just for single phase).

Let me ask you a question, do you love to have power ? If I double the power(2x) you get, would you exchange it for (1x) ? Most of us will, its always better to have more power. But single phase won't produce power as much as 3 phase, actually it has power 1/3 of 3 phase power. So we all now want 3 phase power, i.e, 3 phase voltages producing 3 phase currents. Earlier, single phase AC quantity, be it current or voltage was pulsating/dancing. Now, we have 3 phase quantities, (current and voltage), all pulsating/dancing. And to our excitement, without introducing external phase difference it is producing rotating effect on its own. Wow ! but how ? We have dancing voltages on stator windings, dancing currents on rotor windings producing dancing fluxes. Everything is dancing/pulsating. And the resultant is rotation(not just 90 complete 360 degree cycles) effect.

Ok, as an engineer now we have the task to understand it. Or, we just say, we got rotation, work is done and we leave it ? But the power we had should be controlled right ? Else it might destroy us, we need to understand it so that we can choreograph the dance. But there are several dancers all the phases are dancing and choreography is difficult. We want to make it easy so we need to do something. See, we are static and the AC quantities are dancing, and causing the rotor to rotate. So our frame of reference is stationary, but rotor's frame of reference is rotating along with dancing AC. Similar to us, stator of motor is also static, stator frame of reference is also stationary. We and stator both have same static frame of reference. It is like as if we were sitting on a chair on a stator and watching the rotor rotate, easy job right ? But to control the rotation we must embark on a journey to the rotating rotor. We must step into the rotor and rotate along with it. Now our frame of reference is similar to that of rotor. We are rotating/dancing with the rotor, for us now suddenly every AC quantities are dancing along with us in the same frame of reference. We are static to each other. Assume two person sitting on a rotating chair holding hands with each other, we and AC quantities are exactly like those persons. For us now, the rotor is static, the AC quantities are static. And suddenly all those dance is static. And life is always easy when things are static, easy to deal with those things. This static frame of reference with respect to rotor is the dq frame. So 3 phases dancing produced a single rotating effect, this single effect can also be resulted from 2 vectors acting at certain angle. So when we resolve this rotating effect which is stationary to us now, into its components in the dq frame we get its d axis and q axis. This is how I found dq frame virtually not physically. Do not search for it in the physical domain, you want to conserve that power to do something else.

Animations Explanation

In this video I have tried to demonstrate that pulsating sine wave in time domain is actually having a rotating phasor or vice versa. Imagine the rotating phasor is centered at origin and as it rotates in anti-clockwise direction, it traces out a sine wave in time domain.