Field Weakening: Theory & Misconception

In this video, I go over how the field weakening technique works and a common misconception about it.
0:00 Intro
0:28 Why is field weakening needed?
1:38 How field weakening works
5:18 Field weakening misconception
7:00: Field weakening limit? Positive Id?
9:33: Field weakening motor design
References:
Most images come from a great thesis on the topic by Wen Liang Soong:
"Design and Modelling of Axially-Laminated Interior Permanent Magnet Motor Drives for Field-Weakening Applications"
Python script:
github.com/MatanPazi/VFD/blob...
Brushed DC motor diagram:
ctms.engin.umich.edu/CTMS/ind...

[@dougfoster445]

Subscribed! I'm an electrical instructor and will always support growing channels like these!

[@SirEngelmann]

Thank you, very informative to watch. I never fully understood the reasoning and working principle of field weakening up until now.

[@shaner.5]

WOW, You need a VESC from Bennjamin Vedder, a controller that is open source. Man I would love to pick your brain to help me program my motor, can you explain how to calculate motor inductance difference iq-id? how do I calculate motor flux linkage? These are my measured motor parameters, R = 66 milliohm, Inductance = 366 microhenry with a max voltage of 84 volts, FOC, no field weakening yet.

[@matanpazi3777]

Aloha, well there are several methods. First of all what do you need those parameters for? Does the motor even have saliency (Lq, Ld ratio)? You can identify them using step/sine injections and process the response for example. Or w/ an LCR meter, make sure to rotate the rotor to get min (Ld) and max (Lq) values. Regarding flux linkage, you can pretty much measure the bemf between phases and w/ some scaling get the value.

[@shaner.5]

@@matanpazi3777 with min and max values do i average them after take measurements of one revolution

[@shaner.5]

@@matanpazi3777 I can get the motor to run in foc just not smoothly above 15000 erpm. In bldc mode the motor does fine. From what I understand in FOC mode I should get even more performance out of it, but I just can’t seem to dial it in.

[@eelcohoogendoorn8044]

Your diagram does not address the misconception at all; it merely repeats the assertion in a graphical manner.

[@matanpazi3777]

Would you care to elaborate? Misconception: Field weakening works by essentially lowering the motor's torque constant / back EMF constant. My claim: No, it does not. it's easiest to see that in 5:36, by adding current in the d axis, we can lower the voltage amplitude yet leave the torque constant.

[@eelcohoogendoorn8044]

@@matanpazi3777 Im not saying you are wrong; I am saying you are not presenting a novel argument; just the same assertion in a graphical format. Would love to see those experiments on a real motor you alluded to at the start of the video btw.

[@danilolattaro]

​@@eelcohoogendoorn8044look for "VESC field weakening"