Motor kv and efficiency

[Hummina Shadeeba]

It makes senses that a lower kv would be more efficient as it will have less switching in the esc maybe but would it be any less efficient in the motor? It seems the grim simulator incorporates the whole system and not possible to isolate the motor.


while it’s said a motor will have the same km regardless of winding is that realistic? It would require inductance and resistance will be raised or lower in equal measure. It kinda seems like the “speed” element in the torque x speed equation Is free: why not just spin a motor at 100,000 rpm and gear it down..as long as the iron losses are not greater than copper?

 

[liveforphysics]

Iron losses quickly exceed copper losses if you just keep spinning faster.

Consider the slot area from a cross sectional perspective like you sliced through all the end-turns to just see the copper fill in the slot. Not imagine you can power those little pieces of wire in the slot by directly touching probe leads to the ends of them, and you've got an adjustable current power-supply driving each of the probe leads.

The field you build is the result of the number of amps in each of those wire turns. If you have say 10 wire strands in your bundle wrapped around the tooth 10-turns, you have 100 little pieces of copper laying the slot. If you have 1 turn with 100 strands, you have 100 pieces of copper in the slot. If you have 100turns with 1 strand, you have 100 pieces of copper in the slot.

If you're driving all 100 of those pieces of copper with it's own separate 10amp power supply, or you've got them all in a series string with just a single 10amp power supply driving it, the cross sectional view of the magnetic circuit is identical, the copper loss/heating is identical, the magnetic field and hence torque is identical.

What changes is the inductance gets lower and lower, the rise rate of current gets too fast for primitive current control loops to control phase current. You want to look at the limitations of the controller with respect to voltage and phase current and commutation frequency vs your motors curve for core losses and Kt until you find the best compromise. Field weakening lets you run lower Kv for more starting torque within a given controllers phase current limitations, and then still get high RPM capable operation (at a small efficiency hit). If you have the budget for the silicon needed to drive more phase current, you don't need field weakening, but even then it would still just buy you more over-speed (which may only be useful for bursts due to core loss heating).

 

[liveforphysics]

The motor only feels the magnetic field in the slot with respect to making torque. The magnetic field is exactly proportional to amp*turns. It doesn't matter how you get the amp turns.

 

[Hummina Shadeeba]

With the 100 pieces of copper in parallel being each fed with a 10 amp supply then the total current before split into the parallel strands would be 10,000 amps to get the same torque as the single 100 turns of the single wire doing 10 amps.
The motor constant km. Same ohmic loss to torque. I guess I like to hear it over n over. (Have to get into motor solver program still. For sure before next question. )

Why do strands in parallel not build the magnetic field while in series they do? Why must it be the same electricity? Cant magnetic fields be increased from different electrical sources?

 

[liveforphysics]

Identical power to make your amp turns in a 1t or a 100t.

This is because the current path gets shorter and fatter as turns decrease or longer and thinner as turns increase, this change perfectly balances the I^2*R losses.