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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
ok, I have a somewhat complicated question.
fisrt, does skin effect through copper on high speed motors become a problem? (on low turn count, HF pwm driven, 3 phase motors.)
second, I don't know what the outrunner freq is, but it may be between 8khz and 24 kHz. (which puts skin depth at 1 to 0.6 mm.) wire is 0.045" in dia, copper.
third, is square wave skin depth different from sine?
I wonder if this is worth pursuing, if I could gain 10% more flight time, or 5% less battery mass, it would crush other college flight teams in competition !
My programmable features...im most interested in high efficiency (long flight duration).
a 7 turn motor, 1120 k sub V, on 12.6 to 9.8 volts. About 6k rpm at 55% throttle. its a 3 phase outrunner too.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Could work, copper plated aluminum wire has been used in transformers for a long time. I often wondered why this approach isn't used for e-bikes so the motor can fit inside the wheel hub as a segmented PCB with copper on the back, Al in the middle and another layer of copper on top held in an Epoxy mount for insulation.
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
With that many turns pure silver wire comes to mind.
secondly yes harmonic energy will use lower skin depths but at this frequency I think you will be only chasing a percent or two at max with less than a percent improvement
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
This is a very complicated question. The bottom line is that the motor's back EMF (the voltage it generates when you spin it) has a certain waveform defined by its mechanical construction. For optimum efficiency the driving voltage waveform should look the same as the back EMF. I imagine these RC model motors are designed to have a squarish back EMF, as it's easier to design a square wave ESC.
Any harmonics that exist in the driving voltage and not in the back EMF will just heat the motor without producing torque, and any harmonics that exist in both waveforms, but in the wrong phase relationship, might produce torque in the wrong direction.
Torque is produced by the winding currents, which are driven by the difference between the driving voltage and the back EMF. Since the windings are inductive, the current waveform gets low-pass filtered. This means that most of the torque is generated by the fundamental and the first few harmonics. The higher harmonics aren't really that important.
The Fourier approach helps with skin depth too. You consider the skin depth and AC resistance separately for each harmonic.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Steve Conner wrote ...
This is a very complicated question. The bottom line is that the motor's back EMF (the voltage it generates when you spin it) has a certain waveform defined by its mechanical construction. For optimum efficiency the driving voltage waveform should look the same as the back EMF. I imagine these RC model motors are designed to have a squarish back EMF, as it's easier to design a square wave ESC.
Any harmonics that exist in the driving voltage and not in the back EMF will just heat the motor without producing torque, and any harmonics that exist in both waveforms, but in the wrong phase relationship, might produce torque in the wrong direction.
Torque is produced by the winding currents, which are driven by the difference between the driving voltage and the back EMF. Since the windings are inductive, the current waveform gets low-pass filtered. This means that most of the torque is generated by the fundamental and the first few harmonics. The higher harmonics aren't really that important.
The Fourier approach helps with skin depth too. You consider the skin depth and AC resistance separately for each harmonic.
WOw! these statements are profound! I never realized the wave-form harmonics were so important.
and heres even more. yes as johnf implies, I may be spedding huge effort in engineered changes for insignificant gains. but, the Castle ESCs are super sophisticated. the Chinese and SimonK flashed cheap ESCs (9-20US$) are basically RC servo input to 3 phase H-bridge Mosfet devices. But the Castle ESCs actually (Corporate propaganda implies) measure the motor response, when first throttled from zero to max, and then learn what is the best timing (and advance) and then those settings ive shown above mod to the users Ideal usage.
the Castle ESCs, cost 70USD for mine, and I need two (bicopter), ive taken them apart they have more components inside and more capability than the cheap escs.
ill leave the motors alone and just try to optimize the ESCs settings, which no doubt push the H-bridge generated wave form around as per Steve Connors point.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Keep in mind if you are trying to optimize an inverter for putting out reduced harmonics (i.e. sine wave vs. square wave), that actual inverter may be less efficient so any gains you achieve through driving the motor with a sine wave will be traded off with losses in your inverter circuit.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
Steve Conner wrote ...
Any harmonics that exist in the driving voltage and not in the back EMF will just heat the motor without producing torque
Isn't it the current that generates heat, not the voltage?
As I see it the voltage can almost do whatever at the highest frequencies, because it's filtered out by the winding inductance on its way to making a current. The PWM sampling harmonics get filtered off that way and you're left with a continuous current waveform- and the harmonics of that current waveform will be lossy to some varying extent.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, I should have said:
Any harmonics that exist in the driving voltage but not the back EMF (or vice versa) will cause harmonic currents to flow in the windings that only heat them and generate no torque.
This is almost the same thing, except the inductive nature of the windings means that you don't have to worry so much about the high order harmonics.
EasternVoltage makes a good point. Castle have just punted this problem onto the user by making the PWM carrier frequency user adjustable. Personally I would investigate by building a dyno rig to measure the efficiency directly.
FEMM is routinely used by just about every maker of motors nowadays.
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Skin effect and khz driven electric motors.
