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HomeAircraftHelicopterMain Discussion › PWM freq and motor timing
04-11-2015 08:43 PM  3 years agoPost 1
emederos

rrApprentice

Miami, Fl Dade

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Is it ok to have the pwm freq more than double the motor timing? Or should they be close in numbers to work more efficient together?

Example : my scorpion 3326-1400kv they recommend 5 deg timing,

My esc is set to 8htz PWM freq as scorpion recommends it with this motor but according to the formula:
1400kv x 22.2v x 10/20 = 15.5

This is more in the range of 16htz PWM?

Should i leave it at 5deg timing and 8htz i have now or is it better to increase PWM to 16htz?

I tried both and cant really tell a difference

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04-12-2015 12:55 AM  3 years agoPost 2
Jerry K

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Houston Area

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I am not here to answer the question but you may receive a little understanding from this site.

http://heli.dacsa.net/MrMelSite/index.htm

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04-12-2015 05:38 AM  3 years agoPost 3
EEngineer

rrProfessor

TX

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That's a great link, Jerry.

Would like to explain Mr. Mel's example further, although I can't answer the OP's question precisely....

With regards to Mr.Mel's example, an ESC is indeed bumping the "paddles"(motor coils) at a variety of frequencies of the PWM.

And those freqs. are...8KHz, 16KHz, etc.....not Hz but kilohertz(1000 times faster)...no offense intended.

Brushless motors come in two basic configurations...."inrunners" and "outrunners"...

Brushed motors use "mechanical" commutation(which just needs a steady DC source of current) to make the motor spin(in servo motors..the design uses "H-Bridge" circuitry to make the servo move in both directions).

On the other hand, "brushless" motors don't have any mechanical "commutation", instead the ESC controls the exact timing of the motor commutation.....less parts, brushes wear out, etc.

ESC's come in two "flavors".....active "freewheeling"(Kontronik)...or passive "freewheeling".

Passive uses a diode to dissipate the "back emf of a de-energized motor coil".....

Active, uses a FET to dissipate this back emf....and because the ESC's microcontroller can determine timing....depending upon the S/W....it can "adapt" to the precise motor timing...of one's "setup".

A passive uses a diode to dissipate the back emf that is caused by the collapse of an "energized" coil...can't be controlled be the ESC.

Would say that while Kon are the masters of active, Castle is the master of passive....

cont.

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04-12-2015 05:58 AM  3 years agoPost 4
EEngineer

rrProfessor

TX

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That's the basic "givens"....

If ya'lls eyes haven't "glazed" over yet....stick with me...

Now when there is talk of PWM and motor timing, one has to understand what all that means...right?

And I will try to explain this, as best I can, in "laymens" terms.

At whatever PWM frequency, one's ESC controls the brushless motor speed by varying the timing width of the 8/16/32 KHs pulse train.

The speed of a brushless motor is controlled...by the uP in the ESC by the "pulse width" of the "signals" that are used to control such motor speed....regardless of active/passive "freewheeling".

So, the ESC is "energizing" the coils..as Mr.Mel described.....as in 8/16/32 KHz pulses.

What wasn't mentioned was that the "time-width" of these series of 8/16/32 KHz wasn't mentioned....NP

Mr.Mel mentioned "back emf"...and that's correct.

So...

All brushless motors have a wide variety of "coils....each having such back emf.....and no matter whether such back emf can be reduced is up to the ESC's design.

Differing # of motor coils....on each motor.

Now ya'lls eyes are really glazed over....

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04-12-2015 06:12 AM  3 years agoPost 5
EEngineer

rrProfessor

TX

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One's ESC must determine the precise timing...

to let the "back emf" dissipate....as these motor pulse "trains"
are sent to ones brushless motor.

I one's ESC timing is of a very high rate, the "width" of that pulse..each repeating at 8/16/32....is maxed at full throttle.

Meaning the "width" is at max......

If at max, the timing can "interfere" with one's motor's commutation.

The back emf means that when re-engizing the coil...to align with the "next magnet"...can want to re-align with the previous magnet...

That's how they do "braking"...but only at low throttle...so the ESC can issue "signals" to that the coils tend to "align" with the previous magnet
..

In general...the less "poles", the higher the PWM rate. because
of such coil back emf

The dissipation of back emf, whether done by a passive diode....or the use a FET is the "key" to the max rate....

If such back emf has not diminished, the ESC is "fighting" this.
FWIW

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04-12-2015 12:18 PM  3 years agoPost 6
jason46

rrVeteran

MI

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In general...the less "poles", the higher the PWM rate. because
of such coil back emf
So I'm not an electrical engineer and I'm not going to pretend to know sh*t about sh#t, but I'm about 90% sure the above statement is not correct. If you just think about it, the more poles the more "communication" it will take to turn the shaft the same amount. Also, the formula below widely used everywhere proves it, right?

Nominal Voltage x Kv x Pole Count / 20 = Total / 1000 = value in kHz

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04-12-2015 12:27 PM  3 years agoPost 7
jason46

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MI

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I've had some finicky motors and I've found the only way to find a good pwm setting is try them all and closely watch flight time, motor temp and esc temp. If it is really off the motor will continue to make an audible tone. I find this to be the electrical equivalent to tuning a carb, trial and error.

In regards to Castle ESCs there was a firmware update roughly a year ago that makes a nice difference on high pole(14) motors.

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04-13-2015 06:38 AM  3 years agoPost 8
EEngineer

rrProfessor

TX

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So I'm not an electrical engineer and I'm not going to pretend to know sh*t about sh#t, but I'm about 90% sure the above statement is not correct.
I knew I shouldn't said that...as each ESC/Motor situation is unique.....

The point I was trying to make was that until the "back emf" from the "collapsing magnetic field" is dissipated by "freewheeling diodes"...whether active or passive.....the PWM pulse train will be "fighting" the back emf of a motor coil....

Messes with the "freewheeling" circuitry.

At full throttle, the "timing width" of each 8/16/24 KHz PWM pulses....are varied...at full throttle, the width is at max....

So the ESC shouldn't have the next PWM "pulse" occur before the motor coil is properly de-energized at other than "idle"...

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04-13-2015 07:21 AM  3 years agoPost 9
EEngineer

rrProfessor

TX

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Timing depends on the motor coil design....and how rapidly an ESC can snub the back emf...by means of its "freewheeling" diodes.

The faster the back emf can be "snubbed", the higher the PWM timing can be.

FWIW

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04-13-2015 08:54 PM  3 years agoPost 10
wc_wickedclown (RIP)

rrProfessor

long beach calif

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EEngineer
Timing depends on the motor coil design....and how rapidly an ESC can snub the back emf...by means of its "freewheeling" diodes.
FWIW
you go boy

Insha Allah made in america

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04-13-2015 09:10 PM  3 years agoPost 11
emederos

rrApprentice

Miami, Fl Dade

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What is too hot for an ESC to run? Safe temp?

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04-14-2015 01:01 AM  3 years agoPost 12
Jerry K

rrKey Veteran

Houston Area

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212 degf is what they usually shut down called the high/high or (HH)
180 is approching the wanrning stage refered to as high or (H)

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04-14-2015 05:02 AM  3 years agoPost 13
EEngineer

rrProfessor

TX

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Also, consider that hotter weather is coming.

When one is flying in 40F weather, one's ESC can run a little hotter.

But, when one is flying in 95F weather, one has to "de-rate" the max operating temp.

But that's different subject from PWM timing.

If you can search and indentify the motor FETs that the ESC uses,
you can go to the FET manufacturer website and check the "absolute max ratings" of that particular FET...temp. in this case.

FWIW

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04-14-2015 12:15 PM  3 years agoPost 14
emederos

rrApprentice

Miami, Fl Dade

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I was only asking because the PWM has to alter the Esc temps as what I've read. I'm in Miami Florida averaging 90 degree weather and my esc is only marking 98f when I fly. Just wanted to know the safe range in case the summer comes and I know what to look out for. I'm overkill on my heli with this 100a with a mounted fan.

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04-15-2015 02:16 AM  3 years agoPost 15
EEngineer

rrProfessor

TX

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One feature I'd like to see is the manufacturers use a thermo-couple like the PC motherboard manufacturers do.

And have the thermocouple/s be in contact with the case/s of the motor FETs....and then include the temp. measurement in the ESC's log file.

It's the temp. of the motor FET that matters, not that of the ESC case, as there are a lot of thermal paths from motor FET case to ESC case.

The FET manufacturers have "absolute max. temp" specs that can't be exceeded without destructive results.

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