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PWM Switching rate anybody?

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SIMON CRAGG21/10/2019 20:00:48
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For my first foray into converting IC to electric, I need a 100a ESC. After some excellent advise from this forum, I have got a YGE100a version. As it is set up for Hele operation as standard, I purchased a programme card which is a work of art in itself. After watching several videos on the subject, the only grey area is the PWM switching rate, which can be set anywhere between 8 and 16 HZ. I have absolutely no idea what to set it at! Can anybody help please?.

Chris Bott - Moderator21/10/2019 20:07:25
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Simon have a read here. Switching rates referred to are higher than yours but these ones refer to small high revving quad motors. I'd think that the general principles apply, though.

SIMON CRAGG22/10/2019 01:17:33
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Having done a bit more research it looks like 8 is the favourite, but a bit of trial and error is on the cards I think!

Brian Stevenson 122/10/2019 06:51:51
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Posted by SIMON CRAGG on 21/10/2019 20:00:48:

which can be set anywhere between 8 and 16 HZ. I have absolutely no idea what to set it at! Can anybody help please?.

 

Set it at 16. And it's KHZ, not HZ.

Its maximum switching rate of 16 KHZ is low by modern standards. Is it quite an old controller?

Why I ask is that  with a low switching rate and lots of poles in the motor (a typical outrunner) at high RPM (high throttle)  it may not work at all - the motor will just 'judder' or worse, as you don't notice it, the efficiency at full throttle or near full throttle  and thus the flight duration, will go down.

Don't worry too much about this if you are flying an 'ordinary' plane. But you can run into this problem with pylon racers, hotliners, and EDFs, where RPM tends to be high.

Edited By Brian Stevenson 1 on 22/10/2019 07:03:10

Edited By Brian Stevenson 1 on 22/10/2019 07:04:45

SIMON CRAGG22/10/2019 08:04:51
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Its a Turnigy Aerodrive SK3 5055-430 KV.

I have found a formula for working out the khz but need to establish how many poles the motor has. This is not in the motor specs. Any ideas?.

Formula : KV X V X Poles divided by 20 = optimum khz.

Nigel R22/10/2019 10:39:45
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It's possible to count if you can see inside:

**LINK**

Dickw22/10/2019 11:01:18
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I use YGE ESCs in a couple of hotliners with 4 pole motors running at 60,000 rpm, and I have seen 80,000 rpm when a prop came off (I use on board data loggers).

I have tried both 8kHz and 16 kHz pwm at different times and both work OK (but I am now using 8kHz), so I wouldn't worry too much. It won't do any harm so just try both and pick whichever sounds best!

Dick

Phil Green22/10/2019 11:02:12
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Posted by Brian Stevenson 1 on 22/10/2019 06:51:51:
the efficiency at full throttle or near full throttle and thus the flight duration, will go down

Just to clarify, there is no PWM at full throttle, there is only commutation

John Bisset22/10/2019 11:58:49
212 forum posts

Blimey! Once again the sheer scale of my ignorance of things electrical astonishes me. I happily fly 'ordinary' electric aircraft without knowing any of this. Fascinating, shall delve deeper.

Meantime, being a sucker for TLAs (Three Letter Acronyms) , what does 'PWM' stand for please? I hunted through Chris' link - most interesting but that bit I didn't find.

(I suspect it may be ~obvious, like 'WOT' turned out to be wide open throttle. Or, 'full power' for most purposes, so I was puzzled why a TLA was used)

Gary Manuel22/10/2019 12:09:19
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PWM = Pulse Width Modulation

John Robertson 322/10/2019 23:32:27
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Have a look at this; https://en.wikipedia.org/wiki/Pulse-width_modulation#Servos

Then scroll down for a link to: https://en.wikipedia.org/wiki/Servomechanism

and then: https://en.wikipedia.org/wiki/Servo_control

Geoff S23/10/2019 12:17:44
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Posted by Gary Manuel on 22/10/2019 12:09:19:

PWM = Pulse Width Modulation

Trouble with TLAs is that they can mean different things to different people. At work (thankfully, long ago) PWM was a Process Write Monitor and it still the first that springs to my miind despite the fact I learnt about Pulse Width Modulation much before.

Geoff

Phil Green24/10/2019 14:18:54
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Posted by John Robertson 3 on 22/10/2019 23:32:27:

Have a look at this; https://en.wikipedia.org/wiki/Pulse-width_modulation#Servos

Then scroll down for a link to: https://en.wikipedia.org/wiki/Servomechanism

and then: https://en.wikipedia.org/wiki/Servo_control

...but PWM as used to position a servo isnt really like PWM used for power control in an ESC
Servo PWM is just a signalling system limited to between 5% and 10% duty cycle at very slow 50cps.
An ESC (and almost anywhere else where PWM is used for power control) uses the full 0 to 100% duty cycle, usually but not necessarily at a much higher rate of several khz.

Also, remember that at 16khz PWM each FET is undergoing twice as many switching operations as it would at 8khz and as we know its the linear transition between off & on that can cause heating in the FETs.

At full throttle, assuming the ESC has been correctly calibrated, the PWM stops and the FETs are hard switched for the full commutation cycle, hence PWM rate shouldn't affect full throttle running

Cheers
Phil

 

Edited By Phil Green on 24/10/2019 22:33:56

SIMON CRAGG24/10/2019 14:57:45
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Posted by Phil Green on 24/10/2019 14:18:54:
Posted by John Robertson 3 on 22/10/2019 23:32:27:

Have a look at this; https://en.wikipedia.org/wiki/Pulse-width_modulation#Servos

Then scroll down for a link to: https://en.wikipedia.org/wiki/Servomechanism

and then: https://en.wikipedia.org/wiki/Servo_control

...but PWM as used to position a servo isnt really like PWM used for power control in an ESC
Servo PWM is just a signalling system limited to between 5% and 10% duty cycle at very slow 50cps.
An ESC (and almost anywhere else where PWM is used for power control) uses the full 0 to 100% duty cycle, usually but not necessarily at a much higher rate of several khz. Remember that at 16khz PWM each FET is undergoing twice as many switching operations as it would at 8khz and as we know its the linear transition between off & on that can cause heating in the FETs.

At full throttle, assuming the ESC has been correctly calibrated, the PWM stops and the FETs are hard switched for the full commutation cycle, hence PWM rate shouldn't affect full throttle running

Cheers
Phil

Edited By Phil Green on 24/10/2019 14:34:33

Many thanks for the clarification.

Still in the dark as to whether I set up the esc for 8 or 16khz though!

Denis Watkins24/10/2019 15:18:17
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Posted by SIMON CRAGG on 24/10/2019 14:57:45:

Still in the dark as to whether I set up the esc for 8 or 16khz though!

You can get a ballpark figure Simon using this formula, to better match your ESC.

The motor Kv x Volts x the pole count ÷ 20

The will not give the exact frequency that you set, but choose the nearest .

E.g 1500Kv on 4S 8 pole

1500 x 14.8 x 8 ÷ 20 = 8880

So set 8khz

Edited By Denis Watkins on 24/10/2019 15:20:54

SIMON CRAGG24/10/2019 15:26:54
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Posted by Denis Watkins on 24/10/2019 15:18:17:
Posted by SIMON CRAGG on 24/10/2019 14:57:45:

Still in the dark as to whether I set up the esc for 8 or 16khz though!

You can get a ballpark figure Simon using this formula, to better match your ESC.

The motor Kv x Volts x the pole count ÷ 20

The will not give the exact frequency that you set, but choose the nearest .

E.g 1500Kv on 4S 8 pole

1500 x 14.8 x 8 ÷ 20 = 8880

So set 8khz

Edited By Denis Watkins on 24/10/2019 15:20:54

Thanks Denis

Only trouble is I cannot find out what the pole count is of the Turnigy Aerodrive SK3 5055-430KV motor.

I think it is probably 12, so the formula would be:

430 x 22.2 x 12 divided by 20 = 5.72 or the lowest setting which is 8khz.

Denis Watkins24/10/2019 15:57:11
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Posted by SIMON CRAGG on 24/10/2019 15:26:54:

Aerodrive SK3 5055-430KV motor.

I think it is probably 12, so the formula would be:

430 x 22.2 x 12 divided by 20 = 5.72 or the lowest setting which is 8khz.

From the Hobbyking motor diagram Simon, the motor is 14 pole

Putting you even closer to 8kHz

BTW the motor does run cooler where frequency can be matched like this

John Robertson 324/10/2019 22:10:04
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Simon - you DID ask "... Meantime, being a sucker for TLAs (Three Letter Acronyms) , what does 'PWM' stand for please? ..."!

This forum is a wonderful resource, is it not?

PatMc24/10/2019 22:25:13
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Posted by Gary Manuel on 22/10/2019 12:09:19:

PWM = Pulse Width Modulation

SIMON CRAGG25/10/2019 06:33:04
552 forum posts
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Posted by John Robertson 3 on 24/10/2019 22:10:04:

Simon - you DID ask "... Meantime, being a sucker for TLAs (Three Letter Acronyms) , what does 'PWM' stand for please? ..."!

This forum is a wonderful resource, is it not?

I am really grateful for all the help I have received from this forum. I have learnt an awful lot in a very short space of time. Having been ic since the seventies, its a big step trying to work out the correct combinations when changing a model, (in this case an 80" PT-19). I have yet to get started on the project, but having researched it to death, I am quietly confident that the model will fly even better than it did before!.

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