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Dec 27, 2017, 09:06 PM
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Discussion

PWM signal


I don't understand how exactly your ESC converts the duty cycle of a PWM signal to a certain amount of current delivered to the motor. Is the ESC calibrated such that a full duty cycle is equivalent to the ESC's max current? If not, does that mean you can feed the ESC too high of a duty cycle that it burns out if the signal causes it to draw too much current for it to handle? I am asking all this because I want to make sure the motor and ESC on my flying wing are really working at their maximum capacity when my throttle stick is at max.
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Dec 27, 2017, 09:14 PM
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The ESC generates a PWM signal to the motor windings - among other very complex things like when to commutate the motor windings and send the voltage to a different set of windings. It does not send current it sends voltage which becomes current based on the motor resistance and BEMF. Full on for an ESC is full battery voltage going to the windings. The current generated is determined by the load on the motor.
Dec 28, 2017, 09:40 AM
An itch?. Scratch build.
eflightray's Avatar
Quote:
Originally Posted by Arontbt
I don't understand how exactly your ESC converts the duty cycle of a PWM signal to a certain amount of current delivered to the motor. Is the ESC calibrated such that a full duty cycle is equivalent to the ESC's max current? If not, does that mean you can feed the ESC too high of a duty cycle that it burns out if the signal causes it to draw too much current for it to handle? I am asking all this because I want to make sure the motor and ESC on my flying wing are really working at their maximum capacity when my throttle stick is at max.
The maximum current the motor will draw will be dependent on the voltage supplied, (the batteries voltage), and the load on the motor.

The load on the motor is usually what causes the 'magic smoke' to escape. Such as too big a propeller, or crashing the model and not shutting the throttle quick enough, so the motor is totally stalled.

To ensure a good range of throttle control over the ESC, check the ESCs instructions regarding, 'setting the throttle range'. If the ESC is one from Hobby King, the instruction can usually be found on the ESCs page, under the tab marked 'Files'.

Buying a 'wattmeter' is well worth the money, it can often save the expense of burning out motors and ESCs by showing just how many amps are being drawn when the motor is under load, (ground testing).
It can also show the state of the battery while under load, (voltage).

.
Dec 28, 2017, 10:46 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by Arontbt
... to a certain amount of current delivered to the motor. ...
An ESC does not force feed current to a motor, the ESC applies a (PWM chopped) voltage. At wide open throttle there is not chopping.

Motorcurrent at wide open throttle does not depend on ESC, motorcurrent at WOT is proportional to pitch¹, voltage², Kv³ diameter⁴. It's up to you to choose those parameters to keep current below max.current of battery, controller and motor. You don't have to worry about PWM of throttle/servo signal and PWM of motorvoltage, it's all in the black box.
Dec 28, 2017, 10:48 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by Arontbt
... I want to make sure the motor and ESC on my flying wing are really working at their maximum capacity when my throttle stick is at max.
It is a good practice to have some headroom built in.
About derating motors, controllers, batteries, electronics in general:
Quote:
Originally Posted by eflightray
... Buying a 'wattmeter' is well worth the money ... .
Vriendelijke groeten Ron
Jan 01, 2018, 07:22 PM
Registered User
Thanks guys! Extremely helpful answers and the whole thing makes a lot more sense now.
Jan 01, 2018, 08:56 PM
Registered User
vollrathd's Avatar

From the ESC basics to ???????


For the other readers of this thread that are interested. There is a whole lot going on under the microcontroller hood of our brushless ESC's.



http://ww1.microchip.com/downloads/e...tes/00857a.pdf
http://ww1.microchip.com/downloads/e...tes/01083a.pdf,
http://ww1.microchip.com/downloads/e...tes/01292A.pdf,


Brushless DC Motors & Control - How it Works (Part 1 of 2) (10 min 33 sec)


Brushless DC Motors & Control - How it Works (Part 2 of 2) (7 min 23 sec)
Jan 01, 2018, 11:18 PM
Registered User
One thing I haven't seen mentioned is that motor windings have a lot of inductance so as long as the modulated frequency is sufficiently high, it will smooth out the current draw over time. So a 25% duty cycle square wave will result in a near-constant current draw roughly half that of a 50% duty cycle square wave. This principle also holds true for motors that are spinning, but it's still a good practice to account for the maximum due to startup and in case a motor ever seizes for any reason.

The same principle is why a D-type amplifier can drive a speaker with a square wave of varying duty cycle and it doesn't sound awful (again, provided the square wave is of sufficiently high frequency).
Jan 02, 2018, 11:45 AM
I am a nice guy! Really!
It would be impractical to size the ESC for the stall current of the motor. Even a 20 amp motor will pull well over 100 amps when stalled. An ESC rated for 20% above max motor current is most commonly recommended.
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Jan 02, 2018, 12:25 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by Access
... So a 25% duty cycle square wave will result in a near-constant current draw roughly half that of a 50% duty cycle square wave. ...
Also ...
The lower the motor induction, the higher the PWM chopping frequency. E.g the low induction iron-/slot-less motors should use 32kHz PWM, instead of 8, 12, 16kHz.
A smoother current (higher crest factor) is more efficient, lower (copper)losses, less heat.

from www.consult-g2.com → course


Gelukkig Nieuw Jaar Ron
Last edited by Ron van Sommeren; Jan 02, 2018 at 12:30 PM.
Jan 02, 2018, 08:38 PM
Registered User
Quote:
Originally Posted by Ron van Sommeren
Also ...
The lower the motor induction, the higher the PWM chopping frequency. E.g the low induction iron-/slot-less motors should use 32kHz PWM, instead of 8, 12, 16kHz.
A smoother current (higher crest factor) is more efficient, lower (copper)losses, less heat.

from www.consult-g2.com → course


Gelukkig Nieuw Jaar Ron
But also keep in mind switching losses, so don't set the frequency unnecessarily high (past the point of diminishing returns). Typical ESC uses a MOSFET which must pass through the resistive region every time it is switched on and off. Pretty sure a IGBT is the same way.


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