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What exactly do I control when sending a certain PWM signal to an ESC?

Posted by NickN on December 9, 2017 at 3:01am

Hi All

I want to ask a question that, after reading about it and watching YouTube videos, I still don't have a clear understanding about:

What exactly do I control when sending a certain PWM signal to an ESC? Is it thrust? RPM? Current?

An example:

I have a drone with a 3S battery, 30A ESCs, 2300kV motors, and 5x3 props.

If I now send a PWM signal of 1500us, i.e. 50% throttle, what happens?

- Do the ESCs draw 15 A (= 30A * 0.5) ?

- Do the motors spin at 13800 RPM (= 2300 kV * 12V * 0.5)

- something else?

Say I now change the props from 5x3 to 6x4. Thus it generates more thrust at a given speed, compared to the 5x3 prop.

Everything else (battery, ESC, motor, PWM signal) being equal, what is now different?

- Does the prop generate the same amount of thrust but spin slower?

- Does the prop spin at the same speed but draw more current?

- something else?

Thanks for your help.

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Replies

NickN December 17, 2017 at 5:13am

Thank you for that explanation, it's very insightful.

One thing I still don't quite understand: What causes the current flowing through the motor to increase with increasing load? Since the ESC apparently does not change its behavior depending on the load.

Nando te Riele said:

Using a 30A esc does not mean you are consuming 30A, it only means that the ESC should stay in one piece until 30A (with cooling) not that it actually does 30A at full throttle.

What PWM controls is Duty-Cycle. (normally, not true in governor mode for example)

Or to say it in another way, the Voltage going from the ESC to the motor. (close enough)

Kv= how many RPM you have to turn the motor to generate 1V, not the other way around

If you take the number and multiply by ~0,75 or 0,8 (dependent on the load on the motor) you get close enough for most people)

So you calculation of "13800 RPM (= 2300 kV * 12V * 0.5)" would be in the ballpark if you compensate for losses

2300 kV * 11.7 (voltage under load) * 0.5(duty cycle) *0.8 (losses, etc)=~10764

When you change propeller size from 5x3 to 6x4 your load increases: the motor will draw more current and do less RPM (@ the same dutycyle).
Nando te Riele December 12, 2017 at 1:21am

Using a 30A esc does not mean you are consuming 30A, it only means that the ESC should stay in one piece until 30A (with cooling) not that it actually does 30A at full throttle.

What PWM controls is Duty-Cycle. (normally, not true in governor mode for example)

Or to say it in another way, the Voltage going from the ESC to the motor. (close enough)

Kv= how many RPM you have to turn the motor to generate 1V, not the other way around

If you take the number and multiply by ~0,75 or 0,8 (dependent on the load on the motor) you get close enough for most people)

So you calculation of "13800 RPM (= 2300 kV * 12V * 0.5)" would be in the ballpark if you compensate for losses

2300 kV * 11.7 (voltage under load) * 0.5(duty cycle) *0.8 (losses, etc)=~10764

When you change propeller size from 5x3 to 6x4 your load increases: the motor will draw more current and do less RPM (@ the same dutycyle).
OG December 9, 2017 at 11:04am

No bo not assume any thing ! Know or ask questions, ESC operates using PWM PPM SIGNALS reseved only ! They do not send any signals, what ever loop is being implied from Flight controller it sends and receives signals.
NickN December 9, 2017 at 10:56am

Certainly the PWM signal plays a role. It ultimately determines what the ESC and the motor does.

I'm assuming the ESC runs some closed-loop control loop, where the input (i.e. the setpoint) is the PWM signal. My question is, what is the feedback variable in this control loop which shall be controlled to match the input (in other words, what am I commanding with the PWM signal?). Is it current draw, RPM?

Are you saying it's RPM?

OG said:

you are straining the motor and ESC bring it out of the it's efficiency range causing it to draw more current by adding larger prop the KV RATINGS AND MOTOR SPEED do not change. PWM signals have nothing to do with this ???
OG December 9, 2017 at 5:09am

you are straining the motor and ESC bring it out of the it's efficiency range causing it to draw more current by adding larger prop the KV RATINGS AND MOTOR SPEED do not change. PWM signals have nothing to do with this ???