PWM Duty Cycle with Adafruit Motor Shield V2

[cosined]

Hello forum members,

I have an Adafruit Motor Shield V2 and a linear actuator. The linear actuator's data sheet says that it's not supposed to be used with a duty cycle higher than 25%, otherwise there's a risk that it will be damaged. The motor speed in the Adafruit_MotorShield library is set by

Code: Select all

myMotor->setSpeed(100); 

where 100 can be replaced by any number between 0 (stopped) and 255 (max speed). The speed is actually the duty cycle, right? So am I correct in assuming that to get a duty cycle of 25% or less, I would use an integer between 0 and 63 for the setSpeed argument? I'd appreciate any help!

Calculation
255(0.25) = 63.75 ≈ 63

[adafruit_support_bill]

So am I correct in assuming that to get a duty cycle of 25% or less, I would use an integer between 0 and 63 for the setSpeed argument?

That is correct for setting the PWM duty cycle. But I am not sure that is the duty-cycle the data sheet is referring to.

For motors, it is common to specify duty cycle on a longer time-scale. For example, 25% duty cycle could be interpreted to mean not more than 15 minutes of active use per hour. You might want to clarify that with the manufacturer.

[cosined]

Thank you for the prompt reply! The information sheet that came with it says the same thing you just did. One of the warnings says "Do not exceed the 25% duty cycle of the actuator: If the actuator is used at full load for 2.5 minutes, then it must remain off for 7.5 minutes." That's if the motor is used at full load. If it's used at 25% of the full load all of the time (i.e. 25% duty PWM) will it work for an indefinite amount of time? Now that I read the warning again, it actually seems like the maximum amount of time I'd be able to use the motor is 10 minutes of continuous use at 25% PWM duty cycle. Is that right?

Calculations
2.5 min (1/(25%)) = 10 min

[adafruit_support_bill]

What kills motors is heat. The duty cycle rating tries to strike a balance between the heat buildup rate and heat dissipation rate. Since heat dissipation depends somewhat on the ambient temperature and ventilation, the rating probably errs on the safe side.

I suspect that at 25% PWM power you could run indefinitely if the actuator has enough ventilation. But I'd check with the manufacturer to understand what assumptions their ratings are based on.

[cosined]

Alright, thank you! I appreciate your help, even though my questions veered away from the Motor Shield V2 in the end. I'll contact the seller and see what they did.