I have a small DC gear motor and I'd like the limit the torque it outputs, without impacting the speed of the motor under a normal load. But I also want to keep costs and component count low.
So I was wondering, what effect a simple resistor attached to the motor would have, and why it would be preferable to use a more sophisticated current limiting circuit over that. Ie, how would it affect the speed and torque?
I'm guessing with a motor things are going to be a little more complex than just choosing a particular resistor to limit the voltage and current to a specific level.
Oh, and I'm going to be controlling this motor with PWM.
Oh, and I'm going to be controlling this motor with PWM.
PWM will do the trick! ...Wth a transistor or MOSFET to "boost" the voltage & current... You can't run a motor directly from the Arduino output.
A resistor will work, but it will have to dissipate power (heat) so you have to have the a big enough resistor and it's inefficient. And, you'll probably have to try a few different resistor values to determine what value you need.
Ie, how would it affect the speed and torque?
Torque, speed, and load are related. Often in an "unpredictable" way... For example, if you limit the current to a fan-motor it will slow-down. But if you remove the fan, there is less load and the motor will run faster. If you want to precisely control the speed of a DC motor, you need some sort of RPM sensor.
Yeah, I've got the PWM part nailed down. I will either be using a dedicated controller IC, or mosfets with flyback diodes.
Or are you saying I can just use a lower duty cycle to reduce the torque? That will reduce the speed of my motor under load though. What I really want is a way to prevent the motor from exceeding the current it draws under load, so that if it's stalled there will be insufficient current, and therefore torque.
There is no way to reduce the motor current and maintain the rotational speed. Torque is proportional to motor current, and rotational speed at a given load is proportional to the average DC voltage across the motor terminals. PWM reduces both. If you are looking for some sort of fail-safe protection, there are many ways to limit the current to a "safe" value.
The only way to reduce the torque, but not altering the speed is using a clutch that allows the mechanical connection to "slide"
Just like the torque converter in an automatic transmission - it just uses oil pressure to "slide" instead of a mechanical connection, but same principle all around.
I would recommend just using a current limited supply.
If you look up constant current circuits then use one of those where the constant current is at the limit of the torque you want.
The motor will then run normally until this current is reached and then both the speed and torque will drop as the current limit circuit reduces the voltage to your motor.
Grumpy_Mike:
The motor will then run normally until this current is reached and then both the speed and torque will hold as the current limit circuit reduces the voltage to your motor.
Both the speed and torque will drop as the current limit circuit reduces the voltage to your motor.
I'm not sure how to select one, and I'm not sure if it would limit the current quickly enough in a stall condition to protect someone's hand if it gets in the way of the moving parts. It is only a tiny DC motor, but with a 30:1 gear reduction, so there's probably not much danger, but the whole point is to stop the thing immediately if something gets in the way.
I did search Digikey though. If any of them are suitable, it would be one of these through hole ones which are in stock:
I'm gonna check out the datasheets of a few and see if I can suss out how quickly they'd react, but I doubt it will be fast enough.
I'm not sure how to select one, and I'm not sure if it would limit the current quickly enough in a stall condition to protect someone's hand if it gets in the way of the moving parts. It is only a tiny DC motor, but with a 30:1 gear reduction, so there's probably not much danger, but the whole point is to stop the thing immediately if something gets in the way.