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Topic: DC motor torque decreasing with PWM (Read 5 times) previous topic - next topic

arvindsk

Hello, I am using arduino Uno as an interface between my RC receiver and H-bridge(L293d).
I have been able to control the speed and direction of the motor by PWM. But as the RPM of the motor decreases, the stall torque of the motor decreases as well. I need the motors to power a pan tilt head. Any decrease in torque is not acceptable as the camera and the head is pretty heavy. Is there any way of decreasing the RPM while maintaining the torque?
Any help would be greatly appreciated.

P.S: I got the Uno just yesterday!

MarkT

Can you tell us more about the motor and the supply for it?  A good motor will pull lots of current at stall, perhaps the supply isn't up to this on PWM pulses?  Torque depends on average current, the supply will probably limit peak current, as might the H-bridge.

BTW the fundamental physics means that torque is related to current and RPM to voltage for a DC motor - but this is only actually true if the windings are superconducting - the winding resistance means deviation from this ideal behaviour.  Worth measuring the resistance if you want to estimate the maximum stall current the motor can pull for various voltages.
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jackrae

You could increase torque by increasing supply voltage - which will require a corresponding decrease in PWM duty cycle, since an increase in voltage will also tend to increase motor speed (which you do not want)   The increased peak voltage will drive more current through the motor windings and torque is influenced by current.

John_S

A gear reduction system may be required if you want lots of torque at a slow speed.

dc42

To maintain high torque at low RPM, use an rpm sensor and PID controller software. The PID controller adjusts the PWM to achieve the required rpm. If necessary it will apply full power (100% PWM) to the motor until it is up to the required speed.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

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