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Effect of PWM on Motor Torque

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tanved96:
I was just reading this page http://www.societyofrobots.com/schematics_h-bridgedes.shtml. Here, talking about PWM it's written "that motor torque, under PWM, remains the same whether fully on or only a percentage on. However, varying voltage for speed control reduces torque. So with PWM you have maximum torque yet slower speeds!".

Here's what I don't understand - the torque the DC motor is able to deliver is because of the current passing through it's coils (assume the load to be constant). And in PWM, the effective voltage actually producing the current is lower than that at full duty. So the motor is getting a lower, constant(?) current, due to induction, because of which we get a lower rpm. SO why doesn't it lower the torque generated??

newInRobotics:
With PWM the effective voltage and current are constant no matter duty cycle, because motor is fully OFF, or fully ON. Torque depends on available current only, where RPM depends on available current or amount of time it is ON. What happens is that when motor is ON it resists load (accelerates) and when it's OFF it does not resist load (gets slowed by load/decelerates), hence RPM can be controlled by amount of time motor is ON, while torque stays at max due to max current at ON period.

tanved96:
Thanks a lot! It's clear now  :)

Billy:

--- Quote from: tanved96 on January 26, 2013, 05:50:13 PM ---Here's what I don't understand -

--- End quote ---

No, you understand it just fine.  PWM (on/off ratio) absolutely affects the current (torque), just as you think it does. I don't know where the source material comes from but it re-enforces "just because it was on the web, it isn't necessarily true."

oscyes:
i have been working on my project and i have a problem with my dc motor control , it happens that when i use an h bridge with pwm control for varying my motor speed it loses torque
 
i mean with high pulse width my motor has full torque and speed but when i reduce the pulse width the speed gets slower but also does the torque so i think you were right with yout doubt

how did i know that my motor lost torque?

well i used the motor for raising my robot's arm and when i put low pulse width it cant raise it as it does with fully pulse width and i need to raise my robot's arm at a very slow speed but i cant still do it

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