Controlling DC motors with Pulse Width Modulation

Thread Starter

abdulwahab.hajar

Joined Jun 14, 2016
93
Hello everyone

I've been searching the web about DC motor control, and found PWM is an easy way to do it......
So, I understand what PWM is...... and finding a suitable circuit isn't a problem......
But something that keeps bugging me is; how do we change the speed by changing the duty cycle, I mean when the voltage is at a low 0V shouldn't the motor actually stop???
Does it have to do with the frequency of the signal if you may?

Thank you
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
You have to look at the resultant current, which is not a square wave as the voltage is, but a saw tooth type waveform which has a mean level, which varies with the pulse width.
In any case at the frequency of a typical pulse it would never be capable of stopping between pulses.
Max.
 

MrAl

Joined Jun 17, 2014
11,392
Hello everyone

I've been searching the web about DC motor control, and found PWM is an easy way to do it......
So, I understand what PWM is...... and finding a suitable circuit isn't a problem......
But something that keeps bugging me is; how do we change the speed by changing the duty cycle, I mean when the voltage is at a low 0V shouldn't the motor actually stop???
Does it have to do with the frequency of the signal if you may?

Thank you
Hello there,

The inertia of the rotor keeps it spinning even when power is removed. The inductance keeps the current flowing because in any real PWM system there would still be a path for the current to flow either through another transistor or through a back emf diode. These two effects keeps the armature spinning over short time periods and the result is the speed looks like the average speed and the current looks like the average current.
To get this to happen usually means using a frequency that is high enough to not allow the current to change too much over one cycle. That would mean probably 100Hz or greater.
An approximation would be at 100 percent duty cycle the motor spins at full speed, and at 90 percent duty cycle it spins at 90 percent of full speed, and at 80 percent duty cycle 80 percent of full speed, etc.
 
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