EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: XaviPacheco on October 08, 2018, 06:13:35 pm
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I'm controlling a DC motor via PWM @10 kHz. Nameplate of the motor attached. I'm increasing the duty cycle in steps of 8%. Every step means a high current for a brief moment (for that motor). My question is: Do these steps changes represent a potential problem? Should I lower the step change? Or how can I vary duty cycle smoothly? I'm doing this with a microcontroller, so it's something more software oriented.
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My question is: Do these steps changes represent a potential problem? Should I lower the step change? Or how can I vary duty cycle smoothly? I'm doing this with a microcontroller, so it's something more software oriented.
For the motor? I would say no, as long as the peak current is below the demagnetisation for the motor - which for that motor likely is a few hundred Amps.
For the load it's driving? Possibly but that's up to you to judge.
For the drive circuit? Possibly but it depends on the magnitude and duration of the current pulse and on the design of the drive circuit of course.
What you're seeing is basically what you'd see if you just slapped the motor across a voltage 8% of the total supply voltage (or 8% higher than the current supply voltage). At first all that the supply sees is the resistance of wiring, the brushes and internal resistance of the armature. The current will quickly (but not instantly due to the inductance) rise to U/R and then, as the speed of the motor increases the current will drop until BEMF=U. If you want to limit the current peak then ramp up the voltage (dutycycle) in smaller steps.
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The motor doesn't care. A full DOL start pulls much more current. Your power switches might not like it, though.
BTW, 10 kHz is probably a bit high for this kind of motor (magnetic losses), I'd suggest 2...3 kHz.
* DOL = direct on line = 125 V bang on switching.