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Author Topic: Dealing with DC Motor Stall Current  (Read 5230 times)
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Chester, UK
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Evening All
What I'm attempting:
To power a 370w 24VDC motor using a power supply  (24VDC 20A)

The problem
The stall current of the motor instantly trips the over current protection of the supply.

What I propose
To add a capacitor/battery to the output of the supply to provide reserve capacity to get through the transient load.

My concern
That I'm talking nonsense / what size capacitor would I need?  I'm driving the motor with PWM for speed control.

Thanks
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CT, USA
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I don't think any capacitor would solve your stalling current issue. Any capacitors capable for doing this would need to be hundreds of Farads.

What you can do is adding a current sensing circuity and reduce the drive voltage (or cut off completely) once a stall is detected.

Of course, you can use a beefer power supply as well...
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Left Coast, CA (USA)
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Quote
The problem
The stall current of the motor instantly trips the over current protection of the supply.

By stall current do you mean 'locked rotor' condition with full voltage applied?

Do you know what the stall current value is?

What would you want to happen when there is such a stall condition?

Sounds like the 24vdc power supply protects itself OK as is? Does it reset itself automaticaly or must you manually reset it or do you have to do a manual power off and on sequence?


Lefty



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Chester, UK
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Sorry, I thought I would put the effort in and clearly state my problem..... and then stuffed it up by saying stall when I meant inrush current.  So that short time, at start up, before the motor produces any useful back EMF.  I'd rather not have to spec a power supply to support this short transient condition smiley

Thanks Chaps and Chapesses
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Sorry, I thought I would put the effort in and clearly state my problem..... and then stuffed it up by saying stall when I meant inrush current.

I would think that just allowing for a normal smooth ramp up in software would handle inrush OK, Just don't allow 100% on pwm to be applied instantanously?

Lefty
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I think you need to implement a "soft" start as  Lefty suggested. What I have done in the past is kinda a dual PWM. Sorta like FM modulation if you will.

During the ON time, PWM at a high frequency. Hope that was clear.
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Chester, UK
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Will give it a try.  I'm a little concerned that the power supply will still be asked to deliver more current (however briefly during on times) than it can deliver and the prtotection will trigger....

Thanks
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The previous comments re controlling the ramp rate is definitely required, however your concern about inrush, even at low levels of PWM, may be valid.  Your enquiry re fitting a capacitor has merit but rather than a gigantic capacitor you could fit a pair of relatively small series connected 12 volt sealed lead acid batteries (say 12AH) across your power supply output.   The batteries provide the short duration inrush current and the power supply recharges them during your normal running speed.

Your 20A supply is pretty close to the mark trying to run a 370watt motor. Motors are (or should be) rated at their shaft power so a 370 watt motor is probably a 1/2HP unit  (746watts to the horse power).  Assuming an efficiency of 80%, the input power requirement would be around 460watt, or 19+ amps.

jack
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If you are still having an issue with inrush, try a power NTC thermistor.
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Lefty, not 100% sure, but I think Stall current is around 8x running current.... that figure is popping to my mind

Looked it up, yep, 3-8x depending on type of motor, which.... now that I think of it, would explain why you asked, I guess the figure I always heard was "Up to 8x"
« Last Edit: January 21, 2011, 02:38:58 pm by Valalvax » Logged

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