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TOPIC: motor specs -- capacitance?

motor specs -- capacitance? 2 years 9 months ago #2206

  • david.b
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I've noticed that on applying drive power to SmartMotors, there's a significant initial current inrush; and similarly, when removing drive power from SmartMotors (particularly ones that aren't servoing), there's a significant residual charge to bleed off. (The context here is applying/removing power from -DE option motors using a safety circuit.)

My understanding is that this is due to relatively large capacitors onboard the motor; is this correct?

Short of trying to measure the capacitance, is there anywhere I can look up the capacitance levels for the different SmartMotor models? I work with a few versions, mainly the SM23165D/DT, but some SM17205D and SM34165D/DT as well.
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Re: motor specs -- capacitance? 2 years 9 months ago #2209

  • hsummer
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They are 800 uF or less.
The starting surge on a scope typically only lasts in the millisecond range. but small switch mode supplies sometimes have a problem with it. The smallest Switch mode type you should run should be able to handle the peak expected run time current which is a lot higher than power up inrush. Those models could pull 12 Amps on peak load with hard acceleration. Typical is less than 5 though.
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Re: motor specs -- capacitance? 2 years 9 months ago #2211

  • david.b
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Yes, I've run into the problem you mention because of an undersized power supply; but I've also had issues where the residual charge in the motor was enough to keep other low-power systems powered up for several seconds after I cut supply power. I've solved this by discharging the motor residual charge to ground immediately after cutting supply power (I assume this doesn't hurt the motor?), but this sudden discharge also sends a momentary power fluctuation through the rest of the system... Should a properly-sized power supply minimize this new issue too?

I take it the capacitance values aren't published. Is it safe to assume that larger motors have correspondingly larger internal capacitors than the smaller motors?

Thank you.
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Re: motor specs -- capacitance? 2 years 9 months ago #2213

  • hsummer
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What I listed was about the most on the largest.
Dropping power with a discharge resistor might be a bit easier. You can place a 10K bleed off on there to drop it down.
Last Edit: 2 years 9 months ago by hsummer.
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