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Topic: Korad supply Limitations (Read 640 times) previous topic - next topic


Paul__B

A collapsing field in a motor generates a reverse EMF just like an inductor.  The flyback diode gives that reverse EMF a path to dissipate as the field collapses.
And that is my point.  The two situations are entirely different.  There is no "collapsing field" when you turn off a spinning motor like there is with a simple inductor.  The field in the coils is alternating in an approximately sinewave fashion due to the rotation of the armature, not due to the applied power supply.

The only "stray" inductive effects are due to imperfections in the design and materials of the motor, and are unlikely to generate potentials anywhere near the magnitude of the supply voltage except in "edge" conditions (possibly at stall).  You may get some "kickback" and it is reasonable to use a diode to protect semiconductor switches against this, but it is completely wrong to suggest there will be a substantial effect comparable to controlling a simple inductor.

raschemmel

#17
Aug 15, 2019, 12:53 am Last Edit: Aug 15, 2019, 12:53 am by raschemmel
So your point after all this time is don't treat a motor like a relay or solenoid just because it's inductive.
That doesn't make it an inductor.

SteveMann

Treat all inductive loads the same- use a flyback diode.  There's no downside to including the diode.
I am usually so far out of the box that most people don't know what I am talking about.

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Paul__B

So your point after all this time is don't treat a motor like a relay or solenoid just because it's inductive.
That doesn't make it an inductor.
Something like that.

Even with the inductor, I find it surprising that even educated engineers can resort to "magical thinking" about the situation, with assertions about "current surges" and putting the diode as close as possible to the inductor because the inductor "generates" the surge.

That turns out to be an absurdity.  What generates the transient is not the inductor but the switching device, either a mechanical contact or a semiconductor.  The inductor - as a response - acts to maintain the instantaneous current flow by generating the "back-EMF".  So you provide an alternative path for it to do so through the diode.  It is still the case that the current through the inductor and its connecting wires does not change rapidly.

What does change rapidly is the current through the switching element and the power supply which suddenly drops to zero, and the current through the diode which as a consequence suddenly rises from zero to that same current.

The significance of this is that if interference is going to be caused by electromagnetic radiation from a suddenly changing current, that suddenly changing current is located in the loop formed by the power supply (or the local bypass capacitor), the switching element and the diode but not the wiring between the diode and the inductor.  The need is thus to minimise the length of that supply - switch - diode loop by placing the diode as close as possible to the switch and power supply (bypass).  Suggesting you need to place the diode close to the inductor (or motor) is actually quite wrong!  :smiley-eek:

On the other hand, there is a voltage transient caused by the switching which can capacitively radiate interference.  This impulse is - again - caused not by the inductor but by the switching element so it actually radiates - possibly counter-intuitively - from the switching element to the inductor however to all intents and purposes, all points on the wire connecting switch, diode and inductor experience the same transient so this is not affected either way by the location of the diode.

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