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Fuzzy_Chom t1_iypgbl1 wrote

Really what you're trying to do is vary the motor speed and torque, not so much the current.

At that point, the best way to do this is with a VFD (though you'll need a rated motor). Variable Frequency Drives are sometimes called Variable Speed Drives for just that reason.

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Knackmanic OP t1_iyrmixi wrote

Is it possible to "guess" the flow/pressure just knowing the RPM and supplied power ?

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PerspectivePure2169 t1_iyu1mei wrote

It's possible to calculate it, yes. But under most circumstances not with enough accuracy to use it as the control input. There's just too many variables.

What you really want to know is either flow or pressure, and measuring current is too far abstracted from that.

I can see wanting to use current in place of adding a flowmeter, but pressure transducers are cheap and super reliable, so if that's what you want to control for just measure it directly.

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Knackmanic OP t1_iyu4l2t wrote

Thanks, you basically answered it. Just out of curiosity, why is it so variable? Shouldn't you expect power and RPM a direct measurement? For the sake of knowledge ;)

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PerspectivePure2169 t1_iyu6iwt wrote

A good question.

There's going to be a lot of variation in current vs pressure because there are 3 energy form changes (electric to mechanical to fluid flow) and many outside influences.

Starting with electrical- is the operating voltage 115V? Or is it 121V or only 109V because someone is running a toaster simultaneously on the same circuit? All those will effect current and yet have nothing to do with pressure.

Is the motor warmed up or in a cold room? The shaft and pump seals, how does temperature, mineral buildup etc effect them? How often does the operator clean the machine? Does the grind of the coffee or make of filter influence back pressure or temperature or something else? All this, and yet even if you figure all that out it will change as the parts wear over time.

So what you will run in to is that there are so many things that effect current that you will never be able to entirely trust it as a measurement for pressure.

So the best possible solution is the simplest - avoid all the guessing and go right to the thing you want to know. It also allows you to build a check into your system if you want - of actual known pressure.

That can be helpful for safeties, interlocks, warnings etc.

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