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Originally Posted by e*clipse
Ok - a couple more dummy questions from the peanut gallery...  |
If you are asking, others are wondering so thank you for asking!
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I can see why one would want to *precisely* tune the PI loop for a servo motor. I WANT MY ROBOT arm ***here*** yesterday!
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Agreed
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Other things can have much more relaxed requirements. I'm setting up a VFD for an industrial ag pump. In this case it's actually better to slowly increase speed.
It seems to me there's lots of room for a more relaxed set of requirements in a car. For example, there's all the slop (gear mesh, etc) and inertia of the drivetrain. Then there's the mass of the car. It seems that even with a perfectly tuned servo-motor type control, you'll still be limited by physics for the car.
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A pump is not a big deal. We have applications with small pumps where we use Volts/Hz, a recipe instead of a tuned loop. You can change out a motor without retuning. In an emergency case, you can run two smaller pumps in parallel if you don't have a spare larger pump ... stuff like that.
For a screw conveyor (auger) or a conveyor, you have situations where the equipment stopped while fully loaded. So you need the regular torque (to move the product) plus some extra torque to start things moving, plus maybe more if things have cooled off and the product becomes harder to move. For this, you need the controller to track rotor position and to control the slip. If the slip gets too high (rotor not turning or torque has passed the knee of the curve) the controller needs to bump up the voltage and back off the frequency to avoid stall, then resume ramping up the frequency when the slip goes back to normal. We use a target speed instead of a target torque ... so things are a bit different ... but the controller controls torque to reach a target speed ...
Paul could likely re-phrase that description in terms of Id and Iq.
From my limited knowledge of the what's *REALLY* happening, that's the best I can come up with for the reasoning. The industrial controllers can somehow figure out rotation speed and slip based on measurements of the motor back EMF. We don't use encoders - they are unreliable in our conditions. I'm told that the sensorless control is a bit less accurate, but is accurate enough for our purposes of generating torque for startup. As you say, being out by a degree or so on the angle of the rotor is not an issue for us.
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Could the auto(mobile) tune make use of this?
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Not sure - just looking for the most torque I can get out of my motor
