AC motor rotary capacitor soft start

[artis]

I was wondering about , have there been or are there any systems that would use a large capacitance rotary capacitor/s as a mechanism for soft start of larger AC motors and also they would serve as power factor correction devices? The idea is basically simple, say one has a 3 phase motor and we have 3 identical rotary capacitors connected in series with each of the phases, the capacitors are set at their lowest capacitance when the power is applied and then they can be gradually turned to larger capacitance this way the current through the motor winding would rise proportionally to the rise in capacitance, such a setup would form a series LC but at some resonant point the whole circuit would become resistive and draw lots more current than an inductive circuit , so i guess that is one drawback I can see, are there any other ones?

Maybe the circuit would be useful at startup and when the motor has reached certain rpm the caps could be bypassed by relays directly to grid.

 

[anorlunda]

How about a second smaller motor of another type used to spin up the large motor?

 

[Baluncore]

@artis, you are dreaming.
The value of capacitance needed will be measured in millifarads or farads. Microfarad variable capacitors are simply not available.

The obvious solution would be to use a fixed starter capacitor that is removed from the circuit when the motor approaches half speed, by a centrifugal switch, which is how it is now done with small single phase motors. Larger motors that need a soft start will have a wound rotor with an external controller.

 

[artis]

I know that the capacitance needed for a large current transfer is large, I did not necessarily stated that this is practical bu more of a theory. I was wondering how in theory this would work.

Although these days with tight tolerances between the plates there are some newly made superdielectric fluids that are said to have dielectric constants up to about 1000 and more. I don't have the papers at the moment but if one wants I can find them.
Surely with such a dielectric the necessary plate area would decrease and such a rotary capacitor become more realistic would it not?

the beauty of it is that it would be very simple and robust, unlike the semiconductor voltage/frequency drives.

although for a AC motor at satrtup I guess one easy way to control it is to use a thyristor, which by cutting the sine would also limit the inrush current.

 

[Baluncore]

Surely with such a dielectric the necessary plate area would decrease and such a rotary capacitor become more realistic would it not?

Surely the breakdown voltage will be too low and the polarised capacitors will need to be used in sets of 4 to handle the AC.

One problem with LC resonance is that it can lead to very high voltages. The series capacitors used for starting will need to later become parallel capacitors to correct the PF.

I believe you have very little experience in the field and are looking for a hypothetical solution to a problem that does not exist in the real world. First read up on the prior technology and the history of the field before you get excited by an idea.

Can you please provide a reference to a 1uF rotary capacitor that will handle 400 VAC.

 

[artis]

This is a question , I'm not saying I'm hell bent on making it a reality.
The idea was just for startup so the capacitance needed is lesser, then after certain rpm are reached one can simple bypass the caps with relays and it's a done deal.

I cannot give you a reference for a rotary capacitor at those ratings, but there are plenty of 400v rated electrolytics around. dielectric properties and plate spacing , I don't see a problem there.

Sure if speaking practically I'm not dismissing your claim that such a system would be impractical , one reason could be size. The benefit of being robust doesn't probably outweigh the size issue.https://www.ncbi.nlm.nih.gov/pubmed/29624504
https://www.researchgate.net/publication/324265178_Magnetically_Tunable_Liquid_Dielectric_with_Giant_Dielectric_Permittivity_based_on_Core-Shell_Superparamagnetic_Iron_Oxide

Not sure if that study is real but if it is or at l;east close then that is a huge dielectric constant for a material, especially one that can be liquid.