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I an electronic engineer and renewable energy engineer specifically. I am working on designing a new turbine and needed some help with a much simpler idea I had.

A while back I was working with smaller 500 watt to 1000 watt wind turbines that generated low voltage (24/48 V) However I am working on a new design for a peak of 3.6 kw and I am currently using a chinese PWM controller for a steady 380v output. The alternator generates higher voltages but will require over speed of the generator. I'm looking to custom design a capacitor for a complete one piece regulator for this. I have some questions I could use some help on.

How do I determine the heat dissipation (in watts) of a capacitor I can't seem to find much information on this. Of course I have found tons of literature on how hot it can get and sometimes a max draw but it's rare. I would like to use a very large capacitor to stall the turbine above 380V (to prevent over speed) and to divert extra power to heat. This would be a rare need as the inverters i am using have a input range of 200 - 600v so it's rare that i need to reduce the voltage. It's more for the protection of the bearings. I've been looking at some Air Capacitors but generally those are very high voltage. I'm really needing a cap that is 380v - 480v and can clamp the turbine down when the winds are super high and prevent over speed. I can do this with some smaller capacitors but the heat builds up and they just can't handle it. I've tried some larger capacitors too but I'm just needing some help calculating maximum voltage clamping capabilities.

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  • \$\begingroup\$ I suspect you're going to need to furl, or alternatively feather the blades, rather than applying electrical braking to prevent overspeed. \$\endgroup\$
    – user16324
    Commented Sep 10, 2015 at 22:18
  • \$\begingroup\$ I've heard of "rewable energy" but never really fathomed out what it means and this seems like an ideal time to ask.... the floor is yours. For my part I promise to help if you can justify how energy can be renewed. As a starter can you provide a link to this capacitor that dissipates heat? \$\endgroup\$
    – Andy aka
    Commented Sep 10, 2015 at 22:18
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    \$\begingroup\$ I unfortunately have not found a suitable capacitor I have used the following capacitor with some success but it runs very hot and I know it's degradation is happening quickly. ebay.com/itm/… I have removed the plastic and strapped into a steel project box to try to cool it. I just want to find out if there is some method to find a capacitor that will work for me. My circuit is a 3 phase generator head wound for 380v to full wave rectifier to capacitor to inverter. \$\endgroup\$
    – Scott Stevens
    Commented Sep 10, 2015 at 22:27
  • \$\begingroup\$ I'm unsure if there is more information on the Zener effect of capacitors and some sort of heat dissipation calculation if anyone who really knows capacitors could chime in that would help thank you . \$\endgroup\$
    – Scott Stevens
    Commented Sep 10, 2015 at 22:32
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    \$\begingroup\$ As far as I'm aware capacitors aren't supposed to have a Zener effect... If yours is generating heat that is because it's been damaged (such as by too high a voltage). I guess maybe a damaged capacitor works well as a voltage regulator for your turbine but it's not what anyone would specify to use. \$\endgroup\$
    – Stack Exchange Supports Israel
    Commented May 13, 2018 at 23:28

1 Answer 1

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The alternator generates higher voltages but will require over speed of the generator.

You probably mean to say that you need overspeed regulation to prevent overspeed.

I'm looking to custom design a capacitor for a complete one piece regulator for this.

You are extremely unlikely to be able to build a better capacitor for 380 V AC in any reasonable capacitance than is commercially available.

How do I determine the heat dissipation (in watts) of a capacitor.

Ideal capacitors do not dissipate any power.

Real capacitors have some internal resistance and the "equivalent series resistance" (ESR) value gives you this. Power dissipation will be given by \$ P = I^2 R \$. I for a capacitor is frequency dependent and is given by \$ I = \frac {V}{2 \pi f C} \$. (These are RMS values.) Using these two equations you can work out the power. Note that the capacitor will have a derating based on frequency due to dielectric heating. This information will be quoted on power-factor correction capacitors which are designed to be connected directly across two phases.

I would like to use a very large capacitor to stall the turbine above 380V (to prevent over speed) ...

It is unlikely that you want to stall (stop) the turbine. It is more likely that you want to increase the load to prevent overspeed.

... and to divert extra power to heat.

If you want to generate heat then you need resistors.

I've been looking at some Air Capacitors but generally those are very high voltage. I'm really needing a cap that is 380v - 480v ...

Have a look at datasheets for power factor correction capacitors.

I can do this with some smaller capacitors but the heat builds up and they just can't handle it.

Either the ESR is too high or the dielectric is the wrong type for AC at that voltage.

I've tried some larger capacitors too ...

It sounds as though you are using trial and error rather than electrical engineering principles.

... but I'm just needing some help calculating maximum voltage clamping capabilities.

Capacitors do not clamp voltages. On DC they store charge. On AC they will behave as an impedance and pass current. The voltage and current will be very close to 90° out of phase and so little heat will be generated.

From your comments:

But the problem I am having is over heating and I am trying to properly size a capacitor for the task of using it as a Zener Effect.

Capacitors do not have a Zener effect.

Note: 'V' for volt. 'W' for watt. 'A' for amp.

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