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Double Resonant Precharger Circuit Question

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TeslaRay

Sat Mar 15 2014, 05:37PM

Registered Member #30606 Joined: Mon Jul 29 2013, 01:55PM
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Posts: 5

Good Afternoon Everyone!
We are building a large Double Resonant Tesla Coil for university project and have a question about a Precharger Device.
I see that many coilers are using these precharge devices for their Double Resonant Tesla Coils.
Here is an example from a Eric Goodchild that was found through a quick GOOGLE search.

I guess my question is that is the only reason to use these to provide a soft charge of the main power supply across the bridge of the igbts?
From looking at this, it appears that there is a relay switching in a large value resistor to do this and then once charged up, the resistor is switched out.
Also, what is the purpose of having two large electrolitics in the precharger circuit itself? Is it better to double the voltage in the precharger circuit and then send out a doubled AC voltage to the main capacitors across the igbt bridge?
In that case, is voltage doubling only occur in the precharge circuit and not at the main Tesla coil itself?

Goodchild

Sat Mar 15 2014, 05:42PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Hello and welcome to the forum.

As it happens I'm Eric Goodchild. There are two reason behind those prechargers. One to double and rectify the 240VAC mains to 680VDC. And second to precharge the bus to 680V thru the ballasting resistor. Once we are up to 680VDC we bypass the resistor with a relay.

The caps are part of the doubling circuit, (two caps two diodes).

TeslaRay

Sun Mar 16 2014, 01:53PM

Registered Member #30606 Joined: Mon Jul 29 2013, 01:55PM
Location:
Posts: 5

good stuff

and is there a reason why the doubler is external to Tesla Coil as opposed to putting it right at the igbt bridge?

and in this configuration as you show, is there any special ratio that the doubler capacitors have to be with respect to the bulk storage at the igbt bridge?

Goodchild

Mon Mar 17 2014, 12:01AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

The only reason I do it is to reduce the size of the transmission line to the coil. At 680V the current is much lower for the same amount of power, in addition the power factor for that section of the line is also close to 1, due to it being DC. This helps a small amount with losses.

Dr. Dark Current

Mon Mar 17 2014, 09:56AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

That also didn't make sense to me. In the wall and outside, you have hundred meters of cable, a few meters more will not do anything and you will save caps. Also, a simple resistor and a relay for the soft start is enough. The resistor needs only be rated at a low power (say 20 watts) because only thermal capacity plays a role. This circuit can be integrated into a very small space in the coil.

Thomas W

Mon Mar 17 2014, 11:14AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Goodchild wrote ...

The only reason I do it is to reduce the size of the transmission line to the coil. At 680V the current is much lower for the same amount of power, in addition the power factor for that section of the line is also close to 1, due to it being DC. This helps a small amount with losses.

Thats actually a really good idea, meaning you only need a few meters of 50A cable and you could do the rest in 16A cable instead, saves a resonable chunk of money considering
64A SY flex is around Â£5 per meter here and 16A flex is around Â£1 a meter or so.

HV Enthusiast

Mon Mar 17 2014, 02:16PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Hi TeslaRay. You didn't say what your DRSSTC output power is, but if its a med-power DRSSTC (such as in the range of a few kW), then you can use thermistors instead of a switched in inrush resistor. Simply use two thermistors - one on each leg of your input AC. These work great and are used in appliances for the same exact application. At cold, they have a high resistor (typically 1-10 ohms), but once they heat up and are under load, they decrease to a few hundreths of an ohm. They are rated up to 50+A current. Of course, if you are making a high power DRSSTC, then the pre charging circuit would be a better solution.

Goodchild

Mon Mar 17 2014, 03:17PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Thomas W wrote ...

Exactly right!

I just threw the PF thing in because it's a fun fact. It doesnâ€™t save a whole lot of power. Maybe a few mW saved at the most. The main reason is reduced cable size. Savings in both cost and weight of the cable.

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