Transformer max primary current draw question

[Chris Wilson]

6600 volts, 5.75kVA rating. On a primary supply of 240 volts max primary current draw, start up aside is 23.96 Amps, right? And max secondary current is 1.15 Amps? Just a sanity check on my maths and methodology! Thanks.

[madires]

Primary looks ok, but secondary is 5750VA / 6600V = 0.87A (if the secondary is 6600V).

[Chris Wilson]

Ah, yes, of course. The secondary windings are in series with access to the wire connecting the two bobbins. If I wire them in parallel for 3300V what current would the secondary then supply please? Cheers madires.

[madires]

Twice the current, i.e. 2 * 0.87A = 1.74A or 5750VA / 3300V = 1.74A :-) If you wire them in parallel make sure both secondaries got the same direction/orientation. Otherwise they would cancel each other out.

I hope you know what you're doing since 3.3kV isn't a safe voltage to play with.

[Chris Wilson]

I treat it with a lot of respect, I ask re the current as one person once said the twin bobbin transformer wouldn't parallel up. Well, it does, and assuming it's phased right seems to work just fine (the photo is DC through a home brew rectifier / capacitor board).  The supply has all the safety features Ian White GM3SEK advises in his various tomes on HV supply design, and I am still here Thanks for your concern though, appreciated. If we didn't play with dangerous toys though, we'd be pretty bored, wouldn't we?

[madires]

Good to see a reply from you  The DC voltage is expected to be sq(2) * 3300V = 4667V in case you like to know how to calculate it.

[Chris Wilson]

<LOL> That tallies with the meter then The valve needs about 4800V max, so that'll do fine. I have the primary on a switchable in / out BIG Variac, so for projects with a need for less than this brute can supply it can be toned down somewhat. It gives me thw willies just looking at it, so I do treat it with utmost respect. Until the valve is in use that this thing will supply, I have to just hope it can give the current required, as there's no safe way I can find to load it up.  Thanks for your help.

[ElectroIrradiator]

If you ask 5.75KVA from the secondary, then the primary power will be higher due to transformer losses. How much as a percentage will depend on the transformer construction and the materials used. Above 90% efficiency isn't unusual in a large, modern transformer.

The small cap sitting on top of the transformer suggest it is about 1960 vintage, or perhaps even earlier. Not sure I would trust a transformer this old for a project like this...

On the other hand, since the tube has neither handles nor is it intended to be bolted directly to the chassis, then this must still be considered a QRP project...

[Chris Wilson]

Hi, the cap is actually a thermo switch, bi-metallic type. It's out of a wood glue drying machine and is rated for CCS. I had to carry it upstairs in stages, and I am no weakling, it's a heavy barsteward! It has been dry stored and I put a Variac and mains on the secondary and loaded the primary with car headlamp bulbs, and ran it warm for a couple or three weeks. No flashes so far! It looked horrible when I got it, just wood dust though. Tube is a 3CX3000A7.  Here's the rectifier board I built, with the home brew PCB for the cap board behind it. I am a bit better with PCB manufacture now, that was my very first etch, using vinyl tape as a mask!

[madires]

Nice tube! 7.5V@50A for the filament and up to 5.5kW output. Are you building a pirate radio transmitter? 

[Chris Wilson]

It's an old American Henry RF generator that was used for plasma work. A US friend and I are converting it for amateur usage from its current 27 MHz frequency. He had a nast bike accident whilst here in the UK, so it's something to do to keep him sane whilst he recuperates

[nanofrog]

It's an old American Henry RF generator that was used for plasma work. A US friend and I are converting it for amateur usage from its current 27 MHz frequency. He had a nast bike accident whilst here in the UK, so it's something to do to keep him sane whilst he recuperates

That's what I call friendship.