Big Maul primary voltage circuit discussion

[N3DRB The Derb]

the new Mauls I'm building/rebuilding are by far the biggest I've ever considered, and I'd like to see what others think about the right way to build the primary AC circuitry. I have 2 decks, one with a general radio 18 amp variac mounted on it, and a large 'steering wheel' as Todd called it, and another with a smaller 10A variac on it and a front mounted circuit breaker.

I'm going to re use both these panels so I dont have to do metalwork. I'm going to take the 2 variacs to the local motor rebuilding shop to get serviced as they do top work at super low prices, and do a better job than I ever could. The 18 amp has a frayed wire and some crud buildup at some points in the winding.

On a really big plate supply, (the first transformer, a Thermador - 2400-2000-0-2000-2400, is rated at 1446 KVA), weighs around 90 pounds. how do you big strappers wire for safety and keying? I assume some job rated contactors would come into play, some circuit breakers, and some fuses in the right areas. I've never had a variac in the AC line, would one use circuit breakers in front of the variac and fuses afterward to the primary of the xformer?

I think I know how I'd do it, but the hive mind here always helps me clarify my thoughts. TIA. 

[WQ9E]

Tim,

I would plan for some properly sized inrush current limiters for each of the transformer supplies to reduce the stress on the transformer, breaker, and secondary components.  Of course you may prefer to use the relay switched resistor in series type on the plate supply but I have been quite satisfied with the disc style inrush limiters.

I prefer breakers to fuses but you could use all fuses or perhaps a combination with breakers for the line input and then fuses for individual circuits.  Either will provide protection and the choice of which style is up to you.  Using inrush limiting will allows the breaker or fuse to be sized close to the actual load without concern for carrying a high inrush current.  If you don't use inrush limiting you will probably have to use either "slow blow" fuses or "motor start" rated breakers for some of the circuits.  You definitely should have some sort of fuse or breaker where the AC first comes into your transmitter (prior to the Variac) and then suitably sized fuses/breakers for the individual supplies.

For switching the main voltage input, a nice heavy relay is fine.  A key switch to control the relay is a nice cosmetic touch and does allow some control over unauthorized tampering with the rig.  I like solid state relays for switching plate supplies.  My Johnson Desk had a badly pitted contactor for the plate supply when I got it and since I already had a positive experience with the solid state relays used to control the HV supply in my homebrew amp I replaced the plate relay with a pair of these relays.  Most of them are designed to switch on at the zero crossing point and have built in surge suppression.  They are fairly expensive if bought new but are readily available from many of the surplus dealers at lower price.  They do require a proper heatsink and since all of the ones I am familiar with are single pole devices you will need a pair of them for 240 volt transformers.

It is great to see your jumping into all of these new projects, great work!

Rodger WQ9E

[W7TFO]

Hi Derb,

I favor mercury plunger relays.  They are cheap on eBay, and never crap out.  Putting them between the Variac and the transformer saves the 'double inrush' upon closure.

One of my old transmitters in storage now has a soft-start feature built into the path between the rectifier output and the filter.  A big resistor is shorted out after about 1/2 second to go to full poop.  Looks very buzzardly and uses a simple pull-in coil wound with high reactance so it takes awhile to get the plunger bottomed out.

[The Slab Bacon]

you may also want to consider leaving the HV up constantly and grid-blocking the circuits that are hooked to it (final and modders). This saves the constant on/off inrushes to everything for doing PTT operation.

this way all you have to do is switch the standby grid biass to something below the cut-off points for the tubes. this way you only have to switch a much smaller voltage and very little current, which is a lot easier than 120 or 240 @ many amps.

Or use HV relays if you can find them at reasonable prices and cut the HV off to the plates, but leave the power supply up. However, I dont care for HV relays as they can be troublesome

Either way, be sure to have good strapping bleeders for the HV supply.

[W2PFY]

1446 KVA

Do you mean 1.446 KVA? a 14 KVA single phase transformer would weight about 225 lbs. One rated at 1446 KVA single phase would probable be about ten tons.

 

[N3DRB The Derb]

duh. I put in a K where there is no K. 1446 VA it is. here's some shots of the sweet little things.

they are twice the size of the plate transformers in my first HB rig. By a mile. Both so large I wont be able to mount them on a deck.

the nameplate on the Thermo-Whore reads 3560 - 4680 CT @ 500 ma.

[KD6VXI]

1446 KVA

Do you mean 1.446 KVA? a 14 KVA single phase transformer would weight about 225 lbs. One rated at 1446 KVA single phase would probable be about ten tons.

 

My 12 A 8KV AC vault xformer is tipping the scales at......  Well, I'm not sure.  The "shipping weight" stated my engine cherry picker could pick it up, but it bent the arm when I tried, and the freight company came back the next day with it on a liftgate truck.  It was stated to be 4K pounds, but yeah, right.

My 75 KVA pigs, in oil (I haven't removed them yet) are tipping the scales at about 5-600 pounds a piece.

I saw the original post and thought the decimal was in the wrong place as well


--Shane

[N3DRB The Derb]

Frank,

I'm not going to be ready to use these both for a while. if you come on up and git it and let us buy you dinner at the track,  you can use the Thermo-Whore for the 4X1 rig until you get a replacement set up. 

This I would b most happy to do.  if the rig likes the Thermo Whore and it works FB and winds up being THE replacement, I'll just call up the source and have him send me another one.

let me know OM.

[KE6DF]

If you want to do something slightly exotic, you could use thyratrons as the rectifiers and then use the grids to switch the HV from on to standby.

Something like -50v would shut off the HV, and 0v would turn it back on.

Of course, you have the problem that the bias supply and switch has to run near the HV potential, but the current you switch would be nearly zero.

There may be thyristors (SCRs) that could do this also, but getting ones with a high enough voltage rating might be difficult and expensive.

5557 thyratrons are similar to gridded 866a's and 873 thyratrons are gridded 872a's.

They come up occasionally on epay and they go for less than regular MV rectifiers. And, it seems that the Chinese are still making them for industrial applications.

Has anyone out there built a power supply with thyratrons? I've read articles about it, but have never done it.

[k4kyv]

I use old style screw-in fuses on the primary side of everything, before the main power switch.  Then a set of fuses at the "big switch" feeds the station power circuit.  The main box in the house and the main one in the shack use circuit breakers.  I don't worry about fusing the variac that feeds the RF HV power transformer.  Anything that would overload the RF HV variac to a dangerous level would blow the line fuse that feeds it.

I don't use a variac in the modulator PS line.  They are notorious for degrading the voltage regulation. 

[w1vtp]

Derb

I'm going to be following this thread with great interest.  I have a similar plate iron (see attachment).  John, KC2FXE doesn't switch the primary.  He uses bias on everything.  I don't have the details yet -- Now I'm going to "pin him to the wall" about details. 

Switching the primary is really great for effect (KLACK!!! + dimming of lights for an instant) but it may not make any sense.  I'm thinking a combination of switching the bias to beyond cutoff , removing the RF and AF drive on RX might be a start, but I'm afraid of leaving the mod deck hot and unloaded, so I'm interested if anyone has any thoughts on that (suppose spark gaps would help out there but I don't like relying on that for protecting the mod iron).

Good luck and keep the pictures rolling.

Al

OH yeah -- the label's hard to read.  That's ~ 6KV CT with some taps with less voltage.  The primary can be used on either 120 or 220 VAC

[w3jn]

Tim, remind me to throw a bunch of inrush current limiters at ya when you stop by tomorrow.

[N3DRB The Derb]

10-4. 

[AB3FL]

Now those transformers are definetly "Heavy Metal"  Maybe call one Ozzy and the other one Ronnie!


Tom - AB3FL

[W1RKW]

With my 813 rig I didn't go with an auto step start though I thought about having a step start. Instead both the screen and plate supplies are step started using a variac at power up. The key is to make sure they are zeroed before applying AC.  Starting the transmitter manually doesn't bother me.  It's the amateur way.

I didn't use a circuit breaker as the main safety, used a fuse instead and all sub circuits have their appropriate rated fuses.  It's inconvenient but it's a safety mechanism nonetheless.

Also wired in an AC contactor and incorporated in an emergency stop switch, mushroom type. You want to be able to have someone who does not know your setup to be able to shutdown the system without any safety ambiguity.  I'm actually thinking about wiring everything in the station to an E-stop switch and a shunt trip breaker(s) then all ambiguity is eliminated which would include low voltage equipment.