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[2E0ILY]

I am modifying a Henry 3 phase PSU to run on single phase. That aside, and generally speaking, what are safe and reliable ways to load a circa 4kV 1 amp plus HV supply up to check how well it holds its output under load, without connecting it to the amp RF section itself? I have a HV probe and meter, I am more worried about the correct way to apply some form of a dummy current load to the HV output of the supply. Thanks.

[WB2WIK]

That would be a 4000 Watt load.  Kinda big.  I'd use the tube(s) the supply would normally power.

Of course four long 1000 Ohm, 1000W resistors strung in series would be a good load.

[TANAKASAN]

I would seriously consider contacting the engineering department of your local power company. They're used to handling voltages like this and they have the right test equipment.

One mistake here and you're SK.

Tanakasan

[ZL1BBW]

Crude, but 40 x 110v 100w light bulbs all soldered in series would do it.  Just keep them lifted well away from earth, and mind ya fingers.

[W8JI]

Chris,

Are you sure you are being safe?? This stuff can kill you.

I can't believe the advice being given here. He specifically asked for a "safe and reliable way" to test the supply.

He has what was a 4000 volt choke input supply that he is "modifying", and by the question isn't even experienced enough to know how to test it.  

If he doesn't understand how to test it, he most likely does not know about bleeder resistors and critical inductance and resonance. If he doesn't understand critical inductance and bleeders and resonance, then he shouldn't be modifying a three-phase supply to single phase and he especially should not be poking around with power on.

The only time I was nearly killed was working on a choke input supply. Because the choke went on the wrong side of resonance, the supply would surge to some God awful voltage on start. It blew a wire in two, and that wire snaked out of the supply across a couple feet of air and attached to my wrist.

...and we are suggesting resistors and light bulbs outside the case......  

[2E0ILY]

The resonant choke and cap is being removed, a capacitance only fileter set up fitted, with bleeders. I need to see how well it holds up with current drawn. Yes, I know it's dangerous, and I will take care. If it kills me I die doing something I find interesting and I like doing, but I don't intend to be going anywhere far for a while yet Thanks. I haven't rushed out to clear the shelves of 100 Watt light bulbs (not that I would find any incandescents nowadays in that wattage), nor have I lugged it to the vixen on reception at Scottish Power with a mains lead in one hand and a fiver for beer tokens in the other It's hard enough speaking to a real person when I want to pay them, let alone finding "Fred" in the engineering department willing to test a total stranger's HV ideas for him. I do appreciate the concerns with safety, but like with the first time you cross the road on your own, there's an element of risk in gaining the experience to be confident and become "expert" at anything, one just needs to be initially ultra circumspect.

[K8AXW]

BASIL:  That has to be a ham's worst nighmare! A 3-phase 4kw amplifier.  What in the hell do I do with it?   

[G3RZP]

The choke should not be tuned to resonance or you will get the same effects that W8JI did. The idea is to tune thye choke slightly HF of resonance so it looks like a much bigger choke than it really is: that way, you get above critical inductance at the lower currents. To do that really needs a good bridge to measure the values of C and L, or else an ACCURATE LF generator so you can adjust things at low voltage.

I have done negative lead filtering with this technique and then  used a rectifier to get  a negative bias voltage. BUT...

I don't recommend this tuned choke exercise unless you have experience with HV supplies and really understand what you are doing. Remember too, that the 'tuning capacitor' needs to be able to handle a fair amount of AC, both amps and volts.

[W8JI]

The resonant choke and cap is being removed, a capacitance only fileter set up fitted, with bleeders. I need to see how well it holds up with current drawn.

You have a ready made load available.

Plug the RF deck in, get a dummy load and load to power. Or better yet, run the thing completely non-resonant with fully meshed tune capacitor or shorted tune capacitor, and watch the built in HV meter as you quickly do a heavy load test. Even 30 seconds at 4 kW dc load won't hut a 1000 watt or larger external anode tube from a cold start.

If you don't want to use RF to drive the tube, it becomes very difficult and expensive.

 

[2E0ILY]

OK, certainly seems any alternative will be complicated. I don't know that the 3CX in the RF section is OK or not, guess it will be a double test Thanks for the help.

[KE4DRN]

Not sure of your location, are you close to a
university, tv or radio station?

Contact the engineering dept and see if they
can help you test the unit with a large
dummy load as W8JI recommended.

If you decide to test the unit yourself,
be sure that you have somebody there to
watch you on the outside chance you get into
trouble, it could save your life.

73 james

[W8JI]

OK, certainly seems any alternative will be complicated. I don't know that the 3CX in the RF section is OK or not, guess it will be a double test Thanks for the help.

I take it that is a 3cx3000 series tube, which is 4kW long-term average dissipation with full rated airflow. That tube has a very long life and can handle severe overloads. Because of the anode mass, the tube can easily dissipate 12 kW for a minute or so if you provide 3-5 minutes cool down. It is also exceptionally clean and reliable. You should be able to test just fine by shorting the tank input so the tube drives a short through the blocking cap. This will let you load the supply to normal dc load for a dozen seconds or so and watch the panel meter for HV sag.

I'm assuming you built the supply to run maybe 3 amps or so.

If that is a smaller tube like a 3CX1200 series or 8877, then the same thing applies but at less power. The 3CX1200 style has about the same headroom ratio as a 3cx3000, and can handle about  5 kW short term dissipation without even getting close to heat issues. The 8877 has a bit less thermal mass, so you have to be a bit more careful with overload time.

The 8877's most vulnerable area is the cathode and grid. With excessive grid current, an 8877 can be toasted in about 30 seconds or less. You can also strip the cathode easily.

None of the directly heated tubes have those issues.

[KC8RPD]

Why modify the supply when it's likely more cost effective to run it from a phase converter?  I run my machine tools on a rotory phase converter, and while the three phase power isn't quite as "good" as utility 3ph, it's plenty good enough.  Lots of info around on how to make a converter from a readily available 3ph motor.

[2E0ILY]

I have a single phase to 3 phase converter using a 10HP motor, and use it to run my ramp (vehicle lift), lathe and Brridgeport universal milling machine, surface grinder, etecetera. It struggles with the lift, I doubt it would run this thing. Plus it drinks electricity as it's not very efficient, and it's in my workshop, nowhere mear my radio shack. I suppose I should have tried it, but single phase strayight from the mains is a lot easier for my situation. Thanks for the idea though!

[W8JI]

I have a single phase to 3 phase converter using a 10HP motor, and use it to run my ramp (vehicle lift), lathe and Brridgeport universal milling machine, surface grinder, etecetera. It struggles with the lift, I doubt it would run this thing. Plus it drinks electricity as it's not very efficient, and it's in my workshop, nowhere mear my radio shack. I suppose I should have tried it, but single phase strayight from the mains is a lot easier for my situation. Thanks for the idea though!

I have a "static" converter on my lathe, but it is only really a starting system with a run capacitor. It does NOT supply three phase. It supplies about 50-80 degrees phase shift instead of 120. The phases are around 0 -60 180 degrees, not 0 -120 +120 degrees from a reference wire, but it is good enough to start the motor. It actually gets closer while running if I pull the run capacitor leads.

None of these things, even a motor-generator converter, are suitable for an application like this. The only thing that would electrically work would be horribly inefficient and very costly. It would take a 10-20 HP motor to marginally run the Henry. A static convertor would be an unpredictable mess as load changed, and never be close to making +120 -120 degree shifts.

The cheapest system is to replace the transformer, if someone wants it to run correctly.