I am in the process of designing an AC Power supply at my job to test our Surge Protective Devices. Thus far I have ordered a Multi tap Transformer to tap the 7 voltages that we need. We are going to apply 240, 380, 400, 480, 600, 690, and 1,000VAC at 0.5, 2.5, 5.0, and 10 Amps at separate intervals. I ordered an AC resistive load bank but the manufacturer called me back and said that he couldn't do 690 and 1,000VAC. I am now having to design a resistive load on my own. I am speaking to a manufacturer about high powered resistors and was thinking maybe a Rheostat would be better to alternate the resistances to get the selected current level that I need at the selected voltage. The tolerance for the currents at these voltages are +/- 10%. I have found a manufacturer to order Rheostats from. I was wondering what questions should I ask the manufacturer in regards to the rheostat based off of what I am trying to do ? Also what would anyone recommend for the load ? I just figured the high powered relays and high powered resistors would be alot to order and maybe a rheostat would be more simple. Well thanks to anyone who replies.

 

1/4" carbon rods are available in various lengths. They would require experimental measurement and calibration due to heating and cooling effects.

 

You need to know what current at what voltage, that will give you the Wattage needed for the rheostat.

So 10 amps at 1000v is 10Kw!!

 

You need to know what current at what voltage, that will give you the Wattage needed for the rheostat.

So 10 amps at 1000v is 10Kw!!

Yea I have all of that calculated. Besides letting them know about the tolerance for the current, and the wattage, I cant think of anything else.

 

I am in the process of designing an AC Power supply at my job to test our Surge Protective Devices.

1. What kind of surge protection devices are you talking about, and at what voltage levels do you expect them to kick in?

Thus far I have ordered a Multi tap Transformer to tap the 7 voltages that we need. We are going to apply 240, 380, 400, 480, 600, 690, and 1,000VAC at 0.5, 2.5, 5.0, and 10 Amps at separate intervals.

2. Will the load on the transformer be reactive or resistive, and must each of the taps be able to supply 10 amperes into the load and, if so, for how long?

I ordered an AC resistive load bank but the manufacturer called me back and said that he couldn't do 690 and 1,000VAC. I am now having to design a resistive load on my own. I am speaking to a manufacturer about high powered resistors and was thinking maybe a Rheostat would be better to alternate the resistances to get the selected current level that I need at the selected voltage. The tolerance for the currents at these voltages are +/- 10%. I have found a manufacturer to order Rheostats from. I was wondering what questions should I ask the manufacturer in regards to the rheostat based off of what I am trying to do ? Also what would anyone recommend for the load ? I just figured the high powered relays and high powered resistors would be alot to order and maybe a rheostat would be more simple.

If, at the low end, you want to push half an ampere through a load with 240volts across it, then the load's resistance must be R = E/I = 240V / 0.5A = 480Ω, and it must be able to dissipate P = IE = 240V X 0.5A = 120 watts.

At the other end, if you want to push 10 amperes through a load with 1000 volts across it, then the load's resistance must be R = E/I = 1000V / 10A = 100Ω, and it must be able to dissipate P = IE 1000V X 0,5A = 10000 watts.

So, if you choose that route, it looks like what you'll need to make it fly is a 500Ω 10 kilowatt rheostat.
Good luck with that.

Oops...

If the 100Ω segment of the winding needs to be able dissipate 10kW with 1000 volts across it, then, unless the element is step-wound, the presumption is that the entire winding must be able to tolerate 10 amperes through it.

Such being the case, a 500 ohm rheostat capable of handling 10 amperes must be rated to dissipate P = I^2 R = 100A X 500Ω = 50,000 watts.

 

Good luck with that.

Translation: Not frankly likely.
Bad idea. Way too expensive. Unnecessary complication. Less reliable. And the wattage rating on a rheostat is only good for the whole resistance. When you adjust it to less resistance, the power rating is less than it would be if you used the whole resistance. You are trying to use the wrong method.

 

When we had a similar test load a year back we took an array of baseboard heaters straight out of a home center store and erected them on a wooden frame. We placed the thing outside in the parking lot within a shipping container, which surprisingly turn out to be very affordable to modify and rent. 10KW may add some nice extra heat in the winter but summer time it will overload any AC you might have.

That said, what pray tell would "We are going to apply X VAC at Y Amps at separate intervals" mean exactly? I have an idea what an interval may be, but the rest is completely guesswork unless you flip over all the cards and explain what you mean. Schematics may well help here.

 

Thinking out loud, Two resistors rated for 500 V should be rated for 1000 V in series, for the most part?

 

To avoid having 1000 volts across the resistors have you thought of putting them in series with the transformer primary. It would mean that you would get different effective values as you changed taps on the transformer. I was going to suggest a similar solution to ErnieM's using oilfilled radiators or fan heaters (Modified with the fan wired directly to the mains.) but I suspected that the insulation from the element to earth would not be good enough to connect them in series for 1000 volts. They could be used in the primary without this problem. It would provide a very cheap solution to your problem.

Les.

 

I would not recommend using baseboard heaters beyond their rated vottage.

However a transformer or variac can step down voltage to match what the load wants to see.

 

I would not recommend using baseboard heaters beyond their rated voltage.

True. Baseboard heaters use Calrod with about 1/8th of an inch between hot and case ground. You can't push those to 1000 volts without arcing.