Resistor Wattage - How Long Sustainable?
Resistor Wattage - How Long Sustainable?
(OP)
Hi All,
I've got a project that uses a rectifier, and outputs to 5 locations.
I need to build a load cell for each output. The resistance value will be 13Ohms. Power dissipation through each load will be 32.5KW for roughly 15 seconds.
My question is, how would you go about selecting a resistor capable of this. I have checked around and it looks like 1000W resistors are about the max of what I can find. I assume I wouldn't need a bunch of 33kW resistors....but how do you know the minimum you can get away with without burning up the components for x amount of time?
Thx much
I've got a project that uses a rectifier, and outputs to 5 locations.
I need to build a load cell for each output. The resistance value will be 13Ohms. Power dissipation through each load will be 32.5KW for roughly 15 seconds.
My question is, how would you go about selecting a resistor capable of this. I have checked around and it looks like 1000W resistors are about the max of what I can find. I assume I wouldn't need a bunch of 33kW resistors....but how do you know the minimum you can get away with without burning up the components for x amount of time?
Thx much






RE: Resistor Wattage - How Long Sustainable?
There are short time ratings, but it's going to depend on the resistor construction.
Ohmite and Post-Glover come to mind....
RE: Resistor Wattage - How Long Sustainable?
http://www.avtron.com/load_banks.htm
Is your voltage 2 kV? If so you'll need to consider arcing, etc.
RE: Resistor Wattage - How Long Sustainable?
RE: Resistor Wattage - How Long Sustainable?
What is the point of this test? Is there no alternative? Are you just trying to test the rectifier function? That would take 17ms not 15 minutes.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Resistor Wattage - How Long Sustainable?
What about a bunch of electric hot water heater elements wired up for your test?
Wire up a series string of three 4500 W heater elements, with each element drawing about 18.7 A across 240V (~12.8 ohm element), or in your case 16.9A at 217V drop.
Use 3 'strings' in parallel for 50.7 amps, give or take, depending on the actual resistance of the elements.
(I don't know what nominal voltage elements are spec'd at 240V or 230V or 220V?)
I do know that hot water heater elements are very inexpensive, so inexpensive that I can't recall what I paid two years ago for one.
Dan
RE: Resistor Wattage - How Long Sustainable?
www.cressall.com
The applications guys at Cressall are normally very helpful.
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I don't have an attitude problem. You have a perception problem...
RE: Resistor Wattage - How Long Sustainable?
We put together a bunch of them in a cabinet with a shunt for current measurements, and with switches to enable us to switch strips in and out of the circuit to vary the load as needed.
old field guy
RE: Resistor Wattage - How Long Sustainable?
A heater from a hotwater heater must be submerged in water to keep from burning out. Turning electricity into heat is often the easy part. Getting rid of the heat makes things more complicated. If you generate heat in any resistor without providing cooling it will evetually become a light bulb!
RE: Resistor Wattage - How Long Sustainable?
We have 480 3phase going into the rectifier assembly, 640Vdc out per built in rectifier. Currently the rectifier assembly has 2 rectifiers built in, with the potential for 5 down the road. The test procedure hasn't been confirmed, but the most demanding possibility is a 50A draw on each of the 5 rectifiers simultaneously.
Heater elements are the way to go, cooling can be added to the enclosure holding them. (Fans nothing fancy) We currently use 400W 16ohm elements in our production lines. Not hot water, used for heating air in HVAC units. I was hoping we could manipulate those to handle the requirements above. You'll have to give me a bit of slack, I'm a computer engineer...my background is a bit sore with power calculations past what I've given to figure it out...
:-/
One of the "theory" questions I guess that would help me...I'm not sure how watts are dissipated by these resistances. Is it cumulative how much wattage the load could handle? (Two 400W resistors could continually handle say 600 or 800W vs. 400W regardless of configuration?)
RE: Resistor Wattage - How Long Sustainable?
Two 400W can dump 800W as long as you don't somehow constrain their ability to dissipate the heat.
I once soldered wires to (100) 100W light bulbs for a load bank [saving on the (100) sockets] I then hung them in bunches like grapes. Fired up the controller which ran about 8 seconds before the solder start melting and the unsoldered wires started springing loose. I lost a lot of semiconductor fuses before I could hit the power switch.
Took a large fan and some separation then they worked fine.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Resistor Wattage - How Long Sustainable?
You should connect these elements in parallel go get multiples of this output
So , for example, if you need 30kW (30000W) of power, and you have 5000W 480V elements, then you need 6 of them in parallel to make the 30kW.
If you need balanced 3phase, then connect 2 of these in parallel in each of your 3 phases.
CAVEAT: You are playing with some fairly substantial energy levels here. Ensure that you have appropriate measures in place to deal with the heat levels you will be making, and with the current levels you will be producing!
RE: Resistor Wattage - How Long Sustainable?
(I which this forum allowed for edits after submission ;) )
RE: Resistor Wattage - How Long Sustainable?
Mike Halloran
Pembroke Pines, FL, USA
RE: Resistor Wattage - How Long Sustainable?
RE: Resistor Wattage - How Long Sustainable?
I would be thinking more along the lies of a giant electric jug or electric kettle. These can pour maybe 2.5Kw into a couple of litres of water for considerably more than 15 seconds. The elements are fairly frail too.
So how about using some 240V 1.2Kw standard radiator bars submerged in sufficient water (or oil) to absorb the heat? These bars can dissipate 1.2Kw in air, at 480v they should easily dissipate four times that power if submerged in water.
Perhaps seven 240v 1.2Kw radiator bars in a couple of gallons of water should be well up to the job for a once only 15 second "shot". 4.8Kw per bar x7 = 33.6Kw
As they will certainly not be glowing red hot, the resistance will be lower, so maybe eight bars will be required? A little experimentation should get you fairly close to your requirements, and you can always strip off a few turns of resistance wire to lower the resistance.