5kW variable resistor for UAV project

[breakshift]

Hey. I'm doing a University project, and I need to get my hands on a load bank for a generator, for testing its performance. I need a purely resistive variable resistor capable of dissipating 5kW. I've searched the internet for such components but I've had no luck. My colleague did find two websites, but they are Chinese and French. Due to the time frame of this project I need it asap, so a UK based company is preferable. Does anyone know where I might find something like this?

I'll explain a little bit more about the purpose of the resistor: we're designing a gen-set system whereby an engine is powering a generator. Our job is to construct a test rig for the system and, well, test it! We're looking for a graph of rpm vs output power, so that we know how hard to run the engine to keep the UAV just above stall speed. The resistor will act as an artificial load to test the gen-set, so we can measure the voltage and current and hence power. We also want to simulate environmental changes, such as wind strength, and monitor the effect this has on the engine; and the variable resistor will allow us to do this.

I'm happy to answer questions if there are any.

Thanks very much!

Mike.

 

[gerty]

They aren't variable, but I've used water heater elements before for short term testing. The ones we used were rated at
4500watts @220volts. They wil burn out under long use if not submerged in water.

 

[kinarfi]

Check with your local Caterpillar dealer and Electrical motor repair Companies, Generator Sale Companies, ETC. A few of these should have what you need.
Are you working with 3 phase or single phase or D.C., what voltage are you generating?
Another idea is make a plug strip and plug in a hand full of electric space heater and unplug one at a time, should only need 4 of them at 1300 watts each, for more adjustability, put a dimmer switch on one of them. As far as a graph, paper and pencil and put your info in a computer and let it make you graph.

Knarfi

UAV? unmanned air vehicle?

 

[breakshift]

The generator is this:

**broken link removed**



I'm assuming it's single phase as it doesn't say anything about 3 phase in the description. We'll be sweeping the engine speed from minimum to maximum, so the generated voltage will vary accordingly. For the testing data we'll use LabView.

Thanks for both suggestions, I'll look into both the water heater and space heater ideas

But a high power variable resistor would be ideal, there must be something I can get from the internet! If I can find one on the net I can simply pass the link onto the supervisor and he can order it right away.

Edit: Oh and yeh, UAV stands for unmanned air vehicle.

Also, would commercial space heaters run off voltages other than mains? And more importantly, I imagine they wouldn't like frequencies other than mains...?

 

[Speakerguy]

Water heater elements are the way to go. I've seen people use them for testing pro audio amplifiers in the kilowatt range. Get yourself a bucket of water and several of them in series/parallel combinations to get the impedance you need.

 

[breakshift]

But won't they only work with their specified voltage/frequency?

 

[canadaelk]

5kW is a lot!
When I was a kid (more than 50 years ago) and the carnival came to town I always stole into the operators area. The big machine motors (DC?) speeds were controlled with big contraptions similar to an air-plate variable capacitor. The stator fully submersed in (salt?) water and the rotor-blades rotated to give the desired resistance (speed of the motor).
The noise and steam was incredible! Better than Halloween today.
Well, just a thought.
E

 

[shortbus=]

How about using an electric stove/oven. It's made to dissipate the heat into air, so no water needed.

 

[Speakerguy]

Here is the thread with amplifier testing. The guy says it's capable of 14kW, probably not very long at that power level though.

Measuring Amplifiers - AVS Forum

 

[crutschow]

But won't they only work with their specified voltage/frequency?

Resistive elements such as a water heater or radiant space heater (those without a fan) will work at any frequency down to DC, and any voltage up to their rated value.

 

[kinarfi]

If your running the generator referenced above, you will have a frequency change and varying frequency, which is strange. You could still use the water heater elements in water or the space heaters, but the fan most likely would run at different speeds as you vary the speed of the engine. You could disconnect the fans in the heaters and set up separate fans to move the air. The big commercial load banks are basically the same, just much much bigger and run at 60 or 50 hz and can have the blower run on a separate voltage supply.
here's a link for some resistors. **broken link removed**
kinarfi

 

[kinarfi]

Good thought Crutschow, I was thinking of the milk stool heater with the fan, but the radiant would be the way to go with the varying frequency. At 120 volts, you will have 46 amps and 23 at 240 at 5500 watts.
an Ohmite Rheostat, 1000W; 12In.Dia.; 25 Ohms; Open; 1600Vrms; 6.33A; 0.375In.Dia. Shaft; 335 will cost you about $650

 

[Reloadron]

There are plenty of commercially load banks out there much like these units. The model K490 would fit your needs.

Just about all units like the linked to unit use heater elements as the load(s). You could take the home brew route using elements much like these. Obviously you need to make stand offs and mount them and provide several fans for good airflow over the fins or they self destruct. I would also include an element in series with a pot to offer one variable load step. Maybe 4 EA of 1 KW 220 Volt and one 500 Watt in series with a 3 ohm pot rated for 1 KW (Available from Ohmite). As I see was mentioned. However, the pot needs to be in series with a heater element or when a 1 KW pot is way down around an ohm or less it will toast.

Load banks can get to be an expensive proposition as you can see.

I would also use voltage and current transducers to measure the voltage and current. Something like these for the AC current and then something like these for the voltage. You could use a tach pickup for engine speed and run the tach signal through a frequency to voltage transducer. That would give you 3 signals to plot and you could run them into some paperless chart recorder software.

A much less expensive way to go about this would be to find some heater elements from home driers and use them as loads. You will still need fans and plenty of air flow over the elements.

Ron

 

[breakshift]

Looks like buying heating elements would be too expensive - we thought we might make our own coil wound resistor. Does anyone have any experience with this, and if so could you point me in the direction of a good website for this kind of thing?

 

[Reloadron]

Looks like buying heating elements would be too expensive - we thought we might make our own coil wound resistor. Does anyone have any experience with this, and if so could you point me in the direction of a good website for this kind of thing?

Then you may want to look at some cheap space heaters as mentioned. I have an old radiant quartz heater with 1.5 KW and 750 watt settings I use to load my old 5 KW generator to test it every now and then. Home use space heaters can be had pretty cheap. Even several electric bread toasters might be a start or electric hair driers.

If you choose to wind your own elements or coils you would start by looking for nichrome wire, then choose a gauge and size noting the resistivity and work from there. You will need something to wind it on. Ceramic forms are nice if you can get them. Anyway, nichrome wire would be a start.

Ron

 

[breakshift]

Yeh I'm looking into that now - the finned strip heaters website you mentioned is the kind of thing I need, but based in the UK. It'd be better to buy just the heating elements (instead of buying a heater meant for the home) as it'd be cheaper and easier to work with. Do you know any UK companies selling these? I'm searching google but whenever something interesting comes up it's always 'contact sales department' for prices which is really annoying :/

 

[Gary B]

Check this site out. Power rheostats - 25 W - 500 W, max. 5KΩ / 300 W - 4 kW | FVR / FVRB series - MF Power Resistor

 

[Reloadron]

Actually I linked to Omega Engineering for the finned strip heaters and they also have Omega Engineering UK from what I see here. That may help and they may be a source of nichrome wire in the UK. However, sine I am here and not there I am sure plenty of our UK members could better direct you.

Ron

 

[breakshift]

Thanks very much for the help guys, I really appreciate it. If I'm correct I could use the following element:

Strip Heaters

Using 5 of the 'HCS-237' elements I would have a total power rating of 5kW. As far as I'm aware, as long as these elements dissipate the required amount of power (and I can use a potentiometer in series to control the load more precisely), I can go for the cheapest combination. I'm not sure if these heaters require fans or not, though.

What do you think?

 

[audioguru]

I tested high power audio amplifiers into a very powerful speaker and nearly broke all the windows.
One of the name-brand American amplifiers smoked because it didn't have enough heatsink.