The thing is that I want to test the efficiency of some DC-AC inverters but I have to use an adjustable load because the CEC or EU efficiency weighted calcs request to have the perfomance on 5%, 10%, 20%, 30% and so on.
I want to have a single device that can go like 3kW or 5kW if it is possible to regulate the current of it can test the inverter discharge to make the efficiency curve.
Of course the ideal process will be that it can go step by step on power, like 10W to 10W and so on...
One obvious low-tech approach would be to have a bank of switches, each controlling a string of incandescent light bulbs. If you put binary-weighted loads on the switches, you can switch them on in combinations to get almost any value of load.
For example:
This gets you 0 to 3100 W in steps of 100W. If you want finer steps, add a couple more switches with single 25 W and 50 W bulbs.
Obviously, this is going to generate a lot of heat!
Another approach for finer control would be to get a large Variac and put a single large resistive load (e.g., electric baseboard or electric water heater) on its output. The power consumed will go up proportionally to the square of the Variac output voltage.
I've knocked together some load banks in this power range before and we used switched high power resistors for coarse load steps and then a smaller feedback-driven adjustable load to dial in the current around the resistor tolerances and tempco changes.
You can buy 1kW and higher wirewound ceramic resistors fairly reasonably. Switching those would be simple. Adjustable ones (they have a band that you move and fasten down with power-off) are also available.
Assuming the inverters produce mains voltage and frequency, and if you don't mind a bit of inductance in the load, you might be able to use a 5kVA variac and a single resistor bank to make an adjustable load.
Of course if you have the budget, snazzy rack-mount AC programmable loads are available (for purchase or rent) and can be integrated with a test rig.
