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| I need a method to control an AC load |
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| jackbob:
Hello all, I am working on a project for school and I am looking for a way to control an AC load. Here are some of the relevant details and caveats. The load is just a simple bank of resistors being fed by a transformer. The voltage is low (12V RMS). The load is about 100W with four channels of the same circuit so 400W total. I am looking for a way to control the power to the load resistors and would prefer an efficient way using power electronics or some other clever way. This is a small part of a large project so I would prefer to not make the solution too complicated but I am open to all ideas. Seems simple but I tried using a triac phase controller and ran into a problem. We are using many current sensors in this project which monitor the AC current at various points and the transducers we have do not seem to work with non-sinusoidal current waveforms. After doing some research, it seems only true RMS current transducers can measure the non-sinusoidal distorted current waveforms produced by SCR's and triac's. These current sensors are very expensive (especially considering the project is using 16 of them) and most of them have an unacceptable latency. All this leads to a solution which requires the current waveform to be relatively sinusoidal with the AC voltage. I have thought of rheostats but I hate the way they feel (like a scratchy, rusty potentiometer) and they will dissipate quite a bit of power themselves, especially when their resistance is equal to the load bank resistance (maximum power transfer theorem). I have also considered a small variac but these seem very specialized and almost impossible to find for this application. It is very difficult if not impossible to find small variacs (~100VA ) that can pass a reasonable amount of current. Most variacs are made for mains voltage and would be too large for the application. Also, it would be nice to have a power electronics solution as the load could be adjusted through the computer interface rather than manually with clunky knobs. Can anyone think of any other solutions to control the load power? Your help is much appreciated! |
| capt bullshot:
Simple solution: Use rheostats or an appropriate oversized Variac (current rating must match your expected maximum current, voltage doesn't matter too much) Electronic solution 1: Use a PWM controller to "chop" the current using a back-to-back MOSFET switch. If the PWM freqency is high enough, some simple LC filtering should provide you with a near sinusoidal current. Electronic solution 2: Use a "R mode" electronic load and rectify the voltage before applying the load. If your electronic load is good (fast) enough, the current waveform will follow the voltage waveform in R mode, resulting in a sinussoidal current with some crossover distortion near the zero crossing. The power will be dissipated within the electronic load, no need for resistors. One can build the electronic load on his own, or buy one. I've seen AC electronic loads, but I think they're rather rare. |
| jackbob:
I have thought of both those solutions, I think I just needed to hear them from someone else. I do have some electronic loads but the project itself will likely be demonstrated many times and later given to a different group I dont want to give up my electronic loads with the project :D As for the PWM solution, it seems good but I cannot apply an AC voltage to mosfets, would you suggest passing it though diodes in series with mosfets then using PWM? I cannot add filter tank capacitors past the diodes as then the load will become very nonlinear. Are you thinking of something similar to an active PFC controller or are you saying just use PWM to chop up the instantaneous AC voltage? By passing the voltage through the diodes I will automatically have a deadzone of +-0.6V around the zero crossings but that may still be manageable. |
| Zero999:
How about putting the variac before the mains transformer? |
| jackbob:
Thanks for the suggestion, however, I cant put a variac before the mains because each channel must act independently. The mains transformer has four 12V secondaries and each one needs to serve these variable loads but also needs the 12V available for other functions on the project. To give a little more background so that it makes sense. The project is simulating a small electircal distribution grid and will also simulate protective relay schemes to respond to faults. Each 12V circuit will have the load resistors monitored at different points but will also have another fixed load resistor bank of very low impedance which needs the full 12V to create the necessary current magnitudes. The purpose of this resistor bank is to simulate faults and the current sensors will detect this spike in current and reconfigure the circuits to redistribute the load to isolate the fault. |
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