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Control Single Phase AC power

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creative engineer

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I want to control single Phase AC 2-wire system's power (common household power) while keeping the terminal voltage constant.My intention is to utilize PWM output of my microcontroller to control the electrical energy supplied in a household.

Please suggest some strategy or anyone already done this earlier.

Thanks in advance.
 

betwixt

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Your question isn't clear, please give more information.

If you keep the terminal voltage constant, the power supplied is entirely decided by the load current (connected devices) and you cannot control that from the power source. I'm also not sure what you expect to do with PWM unless it is at significantly lower frequency than the single phase supply and you are essentially cutting and restoring power to control it's usage. That would make it petty much useless for household appliances though.

Brian.
 

creative engineer

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Thank you betwixt for your reply,

Actually,I want to build a feedback control system to keep the power consumption of a household or a particular appliance constant(w.r.t some reference value) by modulating the power supplied.

With this intention,I was toying with the idea of Pulse width modulating the power supply and making the duty cycle a function of control signal generated by the control system above.

regards,
Rupayan
 

betwixt

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I understand what you are trying to achieve but I don't think your idea will work. You would have to regulate the voltage to control the power consumption and that would have bad effects on household appliances. For example, a motor draws large current when starting up so with constant power feed it would drop the voltage and probably fail to rotate. Also consider that many devices (TVs etc) use switch mode supplies that work over a large input voltage range but draw more current as the voltage drops, these would work against your power regulation circuit. Consider also that several appliances may be on the same circuit, drawing more current into one of them would cause a voltage drop throughout the house.

The other worry I have is how you would implement PWM to control the power. Remember you are dealing with an AC supply so using triacs or SCR would only work in phase control mode and the distortion of the waveform may not be tolerated by appliances. You might be able to control the voltage if you are generating the AC yourself with a PWM driven inverter but that would be quite a complicated task and still wouldn't overcome the problems I already mentioned.

Brian.
 

creative engineer

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So,what is your suggestion/advice in order to build such a system?

Regards,
Rupayan
 

betwixt

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I would suggest you didn't build it at all. If your concern is that too much load is being placed on the incoming power lines you could consider an alarm system that warned of overload and maybe had a timed power cut off. I think trying to regulate power consumption would be counter productive when the load could change from nothing to maybe tens of Amps. Please explain why you want this device, why do you need to regulate the power consumption?

Consider the implications of constant power, if your incoming AC is 220V and your power is regulated at say 1KW, and your only load was a mobile phone charger (say 5W), the voltage would have to be stepped up to 44,000V !!! If you then switch on a motor rated at say 500W, the voltage would initially dip to almost zero then recover to 440V and if you connected something needing 2KW (a heater for example) the voltage would drop to only 110V.

Brian.
 

BradtheRad

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When I have controlled the voltage to appliances, it has been in the form of changing fan speed, or light dimmers, etc. That is, individual control of low power appliances.

It is a big risk when we put additional devices (especially if homemade!) between our large appliances and mains power. The risk of ruining something is great, and the expense would be great.
 

mtwieg

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Many modern appliances in a household, especially those with built in switchmode power supplies, are designed specifically to work well with a simple, passive voltage source. If you suddenly make the voltage source "smart," chaotic things will happen. For example a computer's power supply at some point may contain a control loop which wants to draw 500W, but your power modulator may want it to consume a different amount. The results will be the two control systems being unable to agree, and instability will likely occur.
 

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