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3 phase automatic changer

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richa singh

Newbie level 2
Apr 27, 2011
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if low voltage i.e. voltage outside prescribed range is found in any of d three phases this circuit automatically changes to the oder avilable phase.
i m doing dis project for final year need immidiate help... may b wid microcontroller 16F676 ... plzz send some immediate help :-?

Industrial control panels use a proprietry relay for that purpose, not much use to your b tech that though.
If I was to do that I'd use something like a pic12f675 8 pin microcontroller, and use 3 analogue input chanels, one to each phase, you could connect the 3 phases directly through a voltage divider, or more safely through a 3 phase transformer, or more available 3 seperate single phase transformers.
This way you could also monitor the phase rotation.
The pic can then give an output to control 2 relays or contactors so it can select which power source to use.

You don't say if its a 3Ph or 3Ph +N, I suspect its this one. Firstly, you suspect (expect?) that the incoming phases are unreliable, so you have to have a phase voltage to , say 12v transformer/rectifier set, the output of all three are connected via diodes to your power supply regulator. This is so if any phases goes missing your kit will still work. I would then feed the +17V via a 15V diode to a resistor (three times). This is so a +- 10% change in mains gives a 3.7 -> .3V range, which makes it easier to measure. Just stuffing these voltages into a voltage comparator with the same voltages on their comparison inputs from high level pot same again with the low level detectors, bit of logic and some relay drivers, job done. Don't know what the micros going to do?

The micro would reduce comp count and also can supply rotation info, some hyd pumps screw up if reversed, knowing rotation is correct saves a lot of messing about.

practically it is useless to changeover the phase
suppose phase A is 200V now and phase B and C is 220v, the moment you change the load to good phase B for example, its voltage(phase B) goes down to 200V due to the load and the voltage of phase A goes up to 220V !!
the problem is due to voltage drop in the wiring, the solution probably lies in replacing the wiring with higher amps rated cable or the voltage tap on the utility transformer may be set one step higher

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