I Need to design and make a CT (current transformer) to energize a 3ph relay with sufficient voltage and current from low load to full load. I'm thinking of a toroidal transformer with very few windings of 10 or less since I'm not running anything except a relay
A CT is preferred as it allows (I hope) me to monitor all 3 phases at once and output to a 3 phase relay connected to a shown 3ph MIG wire feeder.
So what I hope will happen is when power is ON but the stick welder is not energized the wire feeder is NOT ON either. When the machine is on and power is going in to the machine the CT energizes the 3ph relay powering on the wire feed unit in a pass through type configuration.
I guess my main questions are if a CT divides amperage, is voltage multiplied by the same factor?
A CT with that is "measuring" no load outputs no current either? so not energized i.e. 3 phase delta, 3 live wire going somewhere but the load is yet to be turned on…
From what little that I know/understand Its like voltage transformer in reverse (voltage increases at the expense of reduced amperage). the opposite could be said about a CT from what I little know/understand, amperage is reduced while voltage if left open circuited can quite quickly rise to unsafe levels.
Will 3 wire Delta pass through a single CT and function as normal, reduced or just not possible at all. Will I need to have 3 CTs wired in series with a relay as load w/ or w/o a resistor?
One last Question googled and what not still couldn't find a definitive answer with a formula and example of how to find the output voltage of a CT, I'm still not sure, answers were all to do with current/amps etc…
simulate this circuit – Schematic created using CircuitLab
Best Answer
No that won't work under any normal load conditions because the instantaneous sum of all three currents is zero for a perfectly balanced load.
Just one CT attached on one phase is sufficient because if one phase is taking current then the other two can be presumed to be taking current also.
Will it drive a relay? I don't think it will because the relay coil doesn't represent much of a burden resistor and the toroid core may sature and you'll not get enough to power the relay. However, that is a generalization and you might find one that works BUT you are now starting to have problems deciding which cores will saturate and which won't.
Almost certainly, a more reliable solution is to use a conventional CT with a burden and use the signal to turn on a transistor that then activates the relay.