20KWatts AC swithing w/ 5V = poor's men switch (trapizonga)

[joseLB]

I was needing to switch on/off trifasic power up to 30Amps/phase, 220V => 3*30*220 = ~20KVA.
This shoud be done even when there where no supplied power....
I could only find contactors that where hard to find/buy, expensives, and that needs AC or a higher than 5V voltage on their activators/coils.
So, I come with this "trapizonga", nick name in portuguese for "so much creative" or bunch of "strange ideas"... I hope you don't laught so much about what you will see at this post, as some did here...
movie: don't pay attention for the portuguese, the images tell by themseves... http://www.youtube.com/watch?v=mrL7aZJbig0
a more detailed image is attached...

I'm developing the board with 2 relays normally open/close and MP code embebed in an application. Of course it could be a solid state H bridge or four transistors, but in my project relays would be more indicated.
Pay attention at the movie that the printer motor is driven by 5V - 7805 without heatsink = 300 mA, that means I can have some sort of 12V plain small lead batteries (moto, nobreak or whatever) to turn it on/off independently of power supply.
Also, it's a plain screw that was glued in the motor with a epoxy glue (durepoxi) and while it was becoming hard, I was hand turning and putting the screw in center and with almost no "shift", as can be seen.
question: as this is mostly to turn off our DP center if something goes very wrong, could I be confident on it? Would you?
Jose

3 phase controled disjuntor

trapizonga.jpg (149.4 KiB) Viewed 4962 times

[Jack Flanders]

Tough question! I see nothing wrong with the concept - success will be determined by how well the final version is built. Presumably, it does nothing most of the time. Then when needed it will turn on and operate the breaker switch. So it will be important to keep the mechanism clean (no spider webs, dirt, etc.) If I had to support something like this, I would want to be sure it was inspected regularly and operated in test mode once a month or so...

By the way, I like that word: trapizonga. Sounds similar to the English "contraption"

[Dany]

Hi Jose,

I was needing to switch on/off trifasic power up to 30Amps/phase, 220V => 3*30*220 = ~20KVA.

I have a strong feeling the 3 is wrong in above formula.

This shoud be done even when there where no supplied power....

This is of course the problem... If this requirement was not there then you could use a cascade of 2 relays: the first one (single pole), driven by the PIC, the second one (3 poles), driven by the first one).

By the way, I do not know if then exist: bistable 3-pole relays that are e.g. have a 12 V coil? Problem is here that you can not know what the state of the relay is...

Or even better: a bistable 3-pole relay with 2 12V coils: one to switch it e.g. on, and one to switch it off.

p.s. The mechanical solution will work better if the motor's spindle is at the same hight as the handle of the switch (so, the small rod between the sindle and the switch is placed horizontally).

[John Ferrell]

When you measure each of the 3 phases you have effectively measured everyting twice so the kva needs to be divided by two.
This is especially interesting to me. I have a few places in my Ham Radio operation where remote switching would be very handy. Most involve rotary switches...

"Trapizonga" is a welcome addition to my vocabulary!

[joseLB]

Hi Dany and John

Thanks for your suggestions....
you are right, it's 3 X 110V x 30A = ~ 10KVA, if I use the common (neutral) as is usual here (110V). If I use 220V, the current would be common to phases, so, it's smaller than 20KVA. Anyway, it's still a good power switcher, 10KVA, not?

About the distance from motor to switch arm: as it is, there are about 40 motor turns for about 1/8 of switch arm turn. So, the mechanical gain is about 40 x 8= about 320, not counting atrict and other losses. It's driven as shown by 5V, and the motor I had at hands was just that one. As I reduce the arm, the torque will increase, up the point the motor will not be able to turn. For example, if I put the motor in a height that makes the complete movement of the switch, that means 40 turns for about 1/4 of switch turn, that is, my gain is about half =~ 150.

With this motor power I know I still have some spare power, that means I don't need all that arm long, but I didn't test what would be the smaller one.
trapizonga= very strage thing, full of nuts, wires, etc.....
thanks
Jose

[Dany]

Anyway, it's still a good power switcher, 10KVA, not?

It sure is!