Decrease voltage with heavy load without disconnecting

Thread Starter

Joao Ferreira_1501157560

Joined Jul 27, 2017
8
All the message that I wrote was not assumed in the post.

But I think its pretty obvious from the diagram. Can I do this with no problems?

TO DECREASE VOLTAGE
first connect dpdt ---> safe connect the transformer because there are no 40 A flowing between S1 and S2
1 second later connect spdt and the 40 A start flowing between S1 and S2 and the voltage drop at load falls form 242 to 230 V (12 V transformer acting as an autotransformer)
TO INCREASE VOLTAGE
disconnect spdt
1 second later disconnect dpdt
 

AlbertHall

Joined Jun 4, 2014
12,190
Your diagram does not show the connection to mains power. The DPDT in the NC position shorts the transformer winding. This will result in a large current if there is mains voltage between S1 and S2.
 

Thread Starter

Joao Ferreira_1501157560

Joined Jul 27, 2017
8
connection to the grid is in F (Live) and N (neutral) position.

Shorting P1 and P2 in on purpose, because if by mistake I switch on SPDT prior to DPDT, 40 A will flow between S1 and S2 which would saturate the core and induce a big voltage between P1 and P2. Since n=20, shorting them, when 40 A passes S1-S2, 2 A (40/20) will pass P1-P2 and the voltage drop S1-S2 is zero...
 

tcmtech

Joined Nov 4, 2013
2,867
Seems unnecessarily complicated.

Just set the transformer up as a buck mode auto transformer and use a single SPDT relay to switch between mains feed out to the load and the 230 VAC tap point.

As for snubbers just put them across the relay contacts. .1 uF with a few tens of ohms resistor in series is typically fine.
 

Thread Starter

Joao Ferreira_1501157560

Joined Jul 27, 2017
8
What you are saying is what I have in the diagram, but without the DPDT. In that case the transformer is always on, which is not a good thing.. The DPDT is precisely to avoid that..
 

tcmtech

Joined Nov 4, 2013
2,867
It's incorrect in that how it's drawn makes little sense.
Schematics are as much of a written language as what I am using here right now so when something is drawn wrong it reads wrong and thus makes no sense or says/means something different than it should.
 
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