• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

automatic 3 phase selector help

Status
Not open for further replies.

georgetwo

Member
i tried doing this with 6 op-amps, i ended up damageing the cct.
can any one help me with automatic 3 phase selector that gives out the highest of the three and does its switching so fast that no home appliance will notice (no microcontroller) ?
 

tcmtech

Banned
Most Helpful Member
Your trying to do what with three phase?
We will need far more information first.
 

georgetwo

Member
i am talking about a 3 phase each 220V ac suply from a power generating company. my aim is to connect the three wires + earth into the input of my 3 phase selector, which in turn selects the highest voltage amongs the 3 voltages and suply the entire house with it. If there hapens to be any interruption in the active phase, it switches to another stable phase without power interuption in the house. I need Your help
 

tcmtech

Banned
Most Helpful Member
Are you having major problems with line voltage drops?
What is considered normal line voltage and what level are wanting to switch over at?
Where are you located and how do you end up with three phase power in a home in the first place?

Around here three phase systems are typically reasonably balanced. There is not much more than a few percent difference from one to another.
To do the switching you would need to use SSR's, Solid State Relays, they can react in one half of a cycle. However being your jumping from phase to phase you would have some phase overlap issues that would require a added half cycle delay between shut off of one phase to the connection of another phase. Plus switching phases on the go will cause some possible spiking issues due to the AC current cycles jumping ahead or back 120 degrees time wise.

However to get a reasonable voltage reading off of each phase it would still take a few cycles for the system to accurately detect a low voltage condition and make the needed choice of which phase line is the best next alternative.

To get better voltage regulation you might be better off using a multi tap auto transformer set up that has several output taps at points a few volts higher and lower than the main line.
If the line voltage goes to high the system chooses a lower voltage tap and if the line voltage drops to much it chooses a higher voltage tap.
This is how the older style power conditioners worked. Each tap was about +- 5 % of the main voltage.
Just something to think about.

the more information you provide the better we can figure out what it is your tying to do and why.
 

georgetwo

Member
actually, i live Nigeria (in west africa). electricity suply is very poor.Infact i am typing this message with my phone. In Nigeria, the 3 phases are mostly unstable and most times, one phase might might shot down while the others are working. sometimes the voltage dropes while the others are manageable. i want to stop the stress of running around changing phases. hope u get it?
 
I want to use it as my final year project in partial fulfilment for the award of B. Eng'g, computer engineering. Am also interested in using it for my own persoonal consumption at home, as here in Nigeria power suply is some times unpredictable, it might be unconsumably too high or too low, as such am thinking of a way to design and implement a circuit that can automatically switch between a three phase supply souce. Range should be maximum of 220-240 and minimum of1170
 

Grossel

Well-Known Member
From the first post, it sounds that you're looking for a rectifier. A rectifier will always forward the highest and lowest voltages at any numbers of phases. But this will create DC with ripple. I guess DC is off topic here.

I'm sure there is possible to atach a number of relays together to always forward phases that has voltage. The problem with relays is you cannot really be sure what minimum voltage is to activate the relay.

If using opamps, I can magine this would be a challenge few on this forum can solve. Together with holding circuits, timers, a lot of basic digital circuits and a handful of opamps it should be possible to solve. Wonder how? Me too :)
 

tcmtech

Banned
Most Helpful Member
Op-amp and relay logic is very basic and common sense logic based so micro processing actualy required to make a basic system work.

Its just a mater of comparing the voltages of each phase and then setting a lower limit point where the one that is being used gets dropped in favor of the next highest one. its just simple op-amp comparator logic with the addition of a few timers to filter out glitches and false changeovers.

It could also be easily done with a basic PLC or PLR type system using the units ladder logic to compare the three input voltages and then select the highest one as needed.
 

MrAl

Well-Known Member
Most Helpful Member
Hello there,


One solution might be to use back to back SCR's as the switching elements, with phase and voltage detection to decide on the switching action.
As one phase goes low, turn that phase off and turn the other phase on. If done right this might work if the lines dont go down too often in a day.
It would surely be a good idea to test this in a simulator first however to make sure the switching matrix method works ok. There will most likely have to be phase detection to get the switching action correct.
For the switching, perhaps a micro controller or PROM and associated comparators to make the decisions.

If the switching SCR's dont work because of too much phase overlap, you may have to use an alternate means such as a transistor with bridge rectifier. I think you would have to analyze the effect of switching one phase to the other first anyway though, the effect on the load. The load may have to take a brief interruption in power. If that's not a problem, SCR's would work ok.

So the first thing to do is to analyze the effect of switching phases quickly, assuming it is done very fast. If the effects are detrimental to the load, there will have to be a delay introduced to protect the load at least. If the load can tolerate say a couple cycles delay, then there should be no problem producing this kind of device.

The other interesting thing about the three phase system is that phase A is equal to the inverted phase B added to inverted phase C, or phaseA=-(phaseB+phaseC), which means that if your system is guaranteed to have at least two out of three phases good, you can do a clean switchover with little change to the line voltage! The problem is that if two phases go down at the same time this doesnt work. You also need power transformers too though.

If the phase A goes down at the 90 degree point and you switch to either phase B or C, you'll be switching from plus 100 percent to minus 50 percent line voltage, but if say phase B is inverted first (transformer) the switchover will be much cleaner from plus 100 percent to plus 50 percent, which is much better overall. Thus, if a few transformers can be added that might help alot, although they would have to be able to handle the full load power.

Of course another idea that would be switching transient free would be to build an AC to AC converter. It would take in the three phase line voltages and output a single phase line voltage of the same level as either phase. It would be designed to work even if only one phase was up and running (as a single phase to single phase converter would work anyway). That way if one phase or even two phases went down there would be absolutely no change on the output phase, and when the other phase(s) came back on line they would again help to supply the load power. The switchover would be automatic and transient free. The main difference between this and a three phase to single phase converter is that the input filter caps would have to be increased to be able to handle a single phase input, and the rectifier diodes on the input would have to be able to handle the full load current plus a little more as a single phase input would be designed. This would definitely do it and provide a continuous output to the load with no interruption, but it would cost more as the converter would have to be able to handle the full load power.
 
Last edited:

crutschow

Well-Known Member
Most Helpful Member
I would use three small step-down transformers to isolate each phase, say with 6-12VAC outputs. Rectify and filter the outputs to get a DC level from each transformer. Use three comparators (with sufficient hysteresis to tolerate normal voltage variations) between the three outputs (1 to 2, 1 to 3, and 2 to 3). Use logic gates on these signals to determine the phase with the highest voltage, which is then used to switch the house relays to the proper phase.

But doing that so that the house appliances won't notice is problematic considering the change of phase. Solid-state relays would give the fastest switching and minimize the transient.

Note: Make sure you have the relays interlocked so that not more than one can be energized at a time and that there is a slight delay from one turning on to the other turning off, unless you want to test the mains circuit breakers against a dead short.
 

Reloadron

Well-Known Member
Most Helpful Member
Something here I don't understand:

i am talking about a 3 phase each 220V ac suply from a power generating company. my aim is to connect the three wires + earth into the input of my 3 phase selector, which in turn selects the highest voltage amongs the 3 voltages and suply the entire house with it. If there hapens to be any interruption in the active phase, it switches to another stable phase without power interuption in the house. I need Your help
I am going to have to guess this is line to line wye configuration 3 phase 240 volts. That being the case then any phase to neutral would yield about 139 VAC. In order to have 240 volts for your mains house power it would have to be phase to phase using 2 of 3 phases. It would not be a matter of choosing a single phase of the three.

If the power is Delta (triangle) configured the neutral means nothing and to have 240 volts it must be phase to phase.

Either way you need two phases to have 240 volts assuming 240 VAC 3 phase power.

What am I missing here or not seeing?

Ron
 

tcmtech

Banned
Most Helpful Member
What am I missing here or not seeing?
Perhaps that the OP started this thread about a year ago?
 

MrAl

Well-Known Member
Most Helpful Member
I would use three small step-down transformers to isolate each phase, say with 6-12VAC outputs. Rectify and filter the outputs to get a DC level from each transformer. Use three comparators (with sufficient hysteresis to tolerate normal voltage variations) between the three outputs (1 to 2, 1 to 3, and 2 to 3). Use logic gates on these signals to determine the phase with the highest voltage, which is then used to switch the house relays to the proper phase.

But doing that so that the house appliances won't notice is problematic considering the change of phase. Solid-state relays would give the fastest switching and minimize the transient.

Note: Make sure you have the relays interlocked so that not more than one can be energized at a time and that there is a slight delay from one turning on to the other turning off, unless you want to test the mains circuit breakers against a dead short.
Hi there,


Yeah but there is always a transient as i explained in my previous post. With phase A at 90 degrees it's at 100 percent of output peak (say 312v for a 220vac system) and phases B and C are both at -156v, so switching at that time for example would change the load from +312v to -156v with the fastest switch possible (0 nanoseconds switch time). That's a 468v transient!

This seems to suggest that at the very least some sort of delay mechanism is needed to pick the phase that will be close to the last value the soonest. Inverting phases helps a little, where phaseA 90 degrees to phase B inverted would be a transient from 312v to 156v whichi is a little better (156v transient) which the load system might be able to take since it must do so for some turn on times. That would require power transformers that can handle the full load power however.

Yes the original post was about a year ago. Not sure if he's still interested or not how, but it is still interesting to think about a little.
 
Last edited:

crutschow

Well-Known Member
Most Helpful Member
Most appliances probably have enough capacitance in their power supplies to tolerate a cycle or two of dropout. There might be a little hiccup, but I suspect most things, including a computer or TV will keep going without a significant hitch. So I would suggest a small fixed delay between switching phases and not worry about the relative phase between the two being switched.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top