Triac applicatioon for phase selction

Three triacs have their MT2 terminal connected to the three phases of 220VACV supply. The MT1 terminals are connected together as output. The object is to get supply in the output if there is a failure in any of the phases by firing one of the triacs.

Will there be any problem?

it is safe as long as you double,triple check that only one TRIAC is fired at a time.
or else its going to be a phase SHORT. did you get it?

Thank you for the hint. I just wanted to make sure before I connected the circuit - afraid of a hazardous experience.

Thanks again

use selector switches for phase selection which is much safer than using triac.

---------- Post added at 22:22 ---------- Previous post was at 22:19 ----------

if you are going to do the thing automatically then selector switches won't help or else selector switches are the best option.

I usually do the reverse. I select an entry of a transformer via a triac instead of relay. All MT1's are connected to the phase line and each MT2 to the corresponding transformer terminal.

Obvioulsy, only one triac should be on at any time. Please note that a triac may still be on for a few millseconds after crossing zero voltage if the load is inductive (as in my case) even if there is no gate current, waiting for the load current to become lower than its holding one.

hi KerimF could you please elaborate your method of connecting triac.

could you please elaborate your method of connecting triac

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Sorry, your question is rather general. Could you please be clearer like asking one or several specific questions?

Hi Kerimf,
mrsandy says that his objective in this project is to get supply in the output if there is a failure in any of the phases by firing any one of the other two triacs. you said that you do this in reverse by connecting an entry of a transformer via a triac instead of relay and connecting all MT1's to the phase line and each MT2 to the corresponding transformer terminal will this method help in achieving the objective of mrsandy.
I may be wrong with the question because i am very new to electronics. I am sorry if you find this question silly.

I see... First, your question is not silly at all... Second, I liked pointing out that joining the MT1's is possible.
The difference is that, in his case, a floating power supply is needed to trigger the triac gates. In mine, the voltage of MT1's are rather stable (connected to a voltage source as the mains).

Added:
A floating power supply is easily done by a transformer.
In my case, I use a resistor (or R + C) powered by the mains voltage to provide the gate current pulses.
The control is usually done via an optocoupler as in a regulated battery charger or directly as in a voltage stabilizer (by steps).

Thanks for answering to my question kerimf.

whatever you do, you want to control the device for;

• false switching ,
• failed triggering
• reliable for a wide range of loads,
• test on wide temp. and ingress noise limits
• dead time safety margin
• surge tests

Normally this is done with large oil filled contactors.

Will there be any problem? post #1

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If a very fast transition is required, I guess there must be a load current sensor which indicates when the actual fired triac is off so that another one could be triggered.
Added:
The detection of the voltage zero crossing is important here for the three phases (though one may be enough, if supported by a good algorithm)

There are two ways to continuously trigger a triac to ensure that it will be on, no matter how strong the inductive load is:

(1) The simple way is by using a dc current pulse in each half cycle. But it is power consuming mainly if the triac is not sensitive.
(2) The pulsed method is to replace the dc pulse with a stream of short pulses (as 12us on pulse followed by 68us off).

In any case, the triggering half cycle pulse should end before the next zero crossing.
Also in one cycle, the two triggering phase should be equal.

Kerim

If triac are used to control an inductive load Current and Voltage are out of phase, so even if voltage is 0V there will still be current flowing. Triac turn off is at 0 current not 0 voltage.

Pretest said:

If triac are used to control an inductive load Current and Voltage are out of phase, so even if voltage is 0V there will still be current flowing. Triac turn off is at 0 current not 0 voltage.

Click to expand...

Who said otherwise?!