you need to put this in the electrical section.

I think what happened here is that when you simulated the loss of one 220-V phase, the two relay coils sharing that phase, call it phase A, were still connected in series between phases B and C. Each coil would thus receive 110 V. If the relays were pulled in, they could still remain pulled in even with the reduced voltage. This depends on the drop-out voltage of the coils.

When you simulated the loss of two phases, none of the relay coils had voltage, so they all dropped out.

I'm not sure, but I suspect that if the relays had 127-V (127 ≈ 220/√3) coils and were connected in star, the loss of any single phase would drop out both relays sharing that phase, and your system would work.

Are the three relays less expensive than a single 3-phase failure relay?

YES,THE THREE RELAYS ARE VERY CHEAP THAN THE PHASE FAILURE RELAY

THANKS.

That makes sense. When trouble-shooting problems like this, it often helps to draw out the a complete star-delta diagram, and to make sure what is still connected to what when any item fails. In particular, you may get part voltages on relay coils, lights, etc., that still form circuits in the system.

Three AC relays are O.K to replace a phase failure relay.

We were using this system, at the time, electronic phase relays were not in the markets.

Now your mistake is using three 220 V ac relays. This means that you have connected those relays each across one phase (Say A1 to phase and A2 to neutral) and the neutral.

This is not the correct way of monitoring phase failure. This is wrong.

You should connect each relay between each two phases. Never use neutral.

Therefore your relay coils must be 380 Volt. Not 220 Volt. Because you connect them between each two phases. (Example: One coil betwen R-S, the other coil to S-T and the third coil to R-T)

Then if one phase goes, the corresponding relay will be off and cut-off your control circuit.

When 2 phase goes, it is comic.. If both two of the phases have gone, it means the third phase have also gone. Current, of which coming from one phase, returns on the other two phases. Any of the three phase motor neither works, nor burns in the event of only single phase.

Pls try and let me know abt your results.

Kindest Regards.

With relays you are only able to control 3 phase presence at the input of the contactor. not directly at the motor.

At older times we used a relay at the central point of the star - if a phase was lacking, the relay received a potential and tripped off the control.

The coils were 380V (not 220V) - that also helped out with a lacking phase.

Dear Yuri

Good idea to connect a relay across the star point of the motor and the neutral.

I have never tried this, and will try as soon as possible. (Considering the coil must be 220VAC)

However I think the control circuit for the motor must be the type of not re-starting automatically.

Otherwise you make a blinker starting and stopping in regular periods.

Regards

Nezih

Engineers,i am very grateful to all of u,appreciating all comments made so far,infact u are all great.More comments wil help me and others! I wil put all ur comments into action.Thanks,more comments pls.

Dear friend,are u sure of yr cct?Bcos if yr cct for star delta is okay,u need not have problem as in case 1.

It is very possible to use three single pole relays as phase failure.for u to be sure that yr star delta cct or other connections is wrong somewhere,test thesame cct with the 7.5HP motor,but this time,use a direct on line starter and makesure yr motor terminal connection is in DELTA or STAR formation ACCORDING TO YR MOTOR DATA PLATE.If yr cct is okay each of the relays should trip the power contactor of the motor when u simulate a phase failure fault on any of the relays at a time.

I have practically done this and is working on 7.5kw(10hp) motor.Power cct is different.The relays are supplied wt 3phase thru 3 single pole mcbs to A1 of their coils.The A2 of the relays are connected to neutral.From L1,L2,L3,L1 is fed to the input of NO of first relay,output to input of second relay,output to input of third relay and output of third relay to control mcb of yr control cct,from mcb to 95/96 of yr motor overload relay,from O/L to stop button 1/2,from stop button to start button 3/4 ,from start button to A1 coil of yr power contactor and A2 of power contactor to neutral.U must also latch or selfhold the 3/4 start button contact wt 13/14 auxilliary contact of yr power contactor.

This is a simple cct which works normally if yr power and control cct design is correct for both DOL or STAR-DELTA.In my example above,u can easily simulate a phase failure on any of the 3 relays by switching off any one single pole breaker connected to the 3 phases above and yr motor must stop for CASE1 or CASE2 above.

Patrick Whowha

I am semolina,in fact the last person that comments is really encouraging me that this circuit can work,i really appreciate this,i know using 380V relay is a perfect sample of this project and everything under the sun is possible when efforts and mind is put to it. I will definitely try everything said here,everyone is highly appreciated by my co designer of this project.............THANK U,VERY MUCH!..............