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can i use 40A relay or contactor

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Be aware that VI ratings affect lifespan during open contact for inductive loads , such as HP ratings for motors and switched autotransformers. Omron is best resource for technical info.

What algorithm do you have for switching rates? Or criteria?
A contactor is basically a long life relay.....since it has a more elaborate spring mechanism to make/break the contacts.
So if the lifetime with your relay is good enough, then stick with it, as a contactor is also more expensive.

if you can switch the relay when the loads are all off, then it will last much longer, and you wont need the contactor.......if there is much inductive current flow, then the relay will wear out significant quicker than contactor. Remember "breaking an inductive current" = "nono" to mother nature
i'm using a zero cross detector while turning on the load.......but i want to know whether KT-909 relay will withstand 40A 230V or not......

Like I said read specs from Omron. On how to rate contacts.

Inductive current during switch off derates contacts significantly.

a wiser method uses the relay only to bypass the ZCS triggered triac.

After turn-on and before turn off to save relay contacts from arcs during switching and then it can be adjusted at any rate without fear of premature failure.

consider worst case surge load of 8x which drops voltage for N cycles , during which you decide to change taps. Then grid voltage restores and you decide to switch back 0.6 pf motor loads plus excitation currents of coil.

How many Joules will it dump? That is the rating you need to know.

The timing must be tested to confirm lag of N 1/2 cycles 1 or 2 before commutation with Triac . The benefit is long life to relay and low heat on Triacs which turn on zero V and turn off near 0 Amps.
I should point out that a zero crossing detector to time the operation of a relay is not particularly useful. It can take up to 15mS to energize and 11 mS to release during which the voltage could rise to a substantial amount.

As Tony suggests, use a triac with zero crossing control to do the initial switching then after a short delay (several AC cycles at least) bypass it with the relay. To switch off, do the reverse, open the relay then switch off the triac at the next zero crossing. Doing it that way will avoid any arcing and preserve the relay/contactor contacts and also not have any voltage drop across the triac because it is shorted out.

I have several large water pumps here (~ 20 Amps start current @ 250V) using triac and relay control and they have all been operating maybe 50 times a day since 2006 without a single failure.

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