Digital control of contactors

[Ivan Leon]

We are trying to control a bunch of 3 phase contactors using an HMI. The contactors' coils are activated by two lines of the 240V 3 phase source (L1 and L2).

We are trying to figure out if the relays needed for activating the coils should be rated at 120V or 240V, or if we need another type of relay.

We did a test with 120V relays(SSR) but one of the contactors' coil burned out after leaving it connected(OFF State) for about 3 days, we measured for current leakage on the relays but we didn't get any measure. We did another test measuring the current with a manual activation using a button an we get a reading of 70mA when contactor is ON.

 

[westward10]

Surely the coils will need to be 240v if connected across L1 and L2.

 

[Ivan Leon]

Yea so the contactor coil is rated for 240V, but the problem was the relays that closes the line to power up the coil, as I think they are only rated for 120V. Could that have been the reason the coil burned out?

 

[westward10]

Definitely 240v across a 120v coil will not last long.

 

[Lucien Nunes]

the problem was the relays that closes the line to power up the coil, as I think they are only rated for 120V. Could that have been the reason the coil burned out?

It's possible. If the control relays were SSRs, not rated for the 240V supply they were switching, it's possible that they failed short-circuit in one direction and fed pulsating DC to the contactor coils. I.e. the failed SSR looked like a diode as far as the contactor coil was concerned. Since a large part of the impedance of an AC contactor coil is its inductance, feeding it DC even of reduced mean voltage might result in excessive current and power dissipation.

Obviously this effect would not happen with a conventional relay, as this would be incapable of rectifying the current, but an SSR could easily do this.

Note to any other posters - the OP is in Mexico and the supply configuration is not 230/400V!