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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.

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  • \$\begingroup\$ Measure DCR and inductance \$\endgroup\$
    – D.A.S.
    Commented Jan 5, 2019 at 17:25
  • \$\begingroup\$ Most contactors running in panel etc. Use a 120v or 24v control voltage, not the 3 phase line voltage... Are you sure the coil of the contractor is designed for 3 phase 240? \$\endgroup\$
    – MadHatter
    Commented Jan 5, 2019 at 20:05
  • \$\begingroup\$ Please post the contractor model. \$\endgroup\$
    – MadHatter
    Commented Jan 5, 2019 at 20:06
  • \$\begingroup\$ You must know how to compute Pd from R,L f,V \$\endgroup\$
    – D.A.S.
    Commented Jan 5, 2019 at 22:57
  • \$\begingroup\$ Yes, I am sure the contactors' coils are designed for 240V. These have been used for a long time now. The electrical system was designed by another engineer. We are now just automating the system (bunch of motors and valves). The problem was after wiring the relays and leaving them there for a few days one of the contactors' coils burned out. It does make sense to use 240V relays to activate the coils. I'm no EE so I was a bit confused with the 3 phase terminology but L1 to L2 does measure around 230V. Tomorrow will be going on field, I'll try to get the contactor model. \$\endgroup\$
    – Ivan Leon
    Commented Jan 6, 2019 at 21:18

2 Answers 2

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The relay contacts should be rated for (at least) the voltage and current they are switching and the datasheet should be checked for derating when switching inductive loads. It appears that your contacts should be rated for 240 V AC.

The relay coil should be rated for the control voltage and current type - AC or DC.

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If the contactor coil is connected between 2 legs of a 3-phase-Y 120 Vac system, the voltage across the coil is 208 Vac. Anything switching that coil on and off should be rated for 240 Vac minimum.

70 mA at 208 Vac equals 14.6 W. To me that seems like a lot for a relay coil, but this is not one of my strong areas.

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  • \$\begingroup\$ Note 3 phase leg to leg is going to be 240, not 208. \$\endgroup\$
    – MadHatter
    Commented Jan 5, 2019 at 20:05
  • \$\begingroup\$ @MadHatter You sure? I thought that was the weird US "Split phase" (120-0-120) centre tapped as opposed to three phase which is 120V phase to neutral, 208 phase-phase. Mind you there are weird things in US distribution practise, corner grounds, wild legs, all sorts of weird. If it is two legs of 230V three phase as it would be in the EU, then you have 400V line to line, and so would need relays rated for that, but I think this is a slightly confused American question. \$\endgroup\$
    – Dan Mills
    Commented Jan 5, 2019 at 20:23
  • \$\begingroup\$ Both Europe and America 3 phase is 3 phase your you're measuring leg to leg voltage. In the US, houses get 2 lines and a neutral, basically 2 120v lines from a center tapped transformer that can be joined for 240. His question sounds more like it is an industrial control panel hence the 3phase. \$\endgroup\$
    – MadHatter
    Commented Jan 5, 2019 at 20:32
  • \$\begingroup\$ leg to leg measures 230V, this is in Mexico! Thanks for the response! \$\endgroup\$
    – Ivan Leon
    Commented Jan 6, 2019 at 21:19

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