Relay Contacts Welding

[kku]

I am having trouble understanding why the contacts on a relay are welding. The relay supplies power to the motor of an electric axis via its amplifier. The contacts on the relay switches 230 VAC but the contacts are always closed. The relay coil is energized as long as all the doors at the station are closed. The only time the relays open is when one or more doors at the station is open or if someone activates the e-stop. The motor for the electric axis doesn't always run. It does 4 moves every process cycle (down to pick, up to home, down to place, up to home). Every process cycle is 10 seconds, each move takes about 1 second with 1 second dwell. Are these movements enough to cause the contacts to weld? I mean if the in-rush current caused by the start and stop (on & off) of the motor enough to cause the contact welding? The contacts on the relay is rated at 6 Amp. maximum (resistive load). The line feeding the electric axis amplifier reads 0.4 Amp whe the motor is running. There is a 5 Amp. circuit breaker upstream of the relay and this never trips. Right now we replace the relays every 4 months because they are welding. The safety PLC trips out because the relay isn't opening. Any help or suggestions regarding this will be much appreciated.

[jawolthuis]

Try putting a True RMS amp clamp on the line to see what the inrush current is. You said that the relay is rated for 6 amps resistive, but what is the inductive rating? It sounds like the relay is simply undersized.

[panic mode]

this is common when relay is undersized, try more powerful ones or contactors. relays can handle resistive load up to some current value, bt capacitive or inductive loads are different story (rating is lower, sometimes much lower).

[DanW]

Look at the difference in ratings for resistive and inductive on this relay spec sheet: