Intermittent Relay Issue

failurepower supplyrelay

See the attached schematic.
As I test this out, I will occasionally see the K1-B contact remain closed after I open the "External System Enable" switch. The relay contact will "stick" even if the motor is not running as I open the "External System Enable" switch. So the only current through the contact is that of the power supply. Note that I had run the motors before opening the "External System Enable", I am simply stating they are not running at the time I open the "External System Enable" and see the N.O. remain closed.

My thinking is this relay cannot handle the inductive load, period. As I look at the spec for it and it only lists "resistive" load. But I'm not sure about that.
Also note, these motors are NEVER running at the same time.

Can anyone shed some light on what might be causing this to happen? Could it be too low a current from the power supply? I've see other threads stating you need to properly "wet" the relay contacts? Regarding ample load as relay closes, as I understand it.

Relay Data Sheet <— http://www.mouser.com/ds/2/307/omron_en-g5la-548280.pdf

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Best Answer

When the relay closes, the charging of the 1000-uF/50V capacitor can create a large inrush current -- assuming that the 24-VDC power supply has a large output capacitor that can supply the inrush current. This inrush current could exceed the relay ratings and weld the contacts.

Some possible solutions are the following:

(1) Use a much smaller capacitor value in place of the 1000-uF cap if you don't need the extra holdup time. The LM340T-5 datasheet only calls for a 0.22-uF capacitor.

(2) Add some series resistance before the 1000-uF capacitor to limit the inrush current. Since the contacts have a 10-amp rating, you only need a few ohms to limit the inrush at 24 volts. You may need to check the pulse-energy rating of the resister because now the resistor will absorb the inrush energy: (24V^2)(1000 uF) / 2 = 0.29 watt-seconds

(3) Use a relay rated for a tungsten load, which can handle a high inrush. The inrush current should still be checked against the relay limits (either measure the inrush or calculate the inrush from the capacitor ESR value).