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Bill Vajk
 
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Default Relay contacts sticking

Kelly E Jones wrote:

I've got a relay that controls the submersible pump which supplies
water to my house. (For reasons I won't go into, the pressure switch
and the pump power circuit are separated by a great distance, so the
pressure switch supplies a 30V signal to the relay which actually
switches the 240V to the pump). Yesterday, I noticed water coming out
of my overpressure relief valve, and after some frantic
troubleshooting discovered that the relay had stuck 'on' (the contacts
were in the 'on' position despite no juice to the coil). The relay is
being used within the current/voltage for which it is rated. A gentle
push with a screwdriver separated the contacts, and after that they
seemed to work fine.


Any idea what caused this, and how to prevent it in the future?


Age of the relay (they're only good for some large number
of cycles) and/or a relay not designed for starting that
size of a pump. You have to remember that the LRA (locked
rotor amps = starting amps) of a submersible pump motor
is higher than a conventional motor of the same horsepower
rating. (Same goes for refrigeration and all other
hermetically sealed motors.)

It is advisable to go up one size (motor starters are rated
in horsepower) for hermetically sealed motors. If you're
using a "relay" I would, for long term reliability, recommend
replacing it with a regular motor starting contactor with
replaceable contacts and use your 24 volt circuit/relay to
actuate the motor starter. Real motor starting contactors
are lifetime deals, and maybe then some.

One of the nicer features about regular motor starters is
that they are powerful enough to "wipe" the contacts
every time they make and break. It is a matter of geometry
that, as they close, the contacts slide one over the other.
I doubt your 24 volt relay has that capability and since
the contacts always hit on the same one spot they eventually
become damaged enough to weld themselves together when they
connect larger loads, just as you've experienced. They're
relatively inexpensive but then you have the maintenance
and replacement problem recurring much more frequently.