Stuck contacts in a potential relay.

Shomburg

New User
Location
Ellis, Ks.
Occupation
Wastewater Plant Operator
I run a couple of different potential relays on a sewage lift station. It is a 240v 1ph. The problem I have is after a couple of months, the contacts stick together and won't interrupt the voltage for the start winding. It then overloads the motor causing massive amp draw on the motor. Any help is appreciated. What's causing the contacts to stick? If you barely touch the arm, they unstick.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
My guess, because I've seen it, is that the pump turning on is causing a voltage drop, which then makes the Potential Relay fail to change state. I seem to recall that one of the potential relay mfrs had a recall a few years ago over something like this, Supco or White maybe? I don't remember so don't take that for gospel. But take a look at the voltage drop when the motor starts. If it's too much, the conductors may be under sized, or the service transformer is. It could also be a sign that the pump motor windings are starting to fail and drawing a lot more than normal current, so check the starting and running current too.
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Brewery Engineering Plant Technician - HVACR Electrical and Mechanical Systems
If a new spec. relay does not quickly solve the problem, you are likely looking at a deeper issue. Stuck potential relays are known compressor killers in HVACR, and the damage can happen very quick. Having worked on some Barnes lift stations, the situation is very much the same. You need to verify that the start and run capacitors are good before testing with a new relay.
I would not give a thin dime for a relay made by Supco.
The number of potential relays out there and the cross references and DOCs is large.
We were taught that its all about pickup and dropout voltage. Its normal that these units fail with a lot of duty cycle, and I have definitely seen them sluggish when all else was ok. When caught soon enough motor destruction can be avoided often times.
 
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