I have a wee technical problem that I would appreciate some wisdom on :

A customer of my employer installed a submersible well pump on a farm. The pump is a Davey brand 1.12HP submersible The installation is a fairly typical rural irrigation-pump with a centrifugal pump about 70m down the well. The pump draws a balanced 2.6A per phase in normal running. Starting is DOL, using one of our 4KW DOL starters and an appropriately-rated Thermal Overload relay. Being a centrifugal pump, I am told that the starting load is quite light.

The problem is that the line contactor keeps getting its contacts chewed. Given that it is good for about 4x as much motor power as it is switching, this should not be happening. When the first contactor fried, it could be dismissed as one rogue contactor, but when a second fried too, there is clearly something wrong, but what?

The float switches are sufficiently far apart that frequent on/off cycling can be ruled out. The thermal overload is set to manual reset, so it is not cycling on/off due to the TOR tripping. There is no PFC installed, and there are no co-connected loads. Like most NZ electrical contractors, this one has a Megger and knows how to use it, and all seems well as far as insulation resistance goes.

Thoughts, anyone?

Hi Mark

The two most common reasons for this that I come across, are :

1. Chattering contacts due to control circuit. Fluttering level switches and the like. Arrange the circuit so that the contactor is latched closed. A single pulse from the top float closes the contactor and it is held closed by an early close contact. If the pulses from the float are very quick, it can be possible for the contactor to partially close and then open several times before sealing closed. Using a relay to interface and provide the latch function will be faster and may eliminate the bounce.
2. Low coil voltage. If there is a voltage drop during start, the voltage on the coil of the contactor will be reduced. If the voltage drops more than 15%, it is possible for the magnetic poles to begin to separate and the pressure on the contacts to reduce.
   If the contact pressure is reduced during start, the contact resistance will increase and the contact temperature will rise causing premature failure.

I would suggest that you use a high speed logger to monitor the voltage across the coil during start and this will probably show you the cause of the problem.

Best regards,
Mark.