Electronics > Projects, Designs, and Technical Stuff
Pump motor postmortem
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calzap:
With the exception of overload, the various protection functions of the controller have been put to real-world tests including low and high voltage from the power company.  There is a test button on the overload module, and it seems to work, but I don't know how realistic the test is.

The outflow of the pump goes to a pair of 86 gallon (327 liter) tanks that have air above diaphragms.  We always use two tanks in case a diaphragm breaks, and yes, it's happened.  Air pressure is checked and adjusted occasionally and has never been an issue short of a broken diaphragm.

Outflow goes through a flapper-type check valve to the tanks.  Pressure switch is in the plumbing Tee assembly that conducts water to/from tanks.  Distance from pump to Tee is about 2 m.  Pressure switch is dual-acting, low pressure safety type.  If tank pressure falls below a certain  level, the pressure switch flips to an off-state that requires a manual reset.  Most common causes of this have been loss of prime, controller detecting something amiss, failure of a controller component, or attempt to draw too much water from the system.

If the outflow check valve stuck in the open position, it would probably not be a big deal because there is a check valve in the inflow plumbing.  If it failed in the closed position, yeah, might destroy the pump  ... any significant obstruction between the pump and pressure switch has that possibility ... might even be treated to a steam explosion.  Possible saving grace in our system is that when we're irrigating, it's usually 24/7.  If the check valve stuck closed, the pressure on the other side would soon drop to the safety trigger level requiring a manual reset.  I've never seen a flapper check valve get stuck in the closed position.  I've seen them get stuck in the open position.   Check valve in this system has worked fine in the past and is working fine today.

The system is pretty well tuned to avoid short cycling too.   During high demand, like sprinklers, the pump runs continuously.  During low demand, like drippers, it cycles on and off based on pressure switch cut-in and cut-out pressures.

Mike in California

andy3055:
If you are planning to buy a separate motor and couple it to the pump, try to buy a motor that has only the run capacitor. That will eliminate the centrifugal switch issue. You could always buy one that has no capacitors (TEFC). The advantage is that the interior of the motor will be safe from the elements. I think it depends on the power/size. The motors with run caps are smaller in size, if I remember.
james_s:
Lots of motors without a start capacitor still have a start winding and centrifugal switch, as far as I know the only motors that avoid this are fractional horsepower shaded pole types. To avoid all this on larger motors you can use a 3 phase motor, if you only have single phase power available  you can use a VFD to power it. Those seem to be quite reliable, there are lots of them running things at my friend's machine shop and we have not had one fail yet.
calzap:
Just noticed something on the motor label.  See the picture below.  Well, it didn't burst into flames!   I think "Thermally Protected" refers to the Klixon CSJ31SV overload protector at the end of the motor opposite the fan.  This OP device works by passing the motor current through a wire that, if current is too high, will heat a bimetal disc which will change shape and  open  contacts.  It's really a self-resetting, thermally operated circuit breaker.  Theoretically, it can also protect against high heat in general in the area where it's located.  Unfortunately it was at the end of motor away from the fan and not that close to the stator.  With fan blade failure, heat generated in the coils would not reach the device sufficiently for it to offer thermal protection.

I also examined the centrifugal switch.  The centrifugal mechanism on the shaft was in good shape.  When at speed, a plastic disc (opposite end of the shaft from the fan as seen in pics in my first post) moves along the shaft and presses on a pair of non-rotating plastic posts that move and open contacts.  The contacts were OK, but the plastic posts had some wear.  I don't know if the wear was sufficient to prevent the contacts from opening.

Mike in California

andy3055:
There are so many to choose from but shipping is pretty high, obviously. Unless you can find a distributor nearby to where you live.

http://www.electricmotorwholesale.com/index.cfm?fuseaction=catalog.attributeSearch&catId=0&20093=20093~3&20094=20094~2500-3600&displayCatId=1794&attributes=,20093,20094&attributevalues=,3,2500-3600&attSearchList=,20093~3,20094~2500-3600
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