Detecting AC motor failure through monitoring load current

[buenos]

I have a sump pump (AC motor) that I cannot physically access every day.
I need to monitor it's health. The only thing I can do is to monitor when it runs (logs) and how much current it draws, over wifi from a $10 Emporia smart outlet.
It normally runs for a few minutes and draws 8A current.
Will a failed AC motor consume more current or less current than nominal?
Will the current consumption change a few months before failure, or only at the moment of the fatal failure?
Does the current load change with temperature (weather)?

 

[BradtheRad]

At powerup a healthy motor draws brief surge current, often several times normal running level.

A stalled motor draws overmuch current. If allowed to continue then a winding burns. If it breaks then current drops to zero.
I'd say you want to monitor for a condition where current is greater than 8A and lasts longer than startup. Shut off power before a winding burns.

Also prevent debris from blocking a moving part and stopping the motor.

 

[KlausST]

Hi,

Is it a 3 phase motor?

Klaus

 

[FvM]

Why do you expect motor failure? An AC motor doesn't undergo wear, except for the bearings. Most likely failure mechanisms are rotor stalling by foreign objects or pump running dry.

 

[zenerbjt]

I guess the amount of current it draws will depend on how thick and sludgey the material is that is being pumped, and this could vary day-to-day?

 

[RCinFLA]

Many single phase induction motor pumps have electrolytic based high value starter capacitors. These caps and their associated startup switch are the weakest reliability items.

Should try to make sure the breaker opens if motor does not startup properly and just grunts with high locked rotor current. Check if motor has a locked rotor current (LRA current) on spec plate and make sure breaker is less then 50% of LRA. Normal AC breakers have two trip mechanisms. One is a very high current magnetic field based trip for momentary short circuits. It can be 3 to 8 times breaker current rating. The second breaker trip mechanism is bi-metalic latch that is heated by current flow until it releases latch. Heat based trip is 110% to 150% of breaker current rating and can take ten to thirty seconds to build up enough heat to trip.

Monitoring run current will tell you if it has not been running for some time.

I have a sump pump with an electronic controller. It runs based on float contact but also has a maximum run time timer. Normally it is all based on float switch but if the maximum run time times out and the float is still active then an alarm is tripped.