I have a very small motor - it's a recirculating pump that is 1/150hp. It has ceased to run and only hums when energized. The motor is all sealed up - the mechanism by which the motor drives the impeller is via magnets (i.e., the motor appears to turn a magnet inside a sealed metal housing and a similarly equipped impeller that sits on a fixed spindle has a magnetic base.) So, the motor inside is clearly not turning. There is no way to start the motor (turning the impeller assembly by hand does not result in the motor starting up).

It has a motor run capacitor, rated at 3.5 mfd +/- 10% at 280VAC. I have tested it with a capacitance meter and it is returning a capacitance value of 2.77 mfd. That is outside the accepted range, but not significantly so (the range should be 3.15-3.85). Otherwise, the capacitor appears undamaged. There is a single capacitor in the motor, with 2 leads.

The capacitor specs are available here (it is a KNM1228): Motor Running & Motor Starting Capacitors KNM

According to this thread, a difference of an additional 10% wouldn't be expected to cause the observed failure, at least in a larger motor. Elsewhere, I read that an undersized run capacitor would likely cause the motor to never get up to full speed (vs. not starting at all, which is the current problem).

My questions:

1. Given that this is such a small motor, is there a reasonable chance that this small difference (0.38 mfd) would cause the failure of the motor to run? I.e., should I purchase a replacement and try it or is that a waste of time?
2. Is there a way to test the function of the capacitor? I.e., if the capacitance meter returns a value of 2.77 mfd, does that "prove" that the capacitor is functioning? As an analogy, an expired alkaline battery may return an acceptable voltage value but any current draw on the battery would quickly indicate that it had expired - is there a similar situation w/ capacitors?
3. Other thoughts on troubleshooting a completely sealed motor? I have no wiring diagram and there are 5 wires to the motor from the circuit board.

 

That small a difference should not have had any effect, it should have at least attempted to run.
If it is free to turn any spin of the shaft would normally run it for PSC motor.
But if it has 5 leads that indicates it there may be more to it than a simple PSC motor.
Is there any components on the circuit board?
Max.

 

Hi Max,

Thanks for your input on the capacitor.

I cannot actually see or touch the shaft - the rotor has magnets affixed to it inside the motor casing which magnetically drive the impeller which is external to the motor casing. But, when I turn the impeller I can feel resistance from the magnets so it doesn't feel like the rotor is turning. Nor can I cause the rotor to start by moving the impeller.

And I was wrong - there are 6 leads to the motor (2 are the same color).

Attached are both sides of the small circuit board, it's maybe 2 1/4" square. There is a standard electrolytic capacitor that I didn't test. None of the components appear to have overheated or burned. The two lugs on the side with the capacitors are for AC 110v input. The three wires from the middle of the component side of the board lead to a small switch/timer assembly which operates correctly. I just set the large capacitor back in place so that is why the C2 solder points aren't connected.

 

Obviously not a split phase induction motor.
It appears to be a stepper motor or some other technology.
Max.

 

But, when I turn the impeller I can feel resistance from the magnets so it doesn't feel like the rotor is turning.

That suggests the problem may be mechanical rather than electrical. My guess is the motor shaft has seized either through lack of lubrication or through corrosion due to slight leakage of the pumped fluid.

 

Although if it is a stepper motor of some kind it has normal resistance to turning.
Max.