Pump running backwards?

[mgarf33]

This side conversation was split from HERE. jblizzle

Both the impeller outlet and volute are at high pressure so cavitation would never occur there.

Not even if the motor were turning backwards?

Mitch, have you checked the rotation of the failed replacement motor with the impeller on?
Can you highlight an arrow in its direction?
Something as simple as a miswired run cap can cause reverse rotation.

 

[mas985]

Re: Bad motor --> cavitation damage = useless wet end?

You would have to flip the starter winding to reverse rotation and with a single phase induction motor, that is not possible because of the hard wired connections.

Plus water still pumps in the same direction with a reversed motor just much less efficiently. So it is less likely for a pump to cavitate when running backwards because flow rates are lower so suction head is lower.

 

[mgarf33]

Re: Bad motor --> cavitation damage = useless wet end?

You would have to flip the starter winding to reverse rotation and with a single phase induction motor, that is not possible because of the hard wired connections.

It's quite simple actually. Just move the L2 to start winding side of the capacitor, and leave the run (or main) winding on the other side so it will now have the phase shift.

I've edited this image of a basic cap start, induction run motor.

 

[mas985]

Re: Bad motor --> cavitation damage = useless wet end?

Not that simple. You can't simply move L2 to the capacitor lug and still have the capacitor connected to the main winding. Typically, there is only one connection on the capacitor. Someone would have to deliberately modify the connections to make that happen. Too difficult to happen on accident.

 

[mgarf33]

Re: Bad motor --> cavitation damage = useless wet end?

Oh, but you can. I have a motor on my bench right now that proves it.

If in fact the OP's motor is cap run, and there are only two wires to the capacitor, I will agree with you, it would take much more effort to reconfigure. I don't know what the wiring on his pump looks like.
Some motors connect L2 and Main winding on one side of the cap as shown in the pic above (before I altered it).

The motor will likely never reach speed enough to disengage the centrifugal switch, but it will reverse rotation. I have a motor on my bench wired exact as I altered above with reversal occurring, by simply moving one yellow wire on the start cap.

Again, I would agree with you in the fact that most mfg's will dummy proof the run cap wiring using only 2 wires, so you can't mess it up.

 

[ewkearns]

Re: Bad motor --> cavitation damage = useless wet end?

A bad cap can, indeed, reverse rotation..... So can a bad centrifugal switch and a little help from Murphy's Law...

 

[mas985]

Re: Bad motor --> cavitation damage = useless wet end?

Oh, but you can. I have a motor on my bench right now that proves it.

I still don't envision a scenario where that would easily happen on accident. When changing the motor voltage, that is done on the motor terminal board and no mater how you rearrange those wires, you cannot force the motor to run backwards. Only if a capacitor was replaced and had the necessary terminal connections, then perhaps someone could accidentally rearrange the wiring to get the motor to run backwards. But I don't think the OP changed a capacitor so had no reason to mess with those wires. But even if it did happen, start capacitors are not suited to be used in the main winding so would likely fail very quickly once the motor is under load and drawing high current through the capacitor and main winding.

But as I said before, even if the motor runs backwards, a centrifugal pump still pumps water in the same direction (centrifugal action always throws water from the inner to the outer impeller) albeit much less efficiently with much less flow rate so cavitation would still never occur in the volute or ends of the impeller. That is either debris damage or as someone pointed out, screw driver damage.

I don't think any of this is very helpful to the OP so we should really stay on topic.