Induction Motor Acting as Generator

It will not harm the capacitors. It's not a good idea, but that's not what you asked about. If it were me, I would have an isolation contactor on the caps.

You need a non-reversing motor...

http://www.usmotors.com/TechDocs/ProFacts/Non-Reverse-Ratchet

PDF file...

Or that... Yeah.

Side story: if you use a back spin ratchet, be sure to do your rotation check with the ratchet DISCONNECTED then reconnect it when you have it right. Then remember to do that every time you remove or change the motor. It's not fun to energize the motor into that ratchet.

Since there is enough back flow to generate power for 40 seconds putting in a check valve might put too much of a start up load on the motor. Conversely, the check valve could prevent the system from starting up "dry" resulting in longer life for the pump. More information of the system specs required.

How about installing a check valve at the discharge?

Best call AFAIC, and GA.

Dear Mr. Tornado,

I want to supplement to your views.

Installing a check valve should be considered with a Pressure relief valve, since sudden tripping may cause severe water hammer, more the head, more will be the intensity of water hammer.

DHAYANANDHAN.S

As I have been told by pump guys, a check valve on a vertical pump won't work on the discharge side, it has to be at the bottom, which is often a maintenance headache.

That's known as a foot valve, which should have installed at the get go. That would have been cheaper and easier. Still installing the proper foot valve seems the best solution.

Perhaps there IS a foot valve and it has failed or become blocked open, leading to this problem ??

Unfortunately like many other post on CR4, the OP doesn't give much information as to the history of the problem. Has this been a problem with the pump since it was commissioned or is this a new problem?

I don't see why a properly spec'd out check valve wouldn't work in this case. Very low cracking pressures should be no problem; if the check valve is close to the system discharge, it will have a low cracking pressure and close more securely as the head increases.

Check valves, be they foot or otherwise, represent a loss of flow, ergo an increase in required power for a given flow which then means increased operating costs. On a 650kW pump, a foot valve could represent tens of thousands of kWH per year in added energy costs. Typically a Vertical Turbine Pump is used because if they can keep at least the first bowl immersed in the water it will prime itself, meaning you DON'T need that foot / check valve to avoid loss of prime and you avoid that increase in operating and maintenance costs.

The ONLY down side is that the water left in the column will fall back into the hole (even if there IS a check valve on the discharge) and unless there is a back-spin ratchet, it will spin the motor backward. For a great many people, this is inconsequential, but as was mentioned, PFC caps will keep the motor excited and make it a generator. Assuming it's an Across-the-Line (DOL) starter, it is isolated from the line and harmless, but I still don't like it for safety reasons. A maintenance worker might open something up too soon, thinking it's safe because they killed power, when in fact the motor is still back spinning and the caps are keeping it charged. So he opens a box too soon and ZAP, he's dead. Isolating the PFC caps with a contactor that opens when the main contactor opens, removes the excitation and the motor stops generating after about 1 second.

The capacitor may be way oversized. For a 650kw motor, something like 150kvar might be more like it.

Install another contactor to disconnect your pF correction capacitors if you feel strongly about this!

You said
"This rotation is presumably greater than the speed of the motor as it generates voltage roughly 1.6 times the rated voltage of motor."

You use the word presumably.... have you measured either the RPM or generated voltage?

However, I would be more concerned about restarting the motor if its still back spinning!
Restarting the motor while pump and shaft are spinning the opposite way will break shafts, couplings etc, I hope you have a "back-spin relay" in the control circuit!!

You need to make this right!

Regardless of whether the pump/motor/capacitors can handle the loading short term.

With a motor/pump set of that capaity, the effect of priming the line each time the pump starts is also a significant issue for the pipeline attached. (Unless it's an open discharge outlet above the pump.)

Flow surges each time you start the pump will cause significant pipe movement at each change in direction.

If you don't have valves in place to maintain the line prime, what are you doing to the pump when you start on no-load? Cavitation??

Please review th ehydraulic implications of what is happening in your system also.

<...The problem is there is no check valve at the discharge...> Then, solve this problem.