Oversized motor protection

[MakingAComeback]

My company has plans to repurpose an existing 4 kV, 700 hp motor presently driving a 700 hp centrifugal water pump. The existing pump is being replaced with a smaller pump that requires only about 300 hp. What changes in the motor's existing protective relay settings, if any, would be customary for this arrangement?

 

[waross]

The protection is generally set for the motor. Settings may be varied for different types of loads and may be changed in service to avoid nuisance tripping.
The original recommended settings should be satisfactory. However it may be well to check that the settings have not been changed over the years.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

The only thing I might consider adding is maybe some additional open channel flow protection, ie broken pipe. In your old pump, if a pipe broke and the pump went open channel, the motor could deliver the pressure and flow the pump was rated for into that. Now the new pump is smaller in terms of capacity, but the motor can potentially deliver more that twice what the pump might be designed to handle so a broken pipe might result in the destruction of the pump. If you can program a kW trip into your protection relay, I would do that, set for the kW needed for maximum flow.

"Will work for (the memory of) salami"

 

[jraef]

Wish I had an edit function...

To be clear, in the old system if you went open channel, the motor would almost immediately overload and the protection relay would shut the pump down in enough time to avoid collateral damage. In the new setup, the motor is now so much bigger that this will not happen for a much much longer time, long enough to expose the entire pumping system to dangerous conditions.

"Will work for (the memory of) salami"

 

[squeeky]

The main problem that I have is that should there be mechanical problem from the motor bearings all the way to the pump seals and impeller the pump, the motor would try to deliver the required energy to overcome the increased mechanical load. It would only trip when it reached the 700 HP motor. It would eventually burn out. You don't state starting method. If it is a star delta, you need to worry where the overload sensing is done. If it is done on the incomer, then motor rated current. If it is under then main contactor or in the delta leg, then you must set it at rated current divided by 1.732. Current transformers and an electronic overload would give you the flexibility to play around until you met your start up criteria.