Motor Tripping Problem (Centrifugal Pump)

If it is tripping on overload, that usually means it is over loaded! If you measure the current and it is higher than the FLA on the motor, then you are overloading the motor.

Does changing the pump impeller have an effect? ABSOLUTELY!!!

In centrifugal pumps, "load" is equal to flow. If you increase the flow, you increase the load on the motor. An engineer would (should) have selected the motor so that the BHP requirements of the pump, based on expected flow at expected head at expected pressure, is about 80% of the motor HP (I'm sorry, I don't know the metric equivalent of BHP). If your flow, head or pressure is different from what the original design was, there is no telling what you are demanding of that motor now.

Perhaps trim some more off the problem impellers so they are non-overloading regardless of where the pumps operate along their respective curves.

(The fewer pumps you run, the less ΔP in the piping. This moves the pump operation to the right on the curve. The flow increases more than the pressure decreases; hence the BHP increases and thus can overload the motor.)

"Voltage 400 Volt plus minus 20 (Actual Voltage is 440 Volt)" Does this mean you are operating at 400V +- 20V and the motor is rated for 440V?

Is your phase to phase voltage and current balanced?

Yea, thats right. Motor is rated for 440 Volt.

It was balanced (Phase current difference was 1 amp to 2 amp then) before. We did checked it and tripping was there at that time too. But now phase current difference is 5 amp to 6 amp. How could we overcome That???

As JRaef mentioned in post#1, your motors are being overloaded, as a result of insufficient voltage.

I agree with everything else said here, but would add one more thing--get somebody out there with a (clamp on) ammeter and measure the current. Then you'll know for sure what is going on.

If it was happening on just one pump, I could suspect a defective motor starter/controller, the wrong overload heaters, or just overload heaters that have been in service for a long time and no longer work at the proper rating.

Are you having similar problems elsewhere on you system? If not I'd take a very close look at the supply to the 5 starters. It could be something as silly as a bad connection to the supply Dis board or MCC panel.
Whatever it is, it's common to them all.

if your pump head is more than required ,then also you face tripping problem,because if your pump is running under low head(with respect to design head) than flow will increase,naturally motor amp will increase and motor will tripp.

Keep the valves closed, while starting the pumps. Once the pumps start running at full speed, then only open the valves.

The problem will be solved.

Check your pump curve (if you have one), it appears your operating point requires next size motor, which is 37kW.

Why not upgrade motor & not throttle pumps' discharge valve?

This exercise may require uprating the supply cable, starter contactors and O/L relay & it's settings.

If it is the pump head being too low and valve throttling doesn't help, another solution is to put in a flow restriction orifce in the pipe so that you can move up the pump curve (to the left).

What is the shaft power requirement in kW when the pump pressure increase is multiplied by the volumetric flowrate though it?

Why have you installed 5 nos. of pumps for the said duty, I mean what was the selection criteria for selecting 5 pumps when only 2 run normally? What is the pump capacity-flow rate? Why have you reduced the impeller dia? Have done it in all the pumps?

BB Raina

Is this a new set-up or has it been in use some time.

If the system has been in use some time, what has been done if the pumps were running alright before this problem arose, e.g. pump maintenance, different valving, different diameter pipework, etc.

If it is a new set up, someone has stuffed up in pump/motor selection, incorrect dia. discharge piping (too small), etc.

Others have made valid comments, just to add or repeat a few:

• As it affects all motors the same way, forget the motors and look at the pumps and system.
• Roughness of the casting of the impellers can have a major effect on the power consumed.
• The reduction of impeller diameter should have reduced the power requirements.
• You have apparently cast new spare impellers - the fact that they weigh more even though they are of smaller diameter could have a major effect, especially if the channels are wider.
• If you have the curves for the pumps (original) then let us see them. If not, give the make and model and you can start looking at the generic family of curves to get some idea of what is going on.
• At what differential head do the pumps trip and where is this point on the curve. (I assume you have no flow meter).

Am I correct in assuming that your alignments have been checked thoroughly?

Dear Gurvir,

if I understood yr problem correctly, then before answering yr querry, may I first ask these questions.

1. When u r able to meet yr requirement with one or two pumps, why 5 pumps are installed ?

2. I presume further that u r running lesser flow thru colling tower that is why probably yr requirement of no of pumps have reduced

3. Did u collect data, as flow, disch pr, current, speed at full valve openning if VFD is not there?

4. On what reasons did the Impeller change ?

5. Adding some weight to impellers might not result in tripping, though it somewhat lead to little higher current but will surely not lead to tripping if pumps arenot very small in size.

6. Did you check the no load current, load current of al 5 pumps ?

7. pls check & inform the Shut off Pressures also alongwith all details regarding the pump flow, head, motor hp, speed etc. to enable me to understand and revert accordingly, probably I can help u in resolving this problem.

regds,

vivek srivastava