Pump drawing more current than FLC

[cams1975]

Hi
I have a waste water Grundfos pump which has a full load current of 18amps.
The pump is drawing 30 amps when measured.

Can someone point in the direction of possible casuses for the high current drawn.

regards

 

[gepman]

There are several reasons why this could happen. Some are:

1) The pump could be pumping much more water than the motor was sized for. Try restricting the discharge valve until the amps come into the normal range.

2) The voltage at the motor terminals could be extremely low. Measure the voltage at the motor starter AND at the motor.

 

[cams1975]

thanks

 

[electricpete]

A few more

If recently installed, it could be a motor connection problem. Some motors have multiple connection. parallel/series connection for 240 volt / 480 volt and wye / delta for start / run. Using 480 volt connection for a motor on 240 volt system, or leaving the connections wye during run can give the motor low voltage and cause high currents even when pump is not demanding excess shaft horsepower.

For axial flow pumps. restricting the flow causes current to go up (opposite of radial flow pumps).

A good start to divide/conqure might be to figure out where you are on the pump curve (including adjustments for speed and density). The from pump curve figure out what shaft horsepower the pump is demanding from the motor. If the pump is demanding excess shp compared to motor nameplate, it's probably a pump/sizing/fluid side problem. If the pump is demanding shp less than motor rated but motor is drawing higher current than rated, it is a motor/electrical side problem.

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[Artisi]

I doubt very much that the pump is overloading the motor, Grudfos units are usually non-overloading across the curve and beyond. It is more likely a power connection problem as pointed out by electricpete.

 

[rcooper]

It could be caused by pre-rotation of the pumped fluid before it enters the pump. As a rough guide, have a look in the sump and see if there is any rotation of the contents. A small rotation at the surface could indicate a rapid rotation at the pump inlet.

The pump curves assume the flow into the pump is purely axial with no circumferential movement. Circumferential flow can alter the pump curves and the power drawn significantly.

Is this the only pump in the sump? Is there another pump which does not overload? Have a look at the current drawn by this second pump (if it exists) and see if it differs (higher or lower) than the pump causing problems.

If this is the problem, you will have to do some work in the sump to stop the pre-rotation. It is worth noting that this is not something CFD will pick up on as the rotation develops with time, and the last time I looked at CFD it only gives you a snap shot of the flow patterns, not those which are time dependant.

 

[Artisi]

I cannot see how pre-rotation would nearly double the power draw, pre-rotation usually causes a drop in performance and noisy operation.