How different between under current and low power protection

[PAP]

Dear sir
I have interesting question about the protection of equipment (ie pump) that connect to the motor. In the past I've ever used undercurrent technic to protect the pump from low flow(loss suction) , but now I see some plant use power metering instead. Theorically ,power input to motor = fn( current ^2) so I think that current can be represented for the required power by load (pump) . How about you opinion?
Cordially yours
PAP

 

[jbartos]

Suggestion: A measurement of power reveals more than a measurement of current since the voltage may fluctuate, when it comes to sensitive measurements. Simply, power is voltage times current. If the current is measured only, the voltage may be lower and the current setting still will not trip. Therefore, the pump will be less loaded than the current setting is set to trip.

 

[bigamp]

The current that goes into the motor is the vectorial sum of two components, the kW (real work) and the kVAr (reactive). The kW component represents the work the pump is doing plus losses in the motor. Since the losses in the motor are (hopefully) low compared to the work the kW component will be a good measure of the real work the pump is doing. The reactive component of motor current is reasonably constant throughout the load range, and it is a significant chunk of current. So, pure motor current is not linear with load, but the power reasonably is.

You can still use measurement of motor amps to get an idea of what the pump is doing but you just need to take care with the setting (be sure that it is above the motor no-load current). Measuring power is more positive and precise (and expensive).

 

[buzzp]

I have had a lot of experience in this area. It is my opinion, and most large pump manufacturers, that current monitoring is adequate for larger HP pumps (>5HP) and power monitoring should be used for smaller pumps. In any case, the power is linear with load since it takes into account the POWER FACTOR, voltage, and current. My advise is to monitor current (cheaper) for larger pumps and power monitoring for smaller pumps.