Power signature of AC motors?

Question

Can someone explain how or direct me to informtion regarding the power signatures of AC motors (pumps) responds as load (head) increases? I have a pump in my aquarium that is rated at 120watts, but installed with the plumbing it only draws about 55 watts (measured with a kill-a-watt device). This surprised me, but apparently it shouldnt. Any details would be great, but i'd really love the technical side/equations. Thanks!!

Can someone explain how or direct me to informtion regarding the power signatures of AC motors (pumps) responds as load (head) increases? I have a pump in my aquarium that is rated at 120watts, but installed with the plumbing it only draws about 55 watts (measured with a kill-a-watt device). This surprised me, but apparently it shouldnt. Any details would be great, but i'd really love the technical side/equations. Thanks!!

The kill-a-watt measures both VA and W, as is my understanding, so it should account for the shift.

Answer 1

your kill-a-watt-device is designed to measure the direct power consumption as is in an electrical circuit.
This works great for lamps and heaters.
now since you have a pump in your circuit you need to consider that this is an inductive device.
Such devices cause a phase shift between voltage and current in your circuit which gives you messed up readings.
the other extreme would be a highly capacitive device.

with inductive devices the current delays within the circuit.
with capacitiv devices it runs ahead.

checkout this:
http://en.wikipedia.org/wiki/Apparent_power

your device measures the real power (so i think) cause it calculates P from Volt and Ampere at every given moment.
So it is unaware that the Amperes are a little delayed

So those guys constructing the motor measured it with other equipment being aware of that and putting the apparent power as a value on the case .. what confuses me a little is that the wattage shoud read 120 VA not 120 W on the motor