Power Factor for AC Pumps

[mrhasan]

Originally Posted by sphelps

My understanding, as far as I know, is the power factor can be defined as how effectively a device uses the current available in the circuit. It's not really related to efficiency. AC pumps typically have a lower power factor than say DC pumps or other devices which follow basic V=I/R & P=VI. This relates to impedance which I don't want to get into but basically a high power factor of 1 means the device draws in X amount of amps and uses every last one. A power factor of 0.8 means it draws X amount but only uses 0.8*X while the remainder is fed back into the circuit so it doesn't actually use more power, it just draws more current.

PF does talk about efficiency. In any AC systems, there are two sorts of powers: real and apparent. You are using the real power to drive the motor (82w) but you are actually taking in more power from the outlet.

Electric companies don't bill you on how much your devices consume but on how much you are consuming from the grid. In this case, you are withdrawing 1.85A from the grid. They don't care about what's the power factor of your device and how much you consuming, they care about how much you are withdrawing. In this case, its 115v and 1.85A resulting in 212.75W. I am not sure whether there is any PF accounted in billing (I will get back to you once I get to know about it from my colleague who is yet to come). Even if they do, it is bound to be above 0.9. So you are actually consuming 212.75W (under unitiy pf condition) for the motor while your motor is actually consuming 82W only.

As for DC, there's one huge reason why they are considered so efficient, they don't have any PF involved. PF is due to the phase difference between voltage and current which DC doesn't have and hence its PF is always 1. Rock solid 1. But since we plug in DC to AC, there is AC to DC conversion stage and power factor corrections are not very very effective in bringing pf to rock solid 1. And hence you are getting 0.97 for the DC motor.