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I am attempting to calculate the energy savings of a fan motor after a new runner and inlet cone was installed which claimed to improve fan efficiency. I have logged amp draw before and after the upgrade. The fan is in a large pulp and paper mill and is used for supplying combustion air to a Power Boiler. The site claims to have near unity power factor however I believe the fan motor is very old and probably has a much lower PF. My question is, do I need to take into account the site PF when calculating the energy consumption of the fan? or simply the PF of the fan itself?

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  • \$\begingroup\$ How are you defining "efficiency"? The amount of real power (kilowatts) used by the motor? Or the number of billable power units charged by your power utility - which might be real power, or apparent power, depending on where you live? \$\endgroup\$ – Li-aung Yip May 14 '15 at 17:23
  • \$\begingroup\$ I would guess that for a small change to the motor conditions, you can assume the power factor didn't change, so the amps ratio is the same as the power ratio. The real question with the fan is how much work does it really do? How would you know if it moved 50% more air for the same electricity, did you measure velocity and pressure? Would the furnace run hotter? Would a controller turn the fan speed down? You need to know this too. \$\endgroup\$ – tomnexus May 14 '15 at 18:30
  • \$\begingroup\$ I do not want to get into too much detail of the work the fan is actually performing before and after the upgrade. I am simply interested in the power consumption of the fan before and after the upgrade. The facility is claiming their site is unity power factor due to large refiner motors and therefore they are calculating consumption of the fan with a PF of 1, using measured amps. I believe the consumption of the fan should be calculated using the PF of the fan itself (<1). Is that correct? \$\endgroup\$ – ZAC May 15 '15 at 16:52
  • \$\begingroup\$ Ok. Yes, to calculate power consumption you need the PF, so you need a wattmeter or a digital power monitor. But I think it's safe to assume the power factor hasn't changed very much. \$\endgroup\$ – tomnexus May 17 '15 at 16:53
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Neither you, nor the fan, can tell what the power factor of the site is, just by looking at the electrical socket (or however the fan connects). The supply to the motor is an AC voltage, and the current drawn by the fan motor (and the phase relationship to voltage) is entirely dependent on the motor itself. You are correct that you can ignore the "site power factor".

Note that reading the fan current may not tell you what you expect. It might not change much, but the phase (and thus, the power factor) could, and that reflects the amount of actual work done by the motor.

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