How to calculate my motor input power

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Discussion Overview

The discussion centers around calculating the input power of a motor, specifically in the context of a fan. Participants explore the use of a formula involving shaft power and motor efficiency, while considering factors that may affect the accuracy of the calculation.

Discussion Character

  • Technical explanation, Debate/contested, Conceptual clarification

Main Points Raised

  • One participant proposes using the formula Shaft Power (Supply) / Motor Efficiency to calculate input power, providing specific values for shaft power and motor efficiency.
  • Another participant agrees with the formula but suggests considering additional transmission losses from pulleys and belts, recommending an adjustment of 5-10% to account for these losses.
  • A third participant cautions that the rated power does not necessarily reflect actual power draw, noting that efficiency may vary with different operational conditions such as airflow and temperature, and emphasizes the need for more data or direct measurement for accurate estimation.
  • A later reply supports the use of plate data for estimating maximum power, suggesting it may suffice for the original inquiry.

Areas of Agreement / Disagreement

Participants express differing views on the reliability of the proposed formula and the importance of additional factors, indicating that multiple competing views remain regarding the accuracy of the calculations.

Contextual Notes

Participants highlight limitations in the provided data, including the potential variability of motor efficiency under different conditions and the lack of direct measurements, which may affect the reliability of the estimates.

Puzzled
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Hello,
I have a question about calculating input power.

Does the following formula work?

Shaft Power (Supply) / Motor Efficiency? We are trying to calculate what an existing fan is using but do not have assess to site. We have the shaft power and motor efficiency from the manufacturers data sheets.
Fan shaft power = 3.46KW
Motor Efficiency = 87%
Rated Power 7.5KW

Appreciate your input.
 
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Puzzled said:
Does the following formula work?

Shaft Power (Supply) / Motor Efficiency?
Yes, unless there is an additional transmission loss from pulleys and a belt. Maybe add another 5-10%.
 
Note that just because it is rated at a particular level doesn't mean that's what it is actually drawing. Efficiency at 3.46kW is unlikely to be the same as at 7.5kW, and the fan won't even necessarily draw 3.46 all the time - it'll depend on airflow, backpressure, altitude, temperature, etc. You won't be able to get a very good estimate without actually measuring, or at least having much more data than you've provided here.
 
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Likes   Reactions: CWatters
+1

Using the plate data should give you an estimate for the maximum power but perhaps that's good enough for your purposes.
 

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