# Does a multi-speed fan operate at a different wattage at different speeds? [closed]

This question has been phrased to be a non-use, theoretical question on electrical design, as it has always been intended. Note to moderators: please be careful editing or re-phrasing to reduce the risk of this becoming a usage question.

Will an AC electric fan with discrete speed settings draw less current at lower speeds or will it simply be less efficient, producing a higher percentage of waste heat and a lower percentage of kinetic energy?

Almost certainly the fan will use less electricity at the lower speed setting. A few fixed speed settings are usually implemented by different parallel/series combinations of windings in the motor, so the motor efficiency should not vary greatly between settings.

I did science.

# TEST ARRANGEMENT

Fig. 1 TEST OBJECT, MEASUREMENT INSTRUMENT, AND POWER SUPPLY ARRANGEMENT.

# TEST OBJECT

DIMPLEX desk fan type GDCDF30 TMB. Rated 220-240V - 50 Hz, 40 W.

Fig. 2 TEST OBJECT NAMEPLATE

# MEASURING INSTRUMENT

Efergy plug-in power meter type EMS-AU. Wattmeter accuracy ±2% or ±1W.

Fig. 3 DETAIL OF MEASUREMENT INSTRUMENT

Fig. 4 MEASUREMENT INSTRUMENT NAMEPLATE

# TESTING DATA

Speed setting     Power (W)     Power factor
0              0 W             -
1             24 W             94 %
2             29 W            100 %
3             35 W          100 - 97 % (flicks between these two values)


All values read off after fan was left to run on the speed setting for one minute.

# CONCLUSIONS

The conclusions of this study are as follows.

• Rated power of 40 W is approximate.

• The settings 1, 2 and 3 do not give a linear decrease in power consumed.

Note, I do not have the means to measure the work done by the fan on the air, nor the output airspeed, so I cannot speak as to the total efficiency of the fan in terms of electrical energy converted into moving air.

• This has been yet another evening well spent in my house, brought to you by caffeine and sleepless nights. – Li-aung Yip Apr 28 '14 at 15:54
• You could estimate the work done on the air by measuring the speed of the fan, combined with knowledge that drag is proportional to the square of velocity, and that work is torque times angular velocity, and that if the speed of the fan is constant then torque must balance drag. – Phil Frost Apr 28 '14 at 15:59
• @PhilFrost What household materials can I use to improvise a tachometer? (Is there an android app for that? :P ) – Li-aung Yip Apr 28 '14 at 16:01
• You could lightly touch the blades with something to make a sound, and measure the resulting frequency. There probably is an app for that. – Phil Frost Apr 28 '14 at 16:02
• @PhilFrost: Then the work done is w = K * v^3 (with K a combination of constants such as drag coefficient, integration of angular speed into speed (considering the drag coefficient is identical along the blades, which is probably quite far off)). – njzk2 Apr 28 '14 at 17:52