Motor Power Requirements & Operation Cost Formulas
These are the fan laws. So, don't listen to the hype put out by some of the manufacturers who claim they invented them and produce the only equipment that they apply to.
S = Speed = RPM = Revolutions Per Minute.
CFM = Cubic Feet Per Minute.
W = Watts = Electrical unit of power.
HP = Horsepower = Mechanical unit of power.
1 HP = 745 Watts = Conversion of electrical power to mechanical power.
CFM2 = CFM1 x RPM2 / RPM1 or CFM is directly pegged to rpm.
If RPM is cut in half then CFM is also cut in half. If 1,040 RPM produces 1,200 CFM then 520 RPM will produce 600 CFM.
HP @ S2 = HP @ S1 x (RPM2 / RPM1)3 This demonstrates that the horsepower required to turn the fan is related to the cube root of the speed change. Or if the fan’s speed is cut in half, then the amount of air delivered is also cut in half but the Horsepower required is only 1/8 of the original Horsepower required. OR, a fully loaded ½ HP fan motor running at 1,040 RPM and producing 1,200 CFM will only require 1/8 HP to deliver 600 CFM at ½ speed of 520 RPM. This demonstrates that the power required to turn the fan reduces a lot faster than the reduction in CFM being delivered.
Watts2 = Watts1 x (RPM2/RPM1)3 or Watts2 = Watts1 (CFM2/CFM1)3 This demonstrates that the electrical power required to turn the fan drops by the cube of the speed change. Or drop the speed (RPM) in half and the Power (Watts) required is 1/8 the original power required. Our example: ½ HP fully loaded fan motor running at 1,040 RPM and delivering 1,200 CFM requires 745 watts/2 or 372.5 Watts. Cut this motor’s speed in half and you cut the air delivery in half but the Wattage required is 372.5/8 = 46.6 Watts.
Therefore, a fan motor must run 8 hours at half speed to use the same amount of electricity as it would running at full speed for 1 hour.
Now lets bump this up-against reality. Lets assume that our example fan is in an average furnace. On an average winter day it runs about 1/2 of the time. It therefore uses 372.5 watts / 2 = 186 watt-hours of electricity. Now we install a FanHandler that runs the fan full time. Now lets say there is a call for heat 3 times during that hour and the fan reaches top speed three times for 3 & 1/3 minutes each time (which it probably won’t) 10 minutes per hour = 1/6 hour x 372.5 watts = 62 watts and the other 50 minutes it uses 5/6 of 46.6 watts = 38.83 watts for a total of 38.8 + 46.6 = 100 watts per hour. For a savings of 86 watt-hours. And the home is comfortable, the air cleaner or filters are working full-time at much higher efficiency.
Monthly cost for the FanHandler equipped fan at $0.10 per KWH = 86 watt hours x 24 hours x 30 days per month = 61,920 watt-hours or 61.9 KWH x $0.10 = $6.19 per month. If you ran the full-speed fan round the clock, it would cost 372.5 watts X 24 hours = 8.9 KW = $0.89 per day X 30 days per month = $26.00 per month. If you ran the full-speed fan ½ the time it would cost $13.00 per month. You can use your electricity costs and horsepowers etc. to do the comparisons. It will prove that the FanHandler saves energy!!
Simply put, you can run the fan, with a good motor, for eight hours at half speed for the same cost as running it one hour at full speed.
There are some Permanent Split Capacitor motors being installed in equipment today that are designed to meet a price criterion. These motors will not follow the fan laws and will growl and rumble at low speed. They will not follow the fan laws. They will not reduce amp draw with speed reduction, or the amp draw reduction will not be in line with the fan laws. ECM motors are generally of much higher quality. Are much more efficient and silent running. The cost difference between high quality PSC motors and ECM motors is shrinking. Our recommendation is to install the ecMModulator along with an Genteq high efficiency ECM motor wherever possible.
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