The following equations for calculating horsepower are to be used for estimating purposes only. These equations do not include any allowance for machine friction, windage, or other factors. These factors must be considered when selecting a motor for an application.
Where:
hp = Horsepower
T = Torque (lb-ft)
N = Speed of motor at rated load (rpm)
If the calculated horsepower falls between standard available motor ratings, select the higher available horsepower rating. It is good practice to allow some margin when selecting the motor horsepower. Also note that the motor’s torque output is reduced during a soft start. The load requirements must be related to the soft starter settings.
For many applications, it is possible to calculate the horsepower required without actually measuring the torque. The following equations will be helpful:
Fans and blowers:
Pumps:
Where:
psi= Pounds per square inch
cfm = Cubic feet per minute
gpm = Gallons per minute
Specific gravity of water = 1.0
1 cubic foot per second = 448 gpm
1 psi = A head of 2.309 ft for water weighing 62.36 lbs per cubic foot at 62°F
Efficiency of fan or pump = %/100
