Useful Formulas

Motor Application Formulas

Output

 Horsepower = Torque (lb. ft.) x RPM 5250 Kilowatts = Torque (N�m) x RPM 9550 Torque (lb. ft.) = Horsepower x 5250 RPM Torque (N�m) = Kilowatts x 9550 RPM

Speed – AC Machinery

 Synchronous RPM = 120 x Frequency (Hz) Number of Poles Percent Slip = Synchronous RPM = Full-Load RPM Synchronous RPM x100

Time for Motor to Reach Operating Speed (in Seconds)

 Seconds = Wk2 (lb. ft.2) x Speed Change (RPM) 308 x Avg. Accelerating Torque (lb. ft.) Wk2 = Inertia of Rotor + Inertia of Load x Load RPM2 Motor RPM2 Average Accelerating Torque = [(FLT + BDT)/2] + BDT + LRT 3
 Where: BDT = Breakdown Torque FLT = Full-Load Torque LRT = Locked-Rotor Torque

Shaft Stress

 Shaft Stress (psi) = HP x 321.000 RPM x D3 Shaft Stress (kg/mm2) = KW x 4.96 x 106 RPM x D3
 Where: D = Shaft Diameter (in or mm) HP = Motor Output KW = Motor Output psi = Pounds Per Square Inch RPM = Revolutions Per Minute

Centrifugal Applications

 Affinity Laws Fans and Blowers Flow1 Flow2 = RPM1 RPM2 HP = CFM x PSF 33000 x Efficiency of Fan Pres1 Pres2 = (RPM1 )2 (RPM 2) 2 HP = CFM x PIW 6343 x Efficiency of Fan HP1 HP2 = (RPM1 )3 (RPM 2) 3 HP = CFM x PSI 229 x Efficiency of Fan Where: Pres = Pressure RPM = Revolutions Per Minute CFM = Cubic Feet Per Minute PIW = Inches of Water Gauge PSF = Pound Per Square Foot PSI = Pounds Per Square Inch

Pumps

 HP = GPM x FT x Specific Gravity 3960 x Efficiency of Pump Volume of Liquid in a TankGallons = 5.875 x D2 x H1 gallon (US) of water weighs 8.35 lb. Specific gravity of water = 1.0 HP = GPM x PSI x Specific Gravity 1713 x Efficiency of Pump Where: FT = Head in feet* GPM = Gallons per minute PSI = Pounds per square inch *Head in feet = 2.31 x pounds per square inch gravity. Where: D = Tank diameter (ft) H = Height of liquid (ft)