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 | |
| Shaft Stress (kg/mm2) = | KW x 4.96 x 106 | |
| 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 |
| Pres1 Pres2 | = | (RPM1 )2 (RPM 2) 2 | HP = | CFM x PIW |
| HP1 HP2 | = | (RPM1 )3 (RPM 2) 3 | HP = | CFM x PSI |
| Where: | Pres = Pressure | 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 | 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 | |||
| 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) | ||