|Synchronous Speed = (120 x frequency)/No. of Poles
Poles can be 2, 4, 6, 8, etc.
(Over 95% of motors sold are 2, 4, or 6 pole.)
Horsepower (HP) =(Torque x Speed)/Constant
Speed in RPM
Value of Constant depends on units used for torque
Horsepower Required by Pumps
HP = (Gallons per Minute x Head in Feet)/(3960 x pump efficiency)
HP = (Gallons per Minute x Pounds per sq. inch)/(1714 x pump efficiency)
Fans and Blowers
HP = (C F M x Pressure (Inches of Water ))/(6356 x efficiency)
Normal efficiency range is 50 to 75 percent.
Air Compressor Rule of Thumb
1 HP produces 4 CFM @ 100 PSI
Approximate Full Load Amps (3 Phase Motors)
Amps = (HP x 1.2 x 460)/(Motor Voltage)
Motor Watts (at full load) = (HP x 746)/Efficiency
Divide Watts by 1000 to get KW (Kilowatts)
OPERATING COST CALCULATION
Operating Cost on Motors
Kilowatt Hours = (HP** x .746 x Hours of Operation)/(Motor Efficiency (Decimal))
**Average Load HP (may be lower than motor nameplate HP)
GENERAL FORMULA ÷ ALL LOADS
Annual Operating Cost = Annual Kilowatt Hours X Cost per KW Hour
|Average Hours per Month
|Average Hours per Year
|Average Hours of Darkness per Year
|Approximate Average Hours per Month
(Single Shift Operation)
A fully loaded 20 HP motor with FL efficiency of 91.0% runs 2500 hours per year, at a location where power costs 7.5¢ per kilowatt hour.
What is the annual operating cost?
Annual Kilowatt Hours = (20 x .746 x 2500)/(.910) = 40,989
Annual Operating Cost = 40,989 x .075 = $3,074