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
Torque Units 
Constant Value 
Pound Feet 
5252 
Pound Inches 
63,025 
Ounce Inches 
1,000,000 
Horsepower Required by Pumps
Centrifugal Pumps
HP = (Gallons per Minute x Head in Feet)/(3960 x pump efficiency)
Hydraulic Pumps
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
Average Hours per Month 
= 
730 
Average Hours per Year 
= 
8760 
Average Hours of Darkness per Year 
= 
4000 
Approximate Average Hours per Month
(Single Shift Operation) 
= 
200 
Annual Operating Cost = Annual Kilowatt Hours X Cost per KW Hour
Example:
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
