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 on Motors

Kilowatt Hours = (HP** x .746 x Hours of Operation)/(Motor Efficiency (Decimal))
**Average Load HP (may be lower than motor nameplate HP)


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


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

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