We love electric motors and are happy to provide you the calculators so that you can do calculations with simple click. Please note that calculators and information provided here are for only information purpose.
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 5,252
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 (fan or blower) 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 x460 / 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 = 200
(Single shift operation)
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
Hp new = Hp original x ( Speed New /Speed Original ) x 3
Example:
Original Hp = 7.5
Original blower speed = 900 RPM
New blower speed = 750 RPM
Determine new Hp
Hp new = 7.5 x (750/900) x 3 or 7.5 x (750/900 x 750/900 x 750/900 )
7.5 x .83 x .83 x .83 =
7.5 x .57 = 4.3 Hp
Note: Driving energy requirement (watts) go up and down at approximately the same rate as Hp.
