Motor Engineering Formulas
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The motor engineering formulas are provided here for convenient reference. They are for use only by expert personnel. In some cases they are rules of thumb and may not be applicable to code.

Legend: E = volts I = amps W = watts PF = power factor Eff = efficiency HP = horsepower

Ohm’s Law

Ohm's Law

Three Phase Values
For 208 volts x 1.732, use 360
For 230 volts x 1.732, use 398
For 240 volts x 1.732, use 416
For 415 volts x 1.732, use 719
For 440 volts x 1.732, use 762
For 460 volts x 1.732, use 797
For 480 volts, x 1.732, use 831


AC/DC Formulas

Legend: E = volts I = amps W = watts PF = power factor Eff = efficiency HP = horsepower

To Find

Direct Current

AC / 1 Phase

AC 3 Phase

Amps when Horsepower (HP) is Known

HP x 746

________

E x Eff

HP x 746

__________

E x Eff x PF

HP x 746

______________

1.73 x E x Eff x PF

Amps (A) when Kilowatts (kW) is known

kW x 1000

_________

E

kW x 1000

__________

E x PF

kW x 1000

___________

1.73 x E x P

Amps (A) when kVA is known

kVA x 1000

_________

E

kVA x 1000

__________

1.73 x E

Kilowatts (kW)

I x E

_____

1000

I x E x PF

________

1000

I x E x 1.73 x PF

_____________

1000

Kilovolt-Amp (kVA)

I x E

____

1000

I x E x 1.73

_________

1000

Horsepower (output)

I x E x Eff

________

746

I x E x Eff x PF

___________

746

I x E x Eff x 1.73 x PF

_________________

746

 


AC Efficiency and Power Factor Formulas

To Find

Single Phase

Three Phase

Efficiency

746 x HP

________

E x I x PF

746 x HP

______________

E x I x PF x 1.732

Power Factor

Input Watts

__________

V x A

Input Watts

______________

E x I x PF x 1.732

 

 

 

 

 

 

 

 


Centrifugal Application Formulas

Fan and Blowers

CFM x PSF
HP = ______________________

33000 x Efficiency of Fan

CFM x PIW
HP = _____________________

6356 x Efficiency of Fan

CFM x PSI
HP = _____________________

229 x Efficiency of Fan

Where:

CFM = Cubic feet per minute

PIW = Inches of water gauge

PSF = Pounds per square foot

PSI = Pounds per square inch

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Pumps

GPM x FT x Specific Gravity
HP = _________________________

3960 x Efficiency of Pump

GPM x PSI x Specific Gravity
HP = _________________________

1713 x Efficiency of Fan

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

 

 

 

 

 

 

 

 

 

 

 


Volume of Liquid in a Tank

Gallons = 5.875 x D² x H

1 gallon (US) of water weighs 8.33 lb.
Specific gravity of water = 1.0

Where:
D = Tank diameter (ft)
H = Height of liquid (ft)



Affinity Laws


Motor Application Formulas

Torque (lb-ft) x RPM

Horsepower = ______________

5252

Torque (N-m) x RPM

Kilowatts = _________________

9550

Horsepower x 5252

Torque (lb-ft) = ____________

RPM

Kilowatts x 9550

Torque (N-m) = _________________

RPM


 

 

 

 

 

 

 


IEC-Style Contactors and Starters Selection Guide

Average Amperages for Given Horsepower Motor Ratings

The table below provides the average full-load currents of squirrel cage motors based on NEC (National Electrical Code) Tables 430-148, 149, and 150. These values are given only as a guide – they may vary depending on the type of motor and manufacturer. Refer to the actual motor nameplate for full-load current values.

IEC-Style Contactors and Starters Selection Guide


“Guesstimating” Motor Current Draw*
At 575 volts, a 3-phase motor draws 1 amp per horsepower.
At 460 volts, a 3-phase motor draws 1.27 amps per horsepower.
At 230 volts, a 3-phase motor draws 2.5 amps per horsepower.
At 230 volts, a single-phase motor draws 5 amps per horsepower.
At 115 volts, a single phase motor draws 10 amps per horsepower.
*These will vary by motor type and application
1 HP = 746 watts

Some Images Courtesy Cerus Industrial

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