P EI R
I x RI2 x R
E x I
PI
ER
E2
R
PE
EI
E2
PPI2
P x R
PR
Ohm's Law
MOTOR ENGINEERING FORMULAS
Motor Application FormulasHorsepower = Torque (lb-ft) x RPM
5252
Torque (lb-ft) = Horsepower x 5252 RPM
Torque (N-m) = Kilowatts x 9550 RPM
Kilowatts = Torque (N-m) x RPM 9550
Centrifugal Applications
Where:FT = Head in feet*GPM = Gallons per minutePSI = Pounds per square inch*Head in feet = 2.31 x pounds per square inch gravity
HP = GPM x FT x Specific Gravity 3960 x Efficiency of PumpHP = GPM x PSI x Specific Gravity 1713 x Efficiency of Pump
PUMPS
Gallons = 5.875 x D2 x H1 gallon (US) of water weighs 8.33 lb.Specific gravity of water = 1.0Where:D = Tank diameter (ft)H = Height of liquid (ft)
VOLUME OF LIQUID IN A TANK
HPCFM x PSF
33000 x Efficiency of Fan=
HPCFM x PIW
6356 x Efficiency of Fan=
HPCFM x PSI
229 x Efficiency of Fan=
Where:CFM = Cubic feet per minutePIW = Inches of water gaugePSF = Pounds per square footPSI = Pounds per square inch
FANS AND BLOWERSFlow2Flow1
RPM2RPM1
=
Pres2Pres1
(RPM2)2
(RPM1 )2
=
HP2HP1
Where:Pres = PressureRPM = Revolutions per minute
(RPM2)3
(RPM1 )3
=
AFFINITY LAWS
Three Phase ValuesFor 208 volts x 1.732, use 360For 230 volts x 1.732, use 398For 240 volts x 1.732, use 416For 415 volts x 1.732, use 719For 440 volts x 1.732, use 762For 460 volts x 1.732, use 797For 480 volts, x 1.732, use 831
Single Phase
746 x HPE x I x PF
Input WattsV x A
AC Efficiency and Power Factor FormulasTo Find
Efficiency
Power Factor
Three Phase
746 x HPE x I x PF x 1.732
Input WattsE x I x PF x 1.732
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
MM
©2004 Cerus IndustrialCerus Industrial provides these formulas 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
"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
0
1
2
3
4
5
6
INCHES
1-800-962-3787Y our S ourc e F or Motor C ontrols
CERUSINDUSTRIAL™
AC/DC FormulasTo Find
Amps whenHorsepower is Known
Amps whenKilowatts is known
Amps whenkVA is known
Kilowatts
Kilovolt-Amps
Horsepower(output)
HP x 7461.73 x E x Eff x PF
kW x 10001.73 x E x PF
kVA x 10001.73 x E
I x E x 1.73 x PF1000
I x E x 1.731000
I x E x Eff x 1.73 x PF746
Direct Current
HP x 746E x Eff
kW x 1000E
I x E1000
I x E x Eff746
HP x 746E x Eff x PF
kW x 1000E x PF
kVA x 1000E
I x E x PF1000I x E1000
I x E x Eff x PF746
AC / 1phase AC 3 phase
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.
440 to 480 VAC
1/101/81/61/41/31/23/41
1 1/2235
7 1/2101520253040506075
100125150200250300350400500
3.03.84.45.87.29.813.816.020.024.034.056.080.0100.0135.0
----------------
-----
4.04.86.49.011.816.626.438.048.072.094.0118.0138.0180.0226.0
-----------
1.51.92.22.93.64.96.98.010.012.017.028.040.050.068.088.0110.0136.0176.0216.0
-----------
-----
2.02.43.24.55.98.313.219.024.036.047.059.069.090.0113.0133.0166.0218.0
--------
-----
2.23.24.26.06.89.615.222.028.042.054.068.080.0104.0130.0154.0192.0248.0312.0360.0480.0602.0
----
-----
2.53.54.05.06.08.514.021.026.034.044.055.068.088.0108.0
-----------
-----
1.01.21.62.33.04.26.69.012.018.023.029.035.045.056.067.083.0109.0135.0156.0208.0
-----
-----
1.11.62.13.03.44.87.611.014.021.027.034.040.052.065.077.096.0124.0156.0180.0240.0302.0361.0414.0477.0590.0
HP SinglePhase
SinglePhase
SinglePhase
TwoPhase
-----
4.46.48.412.013.619.230.444.056.084.0108.0136.0160.0208.0260.0
-----------
ThreePhase
TwoPhase
TwoPhase
ThreePhase
ThreePhase
110 to 120 VAC 220 to 240 VAC
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