3 Phase Induction Motor Current Formulas [RLA and FLA]
The 3 Phase Induction motor is a rugged, reliable and efficient type of motor which is widely employed in electrical industries.
Contents
List
3 Phase Induction Motor current formulas  

RLA from HP 
HP * 745.7
/
E * %Eff* PF * √3

FLA from HP 
HP * 745.7
/
E * %Eff * PF * √3
* SF 
RLA from watts 
watts
/
E * PF * √3

FLA from watts 
watts
/
E * PF * √3
* SF 
where
HP = Horsepower of motor
E = Volts
%Eff = Percentage Efficiency
PF = Power Factor
√3 = A factor used for 3 phase. You can also use 1.73 instead.
How to calculate RLA from HP
RLA or rated load amperes of the motor is a ratio between HP * 746 to the product of voltage, efficiency and power factor.
Example: A 2 HP, 400 V 3 phase induction motor, having efficiency 85% is operating at a power factor of 0.9. Calculate the amps.
Solution: [ 2 * 746 ] / [1.73 * 400 * 0.85 * 0.9 ]
= 2.81 A
How to calculate FLA from HP
FLA or full load amperes formula of the motor is identical to RLA with the difference of SF. The formula is multiplied by service factor to obtain the FLA.
Example: A 1/2 HP, 330 V 3 phase motor running at a power factor of 0.87 has an efficiency of 90%. Find the FLA when service factor is 1.25.
Solution: { [ 0.5 * 746 ] / [1.73 * 330 * 0.87 * 0.9 ] } * 1.25= 0.83 * 1.25 A = 1.03 A
How to calculate RLA from watts
Formula = watts/[E * PF * 1.73]
Example: A 2 kW, 400 V motor operates at power factor of 0.9. Find the rated amperage.
Solution: I = 2 kW/[400 * 0.9 * 1.732]
= 3.2 A
How to calculate FLA from watts
Formula = {watts/ [ E * PF * 1.73] } * SF
Example: A 1 kW, 330 V machine operates at power factor of 0.88. Find the FLAs. When Sf is 1.15
Solution: I = {1 kW/[ 330 * 0.88 * 1.73] * 1.15
= 2.28 Amps
Other useful equations and calculations: