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.

List

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:

Summaryinduction-motor-formulas

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