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Motor Start-Up Current Calculator, Formula, Motor Start-Up Calculation

Motor Start-Up Current Calculator

Enter the values of rated power, R_p(W), voltage, V_(V), power factor, PF and the efficiency, E to determine the value of start-up current I_su(A).

Motor Start-Up Current Formula:

Motor start-up current, also known as inrush current, is the high initial current drawn by an electric motor when it is first turned on.

This current is significantly higher than the motor’s normal operating current and can be several times greater.

The high start-up current is due to the need to overcome inertia and the absence of back electromotive force (EMF) in the initial moments of starting.

Managing this current is crucial to avoid tripping circuit breakers, causing voltage sags, and reducing the lifespan of electrical components.

During start-up, the motor must generate enough torque to overcome both static friction and any load inertia, which typically requires higher current.

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The start-up current I_su(A) in amperes is equal to four times the rated power, R_p(W) in watts, divided by the product of the square root of three, the voltage, V_(V) in volts, the power factor, PF and the efficiency, E.

Start-up current I_su(A) = 4 * R_p(W)/ 1.732 * V_(V) * PF * E

I_su(A) = start-up current in amperes, A.

R_p(W) = rated power in watts, W.

V_(V) = voltage in volts, V.

PF = power factor.

E = efficiency.

Motor Start-Up Current Calculation:

Calculate the start-up current for a motor with a rated power of 2W, voltage of 400 volts, power factor of 0.8, and efficiency of 90%:

Given: R_p(W) = 2W, V_(V) = 400V, PF = 0.5, E = 90%.

Start-up current I_su(A) = 4 * R_p(W)/ 1.732 * V_(V) * PF * E

I_su(A) = 4 * 2 / 1.732 * 400 * 0.5 * 0.9

I_su(A) = 8 / 249.552

I_su(A) = 0.032A.

Determine the rated power, if the start-up current is 0.05 amperes, the voltage is 480 volts, the power factor is 0.85, and the efficiency is 95%:

Given: I_su(A) = 0.05A, V_(V) = 480V, PF = 0.85, E = 95%.

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Start-up current I_su(A) = 4 * R_p(W)/ 1.732 * V_(V) * PF * E

R_p(W) = I_su(A) * 1.732 * V_(V) * PF * E / 4

R_p(W) = 0.05 * 1.732 * 480 * 0.85 * 0.95 / 4

R_p(W) = 0.05 * 334.611 / 4

R_p(W) = 4.1826W.

Enter Rated Power:		W
Enter Voltage:		V
Enter Power Factor:
Enter Efficiency:

Result – Start-Up Current:		A

Motor Start-Up Current Calculator, Formula, Motor Start-Up Calculation