Motor Starting Current Calculator

What is motor starting current?

When an induction motor is switched on at standstill it draws several times its rated current until it speeds up. This is the starting, inrush or locked rotor current. Motor datasheets give it as a ratio to rated current (I_a / I_n).

Calculation

First find the rated current. For a three phase motor:

I_n = P / (√3 × V × η × cos φ)

P is shaft power, η efficiency and cos φ the power factor. Then, depending on the starting method:

• Direct on line: I_a = k × I_n
• Star delta: I_a = k × I_n / 3
• Autotransformer: I_a = a² × k × I_n (a is the tap ratio)
• Soft starter or drive: the current limit that is set

Example

A 15 kW, 400 V motor with 90 percent efficiency and a power factor of 0.85 has a rated current of 15000 / (1.732 × 400 × 0.9 × 0.85) = 28.3 A. With a datasheet starting ratio of 6 it draws 170 A direct on line and 56.6 A with star delta. Direct on line, the apparent power is 117.6 kVA.

Lower current means lower torque

With star delta the starting torque also falls to one third. With an autotransformer torque falls with the square of the tap ratio. On machines that are heavy to start, a reduced current method may not get the motor moving.

Limits

The starting ratio is specific to the motor and must come from its datasheet. The tool estimates the steady current only. The first instant peak, the run up time and the voltage dip are not calculated. This result alone is not enough to select protection, cables or a generator.