# Star delta contactor selection

- Oct 21, 2019-

Generally, there are two types of contactor. The current of the contactor connected in a triangle with a 37KW motor is approximately selected as 37*2≈74A. Then, check the product model manual and include 74A in the selected model.The current of the contactor as star connection should be 74/ square root of 3 ≈43A. As before, 43A should be included in the selection range.The rated current of the thermal relay is the same as that of the motor.The air switch is 100A.

A typical star delta starter has three contactors, main contactor, angular contactor and star contactor.The theoretical analysis is as follows:

1. When the triangle works, phase current I(phase)=U(N)/Z

Z is the reactance of each phase winding

Line current I(line)=I(phase)*√3

U(N) is the grid voltage

2. When the star is working, line current I (line) '= phase current I (phase)' = U (N) /√3/Z

For the motor that normally works in a triangle, its rated current is the line current when the triangle works.

If the motor is in star operation, its working current is

Line current I (line) '= phase current I (phase)'

U (N) /√3/Z

I (phase) /√3

I (line) /√3/√3

I (line) /3

That is, the line current of the star working is 1/3 of the line current of the triangle working

For example, a 55KW motor with rated current of 105A is required to start at star Angle.If the main contactor is connected to the main loop and the angular contactor is connected to the phase line loop, the current flowing through the main contactor is 105A during triangular operation, and 110A contactor is selected.The current flowing through the angular contactor is 1/√3 of the rated current, about 105/√3=60A, and 63A contactor is enough.The star working current is 1/3 of the rated current of the motor, namely 105/3=35A, so 40A star contactor is enough.