## Monday, February 15, 2021

Star-delta starting is widely used in electrical engineering for starting three-phase asynchronous electric motors. Free resources are readily available for star-delta starting wiring diagrams.

But why choose such a device? This page gives you an explanation of the electrotechnical point of view and the mathematical approach which leads to the understanding of the star delta starting device.

Problem

There are three identical receivers with impedance Z, a power factor Cos phi and a three-phase network of simple voltage V; phase-to-phase voltage U.

Write the formula which gives the intensity in line when we couple the three receivers in a star, then when we couple them in a triangle.
Studied the I triangle / I star ratio.
Write the formula which gives the power in star then in triangle.
Studied the P triangle / P star ratio.

Writing convention: a triangle (triangle symbol) as an index of a quantity means that we consider this quantity in the triangle coupling mode.
A star (Y or star symbol) in index of a quantity means that this quantity is considered in the star coupling mode.

Star position
Consider the following diagram where our three receivers are star-coupled (connected):

.

Line Intensity
We have: Line Intensity = Receiver Intensity (IL = IZ) or:

.

Star power
Let's calculate:

.

Triangle position
Consider the following diagram where our three receivers are coupled in a triangle:
.

.

Line Intensity
We know that :

.

Power in triangle
Let's calculate:

.

Study of reports
Study of the Itriangle report on the star

.

Study of the Ptriangle on PÃ©toile report

.

Conclusion
This property is used for starting three-phase asynchronous motors where at the first stage the windings are star-coupled (In and P 3 times lower) and at the second stage the delta coupling is carried out.
The result in the same way is that the starting torque in star is three times lower than in delta.

Star-delta starting is widely used in electrical engineering for starting three-phase asynchronous electric motors. Free resources are readily available for star-delta starting wiring diagrams.

But why choose such a device? This page gives you an explanation of the electrotechnical point of view and the mathematical approach which leads to the understanding of the star delta starting device.

Problem

There are three identical receivers with impedance Z, a power factor Cos phi and a three-phase network of simple voltage V; phase-to-phase voltage U.

Write the formula which gives the intensity in line when we couple the three receivers in a star, then when we couple them in a triangle.
Studied the I triangle / I star ratio.
Write the formula which gives the power in star then in triangle.
Studied the P triangle / P star ratio.

Writing convention: a triangle (triangle symbol) as an index of a quantity means that we consider this quantity in the triangle coupling mode.
A star (Y or star symbol) in index of a quantity means that this quantity is considered in the star coupling mode.

Star position
Consider the following diagram where our three receivers are star-coupled (connected):

.

Line Intensity
We have: Line Intensity = Receiver Intensity (IL = IZ) or:

.

Star power
Let's calculate:

.

Triangle position
Consider the following diagram where our three receivers are coupled in a triangle:
.

.

Line Intensity
We know that :

.

Power in triangle
Let's calculate:

.

Study of reports
Study of the Itriangle report on the star

.

Study of the Ptriangle on PÃ©toile report

.

Conclusion
This property is used for starting three-phase asynchronous motors where at the first stage the windings are star-coupled (In and P 3 times lower) and at the second stage the delta coupling is carried out.
The result in the same way is that the starting torque in star is three times lower than in delta.