When hearing about a star delta starter, you may remember about a circuit to start an electric motor. Also, the name implies, it uses three phase star circuit and three phase delta circuit.
(You won’t meet a DC circuit with either star or delta circuit).
This star delta starter can be used to start a three phase induction motor. Even if we use a star delta starter, it doesn’t mean that our induction motor is connected to a star circuit and delta circuit at the same time.
We use both circuits alternatively. You will understand later, keep reading.
Why Do We Use a Star Delta Starter?
An induction motor is well-known for its sharp current surge at its starting phase. This surge can disturb voltage on the supply line.
Most common method to start an induction motor is to use Direct On Line (DOL) Starter and this can cause a huge spike in current, until the motor reaches its steady state it uses a specified rated current.
We can work the other way around by connecting the motor to the reduced voltage supply when we start the motor and switch the supply to the full voltage when the motor reaches steady state or specified speed.
What is Star Delta Starter?
Star delta starter means we connect the stator with a star connected three phase supply until it reaches its specific percentage of maximum speed, then we switch the connection of the stator from the star circuit to delta circuit.
To simplify that:
A star delta starter is to energize the stator with a star circuit three phase supply then switch it to delta circuit after it reaches a certain speed.
Observe the circuit diagram below:
What is the advantage of using this circuit? This circuit is famous for its reduced starting current without any extra parts.
For how much it reduces the starting current?
Star delta starter only needs approximately 30% or ⅓ of inrush current if compared to the DOL Starter.
What is the inrush current?
Inrush current, input surge current, or switch-on surge is the maximal instantaneous input current drawn by an electrical device when first turned on.
This is why a typical squirrel-cage induction motor will draw multiple times of its running current or steady-state current.
How Does a Star Delta Starter Work?
Since both star or delta circuits are a three phase circuit, we will have three poles. In order to build a proper switching mechanism, we will use a Triple Pole Double Throw switch.
Three poles are connected one by one to the three phase supply, while the double throw will switch between star and delta circuit.
To start the motor, the stator windings are connected to the star circuit.
This is where we calculate how much current is needed to start the motor. And of course, our focus here is to prove that less current is needed to start an induction motor with a star circuit.
Observe the equivalent circuit below.
Where we have parameter below at steady-state condition:
Since we use the star circuit, then the star phase current (IP1) is equal to the line current (IL1).
Since it is star circuit then our voltage across each phase for each winding is
Using Ohm’s Law, the current through each phase for each winding is
As stated above that the star phase current (IP1) is equal to the line current (IL1), we can conclude that
Let us compare the star circuit above with the delta circuit below to start a motor.
The winding of the stator is connected to the delta three phase supply circuit.
As the same as star circuit, we have parameter below at steady-state condition:
Delta circuit is opposite of a star circuit, the delta line current (IL2) is
The delta phase voltage across each winding is
Using Ohm’s Law, the delta phase current through each winding is
As stated above that the delta phase voltage (VP2) is equal to the line voltage (VL), we can conclude that
As a conclusion, we compare the needed current to start a motor using delta supply circuit to the star supply circuit.
As you can see, the current needed to start an induction motor with a star circuit is only ⅓ of the delta circuit.
But what is the drawback of a star delta starter compared to the DOL (Direct On Line) starter?
Ignoring all the variables except for voltage in the torque formula, we know that torque is proportional to the square of the line voltage (VL). We compare the starting torque of an induction motor with the star circuit to the delta circuit.
With the same motor, the star delta starter produces ⅓ of the torque using DOL starter.
What are the Advantages?
- The very crucial one, is to reduce the current needed to start a motor about ⅓ of DOL starter.
- Does not need an external device to regulate the supply after the motor reaches steady state.
What are the Disadvantages?
- The drawback of reduced current is the reduced starting torque to ⅓ of DOL starter.
How Can We Use a Star Delta Starter?
We can freely use this star delta starter if we don’t need a high start torque application, every application that only needs low torque as long as it runs.
It is not recommended to use this type of starter if your application needs high start torque, just use the usual DOL starter.
How do we know that we don’t need high start torque?
It depends on your initial load, if it is heavy then DOL starter is recommended over the star delta starter, and vice versa.
It will be very difficult to start a motor with a star delta starter if the motor is fully loaded from the start.
It will be very efficient to start a motor with a star delta starter if the motor is loaded with none to light load then increases over time until steady state.
