Star Delta Starter Explained - Working Principle

Hey guys, this is Paul from TheEngineeringMindset.com. In this video, we will learn how Star Delta

star

ters for three-phase induction motors work. Remember, electricity is dangerous and can be fatal. You must be qualified and competent to carry out any electrical work. This is a real world Star Delta Starter. And at the end of this video, you'll be able to tell me what each part does and how the whole system works together. Now for this video, I'm going to use the old red, yellow, and blue color code for the phases simply because I think it's easier to see.

However, I will also show you versions with colors used in the US, EU, UK and Australia a little later in this video. Three-phase motors are used in almost all commercial and industrial buildings. Inside a three-phase induction motor we have three separate coils that are used to produce a rotating magnetic field. When we pass an alternating current through each coil, the coil will produce a magnetic field that changes in intensity and polarity as the electrons change direction. But, if we were to connect each coil to a different phase, then the electrons in each phase will change direction back and forth at different times compared to the other phases.

This means that the magnetic field will change intensity and polarity at a different time than the other phases. To distribute this magnetic field, we need to rotate the coils 120 degrees from the last phase and then combine them in the motor stator to produce the rotating magnetic field. This rotating magnetic field will cause the rotor inside the coils to rotate. And then we can use it to drive fans, pumps, etc. On the top, or sometimes on the side of the motor, we have an electrical terminal box. I'll move that box here so it's easier to see. Inside this electrical terminal box, we have six electrical terminals.

Each terminal has a letter and a corresponding number. We have U-one, V-one and W-one and then W-two, U-two and V-two. We have our phase one coil connected to the two U terminals. Then we have the phase two coil connected to the two V terminals. And then we have the phase three coil which is connected to the two W terminals. Below is a Real world example of the electrical terminal box of an induction motor. Now I'm going to test your understanding of this a little bit later in this video. Note that the coil terminals are in a different arrangement from the top to the bottom side.

We'll see why this is so in a moment. We now introduce the three-phase power supply and connect them to their respective terminals. We always connect the power side to the U-one, V-one and W-one terminals. Now for the motor to run, we need to complete the circuit. And there are two ways to do this. The first way is the Delta configuration. To do this, we connect through the terminals U-one to W-two, V-one to U-two and W-one to V-two. This will give us our Delta configuration. Now, when we pass an AC current through the phases, we see that electricity flows from one phase to another as the direction of the AC power is reversed in each phase at a different time.

That's why we have the terminals in different arrangements because we can connect and allow electricity to flow between the phases as the electrons reverse direction at different times. By the way, if you want to learn more about how three-phase electricity works, we've covered it in great detail in our previous videos. Look at them. The links are in the video description below. Now the other way in which we can connect the terminals is to use the Star configuration. In this method, we connect between W-two, U-two and V-two on only one side of the motor terminals. This will give us our Star equivalent design.

Now, when we pass alternating current through the coils, we see that the electrons are shared between the phases at the terminals. So looking again at this real world example of motor terminals, can you tell me what method is being used? Three, two, one, right, that's the Delta configuration. The two ways we just saw of setting the engine to Star or Delta are fixed methods. To change them we have to physically cut off the power, open the motor terminals and then rearrange them. Now, this isn't exactly practical to do. So how can we automate this? To do this, we need to use some contactors.

They now come in various designs. But if we look inside one, then the basic operation is simply a switch that can open or break a circuit to control the flow of electricity in all three phases simultaneously. So we take our main contactor and then connect our three phase supply to one side and then connect the other side to the respective terminals inside the induction motor electrical terminal box. We then take a second contactor that will be used for our Delta circuit and we also feed our three phases into it. From here, we connect our phase one to terminal V-two, which is the phase two coil.

We then connect our phase two to the W-two terminal, which is the phase three coil. And finally, we connect our phase three wire to the U-two terminal, which is the phase one coil. Now we take another contactor, which we will use for our

star

circuit, and connect our three-phase power to it. At the top, we simply connect the three phases together. I'll just remove the housing from the contactors so we can see what's going on inside. Now we start in the Star configuration. And for this we activate both the terminals of the main contactor and the Star contactor so that they close to complete the circuit.

Now, when we pass electricity through the circuit, the electricity passes through each phase and coil and then exits through the motor terminals and reaches the star contactor, where the path of the electrons is shared. This allows them to enter and exit another phase as the direction of each phase changes. The Star connection method will run for a few seconds before switching to Delta. For the Delta connection we disconnect the Star contactor and then close the Delta connection. All this happens very quickly. Now we have the electricity flowing but its direction is divided. It flows to both the main contactor and the

delta

contactor.

Electricity in the main contactor path will flow to the motor coils. And the electricity that took the Delta contactor path will flow to the opposite side of the motor terminals. Each will then flow between different phases as they reverse direction. To control the switching from star to

delta

contactors, we simply use a timer to control it. This will automatically change the settings after a set period of time. Additionally, there are more advanced versions that will control the amps or speed of the motor. If you are in the US, these colors may be used. This is for a 208 volt three phase supply but the colors will be different if you use a 480 volt three phase supplier.

In the UK, EU and Australia these colors are used for phases. If you're in the UK, you'll probably still find versions in red, yellow and blue. This is an old and outdated color system. But the old facilities will continue to exist. Going back to the real world photo of a Star Delta Starter, can you tell me which part is which? Three, two, one, right. This is the main contactor, this is the Delta contactor, this is the Star contactor and this is the timer. Notice that on the Star contactor, they just connected two wires to the same terminal to create that Star point.

So why do we use Star Delta

starter

s? We use the Star Delta

starter

to reduce the inrush current when the motor starts. When a large induction motor starts at Delta, the starting current can be five times greater than the full load current that occurs when the motor stabilizes and operates normally. This huge increase in current will cause many problems. The building's electrical system will be affected by this sudden large demand. Electrical infrastructure will rapidly rise in temperature, leading to component failures and even electrical fires. The sudden high demand causes voltage drops throughout the building's electrical system.

This can be seen visually because the lights will dim. This can cause many problems for sensitive equipment such as computers, servers, and security systems. So to reduce the starting current, we simply need to reduce the starting voltage. The star configuration will reduce the coil voltage to about 58% compared to the delta configuration. A lower voltage will lead to a lower current. The current in the coil while in Star configuration will be about 33% of the Delta configuration. This will also lead to a reduction in torque. Star configuration torque will also be around 33% compared to Delta. So let's look at a simplified basic example of what's happening here to understand it better.

Now let's say we have the motor connected in Delta with a typical European supply voltage of 400 volts. That means that when we use a multimeter to measure the voltage between any two phases, we will get a reading of 400 volts. We call this line-to-line voltage. By the way, if you don't have a multimeter, I recommend getting one for your tool kit. It is an essential piece of kit for finding any electrical faults and developing your understanding of electricity. Links below to find out which one to get and from where. Now, if we measure across both ends of a coil, we again measure the line-to-line voltage of 400 volts.

Let's say each coil has a resistance or impedance, since this is 20 ohm AC power. That means we will get a current reading on the coil of 20 amps. We can calculate that from 400 volts divided by 20 ohms, which equals 20 amps. But the current in a line will be different. It's going to be 34.6 amps and we get that from 20 amps divided by the square root of three, which gives us 34.6 amps. This is because the phase is connected to the two coils. Now if we look at a star configuration, again we have a line-to-line voltage of 400 volts and we see this if we measure between any two phases.

But, with a Star configuration, all of our coils are at the Star point or the neutral point. It is from this point that we can draw a neutral line if necessary. So this time when we measure the voltage at the ends of either coil, we get a lower value of 230 volts. This is because the coil is not connected directly between two phases like the Delta version was. One end is connected to the phase, but the other end is connected to the shared point or neutral point. Therefore, the voltage is shared and will be lower because one phase is always reversed.

We can see the reading of 230 volts by dividing 400 volts by the square root of three, which gives us 230 volts. As the voltage is lower, the current will be lower as well. If the coil again has 20 ohms of resistance or impedance, then the current is calculated as 230 volts divided by 20 ohms, which gives 11.5 amps. Therefore, the line current in this design will also be 11.5 amps. As we can see with a Delta connection, the coil is exposed to the full 400 volts between two phases. But the Star connection is only exposed to 230 Volts between the phase and the neutral point.

So we can see that the Star uses less voltage and therefore less current compared to the Delta version. And that's why we use it first. Alright guys, that's all for this video. But to continue your learning, watch one of the videos on screen now and I'll see you there for the next lesson.