How Star Delta Starters work for three-phase
induction Motors.
Remember, electricity is dangerous and can be fatal. You should be qualified and competent to carry out any electrical work. Now, this is a real-world star Delta Starter. And by the end of this video, you will be able to tell me what each part does and how the whole system works together. Now for this video, I'm going to be using the old red-yellow-bluecolour coding for the phases simply because I think it's easier to see. However, I will also show you versions with colours used in theUS, EU, UK and Australia a little later into this video. Three-phase motors areused in almost every commercial and industrial building. Inside a three-phase induction motor we have three separated coils which are used to producea rotating magnetic field. When we pass an ACcurrent through each coil, the coil will produce a magnetic field which changes in intensity as well as polarity as the electrons change direction. But, if we were to connecteach coil to a different phase, then the electrons in each phase will change direction between forwards and backward at different times comparedto the other phases. This means that the magnetic field will change in intensity andpolarity at a different time compared to the other phases.
To distribute this magnetic field, we need to rotate the coils 120 degrees from the last phase and then combine these into the motors stator to produce the rotating magnetic field. This rotating magneticfield will cause the rotor which sits inside the coils, to spin. And we can then use this todrive fans, pumps, et cetera. On the top, or sometimeson the side of the motor, we have an electrical terminal box. I'll just move that box hereto make it easier to see. Inside this electrical terminal box, we have six electrical terminals. Each terminal has acorresponding letter and number. We have U-one, V-one, and W-one and then W-two, U-two, and V-two. We have our phase-one coilconnected the two U-terminals. We then 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. Here's a real-world exampleof an induction motor's electrical terminal box. Now I'm going to testyour understanding of this a little later into this video. Notice the coil terminals arein a different arrangement from the top-side to the bottom-side. We'll see why that is in just a moment. We now bring in thethree-phase power supply and connect these to theirrespective terminals. We always connect the supply side to terminals U-one, V-one and W-one. Now for the motor to run, weneed to complete the circuit. And there are two ways to do this. The first way is the Delta configuration. For this, we connect across the terminals from U-one to W-two, V-oneto U-two and W-one to V-two. This will give us our Delta configuration.
Now when we pass an ACcurrent through the phases, we see that electricity flowsfrom one phase into another as the direction of AC power reverses in each phase at a different time. That's why we have the terminalsat different arrangements because we can connectacross and allow electricity to flow between the phases as electrons reversedirection at different times. By the way, if you want to learn more about how three-phase electricity works, then we've covered this in great detail in our previous videos. Do check those out. Links are in the videodescription down below. Now the other way that wecan connect the terminals, is to use the Star configuration. In this method, we connectbetween W-two, U-two and V-two on only one side of the motor terminals. This will give us ourStar-equivalent design. Now when we pass ACcurrent through the coils, we see that the electrons are shared between the phases at the terminals. So looking again atthis real-world example of the motor terminals, can you tell me whichmethod is being used? Three, two, one, correct,it's the Delta configuration. The two ways we just sawof configuring the motor in Star or Delta are fixed methods. To change them, we have tophysically cut 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. Now they come in various designs. But if we look inside one,then the basic operation is just a switch that caneither make or break a circuit to control the flow of electricity in all three phases simultaneously. So we take our main contactor and then we connect ourthree-phase supply to one side and then we connect the other side to the respective terminalswithin the induction motor's electrical terminal box. We then take a second contactor which will be used for our Delta circuit and we feed our threephases into this also. From here, we connect ourphase-one to terminal V-two, which is the phase-two coil. Then we connect ourphase-two to terminal W-two, which is the phase-three coil. And finally, we connectour phase-three wire to the U-two terminal,which is the phase-one coil.
Now we take another contactor, which will be use for our Star circuit, and we connect our threephase power into this. On the top, we just connectall three phases together. I will just remove thecasing of the contactors so we can see what's happening inside. Now we start in the Star configuration. And to do that, we activate both the main and Star contactor terminals so that they close tocomplete the circuit. Now when we pass electricitythrough the circuit, the electricity passesthrough each phase and coil and then out through the motor terminals and into the Star contactor where the path of the electrons is shared. This allows them the flowin and out of another phase as the direction of each phase changes. The Star connection methodwill run for a few seconds before switching over to Delta. For the Delta connection, wedisconnect the Star contactor and then close the Delta connection. This all happens very quickly. Now we have the electricity flowing in but it splits direction. It flows into both the main aswell as the Delta contactor.
The electricity in the main contactor path will flow into the motor coils. And the electricity whichtook the Delta contactor route will flow to the oppositeside of the motor terminals. Each will then flow betweenthe different phases as they reverse direction. To control the change overfrom Star to Delta contactors, we simply use a timer to control this. This will automaticallychange the configuration over after a set amount of time. Additionally, there aremore advanced versions which will monitor theAmps or motor speed. If you are in the US, then you might find these colours being used. This is for a 208-volt three-phase supply but the colours will be different if you're using a 480-volt,three-phase supplier. In the UK, EU and Australia,you'll find these colours used for the phases.
If you're in the UK, then you're probably still gonna come across versions with the red, yellow,blue colours being used. This is an old outdated colour system. But old installationsare still gonna exist. So coming back to the real-world photo of a Star Delta Starter, can you tell me which part is which? Three, two, one, correct. This is the main contactor,this is the Delta contactor, this is the Star contactorand this is the timer. Notice on the Star contactor, they've just connected twowires into the same terminal to create that Star point. So why then do we use Star Delta Starters? We use the Star Delta Starter to reduce the in-rush current when the motor starts. When a large inductionmotor starts in Delta, the starting current canbe over five times higher than the full load current which occurs when the motor stabilizesand runs normally. This huge surge in currentwill cause lots of problems.
The building's electricalsystem will be hit by this sudden large demand. The electrical infrastructure will rapidly increase in temperature leading to component failureand even electrical fires. The sudden large demandcauses voltage drops throughout the building'selectrical system. This can be visually seenbecause the lights will dim. This can cause many problemsfor sensitive equipment such as computers, serversand safety systems. So to reduce the starting current, we simply need to reducethe starting voltage. The Star configuration willreduce the coil voltage to around 58% compared tothe Delta configuration. A lower voltage willlead to a lower current. Current in the coil whilein Star configuration will be around 33% ofthe Delta configuration. This will also lead toa reduction in torque. The Star configuration torque will also be around 33% compared to Delta. So let's look at asimplified basic example of what's happening hereto understand it further. Now let's say we have themotor connected in Delta with a typical Europeansupply voltage of 400 volts. That means when we use a multimeter to measure the voltagebetween any two phases, we will get a reading of 400 volts. We call this a line-to-line voltage. By the way, if youdon't have a multimeter, then I highly recommend youget one for your tool kit. It's an essential piece of kit for any electrical fault finding and building yourunderstanding of electricity. Links down below for whichone to get and from where. Now if we measure acrossthe two ends of a coil, we again measure the line-to-linevoltage of 400 volts.
Let's say each coil hasa resistance or impedance as this is AC-power of 20 Ohms. 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 aline will be different. It will be 34.6 Amps and we get that from 20 Amps divided by the square root of three which gives us 34.6 Amps. That's because the phase isconnected to the two coils. Now if we look at a Star configuration, we again have a line-to-linevoltage of 400 Volts and we see that if we measurebetween any two phases. But, with a Star configuration, all our coils meet at the Starpoint or the neutral point. It's from this point that we can run a neutral line if we need to. So this time when we measure the voltage across the ends of any coil, we get a lower value of 230 Volts.
That's because the coilisn't directly connected between two phases likethe Delta version was. One end is connected to the phase, but the other end is connected to the share point or the neutral point. So the voltage is therefore shared and will be less because onephase is always in reverse. We can see the 230 Volt reading by dividing 400 Volts bythe square root of three which gives us 230 Volts. As the voltage is less,the current will be too. If the coil is again 20 Ohmsof resistance or impedance, then the current iscalculated by 230 Volts divided by 20 Ohms which is 11.5 Amps. The line current in this design will also therefore be 11.5 Amps. So 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 thephase and the neutral point. So we can see that theStar uses less voltage and therefore less current compared to the Delta version. And that is why we use it first.
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