How 3 Phase Transformers Work – why we need them

sponsored by madx, this is a three

phase

transformer, it has a Delta primary and a secondary and is rated for 2 KVA, confused, well don't worry, I'll explain all that to you. The transformer basically just takes one AC voltage and converts it to another voltage. We can get one, two, or even three different voltages from the secondary side, but there are no moving parts inside and no wires connecting from one side to the other. You've probably seen these big green boxes on the side of the road in the outback. It is a three

phase

transformer that supplies power to commercial buildings because they have a lot of lighting and the equipment inside your home has a lot less stuff, so you only get a single phase transformer.

It can be mounted on a pole or on a platform. We can see that there are three cables. going into the house but this is not a three phase transformer it is usually connected to a single phase and the neutral of the distribution net

work

then reduces this voltage to a much safer level inside we basically have a primary coil that connects through the phase. and the neutral, then we have another completely separate coil called the secondary and the two hot wires connect to the ends of this coil and the neutral connects to the center. These cables run into the property to your electrical panel.

The primary side has a single phase AC supply. the secondary side is also a single phase between the two hot wires giving us 240 volts, the current and voltage flow back and forth between these two since it is alternating current, but if we connect from the neutral to one of the hot bus bars we get 120 volts because we are using only half of the secondary coil, the other side also provides 120 volts and uses the other half of the coil. If we plug an oscilloscope into the outlet, we would see a sine wave. This sine wave has a positive. and the negative half of the AC sine wave if we connect to each bus bar and the neutral we see that one side is positive 120 volts while the other side is negative 120 volts, the difference between these gives us 240 volts, for What this is is just one phase being split into two, however you will often see banks of two or three

transformers

providing three phase power to commercial buildings, but where do the three phases come from?

Well, the power station generates three-phase AC electricity, the generator basically just spins. a magnet passes over a coil of wire, the magnetic field pushes and pulls the electrons in the coil back and forth as the stronger part of the north and south poles passes through the coil, this creates the wave sine wave with a positive half and a negative half and this is a single phase, the generator spins the magnet fast enough so that the sine wave repeats 60 times per second and that gives us a frequency of 60 htz, but notice that the power output is not constant with a single phase, we could add another coil which adds another phase and helps improve this or we can add a third coil which gives us three phases and we see much better constant power output.

Let's notice that we have three phases, but six wires. We also notice that current always flows forward in one coil and backward in another, so we can combine. the coils and current will share the wires only using

them

when necessary. We could try to send this power directly to the property, but it's too far away, so the resistance of the cable means we'll waste a lot of power just trying to get it. However, if we increase the voltage, we can send the same amount of power with less current, so we lose almost nothing. The power station feeds a step up transformer, this increases the voltage to hundreds of thousands of volts and this will keep the current low for longer.

On the long distance, when it reaches the city, it enters a substation and the voltage is reduced in a step-down transformer. This continues on the subtransmission lines and could power some larger industrial or commercial sites with their own dedicated substations, but otherwise continues. to the distribution substation where the voltage is reduced again and then distributed along the streets to the properties, houses will be connected to one of the phases while commercial properties will be connected to all three phases. Now, if you

need

to buy a transformer, consider Madx. They are basically like a Transformer Superstore, they have all sizes

transformers

in stock and ready to ship all over the USA, from 15 KVA to 15 MVA, they have both dry and oil filled types, if you

need

a transformer, quickly call madox transformer.

Also check out his channel for great tips and tutorials. I'll leave you a link below. Look at

them

. Three-phase transformers come in many designs, but inside we basically only have three single-phase transformers joined together, one transformer is simply two. separate coils of wire placed around a steel core when current flows through a wire, it produces an electromagnetic field around the wire when the current changes direction, the magnetic field also changes direction when we wrap the wire in a coil, The magnetic field joins together and forms a stronger magnetic field when the alternating current passes through the coil, the magnetic field will increase and decrease as well as change polarity as the current alternates direction, if we place another coil very close , the magnetic field will interact with the electrons in the second coil and the magnetic field induces a voltage in that coil if the path is complete on the secondary side then a current will also flow;

However, much of the magnetic field is being wasted, so we use a steel core to concentrate and direct the magnetic field, making it stronger. and more efficiently, the magnetic field will move around the core, this will induce eddy current inside the core material, this wastes energy and also generates heat, and we don't want that, so we use a lot of thin rolled steel sheets to build the nucleus. It helps to keep the Eddie currents as small as possible so when we apply a voltage to the primary side we get a voltage on the secondary output side and the frequency will remain the same if both coils have the same number of turns then the voltage and the output current is the same as the input voltage if the secondary side has less turns than the primary we get a lower output voltage but a higher current and this is a step down transformer if the secondary side has more turns than the primary then we get a higher output voltage but a lower current and this is a step up transformer in each case the power transferred is the same value for example if this step down transformer supplied 200 volts and 10 amps to the load then we have 2000 volt amps on the secondary side.

The primary side would see 5 amps and 400 volts, which is also 2000 amps vol or 2 KVA, so the power transferred is the same, but the voltage and current change. Now we could have three separate transformers, but to save material and space costs, we often combine them. three coils on the primary are connected to the phases in Delta or Y configuration. The coils on the secondary side can also be Delta or Y, so we can have a Delta Delta a and and a and Delta or a Delta and Transformer. The nameplate on the side of the Transformer tells you how it is configured and we connect one end of each coil to each other.

The other end connects to a phase notice. It looks like the letter Y so it is easy to remember from the center point. From the coils we can connect a neutral cable and also a ground connection, this allows us to connect to a single phase or to all three phases. We can see with this transformer that one side of each coil connects to a dedicated terminal for each phase, but the other end of each coil connects to the same point using four terminals, so we know this is a Y connection with Delta. All coils are connected end to end with phases connecting between the coils.

This forms a triangle that looks like the Greek symbol for Delta. There is no neutral with this design, so it stands alone. However, for three-phase loads there are some variations such as high leg and open Delta, but I will explain them later in the video. With this transformer we can see that each coil connects to one terminal, but the other end of each coil connects to a different terminal. terminal, so we know it is a Delta connection, the coils could be on either side of the core, but we usually find them placed concentrically, one surrounding the other, so perhaps we have a transformer mounted on a platform that feed a small commercial building, stickers and nameplate. tell us it is a 12,470 volt Delta primary and a 20820 volt secondary and the transformer is rated at 150 KVA so we have three phases and three wires entering the primary side of the transformer with 12,470 volts between each phase.

The secondary side is connected with four wires. leaving the transformer, it is a step down transformer with 208 volts between any two phases or 120 volts between any phase and the neutral. It can handle up to 150,000 volt amps total, so that's 50,000 per phase or per set of coils, which means we can supply up to 416 amps on each phase and through each coil on the secondary and that would cause four amps to flow at through the primary coils and that causes 6.9 amps to flow through each line, which is our maximum limit for the transformer. The actual KVA transferred depends on how much equipment you connect and power we simply cannot exceed the transformers limit otherwise it will overheat, cause a short circuit and simply burn out.

Our next EX example is this larger commercial building that might need to power 480 volt three phase motors from 277 volts. fluorescent lighting 28 volt appliances and 120 volt outlets outside the building we find a skid mounted transformer with a capacity of 12,470 g/ 7,200 and that is on the primary side and on the secondary side we have 480 v/ 277 Vol and the transformer has a capacity of 500 KVA and this is supplying the building, we know that it is a year transformer with four wires entering the primary side and four wires leaving and entering the building. It also tells us that the primary side is grounded, so there is 12,470 volts line-to-line or 7,200 volts line-to-neutral on the primary side and there is 480-volt line-to-line or 277-volt line-to-neutral on the primary side. secondary side.

Now this could go into the building and connect to a distribution board and from here it could power a panel that powers a 480 volt three phase motor and maybe powers a panel that supplies a smaller dry type step down transformer and that transformer charges a panel 208/120 volts. This smaller transformer is a 480 volt Delta primary and a 28 volt and 120 volt secondary and the transformer is rated for say 30 KVA so this provides 28 volts three phase or 120 volts phase to neutral within this transformer. We have three coils and the terminals labeled H1, H2 and H3. This is our high voltage side where the phases are connected for the Delta connection, then we have X1, X2 and X3 on the bottom. provides our three output phases and there is also a x0 terminal for the neutral notice on the coil these black jump wires and the seven connection points on each phase are called taps.

We can move the jump wires to increase or decrease the length of the coil. Let's say we have 84 turns on the primary with five taps and 20 turns on the secondary, we measure the supply side and we have 480 volts, so the data sheet says to use tap three on the coils of all phases, This has 80 turns and provides the designed 120 volts. however if we measure 54 volts on the supply side we would get 126 volts on the secondary so we need to step up the coil and use a tap with 84 turns to reduce the output voltage to 120 volts but if we only had the 456 volts on the primary then we will need to use tap 5 with 76 turns to get the 120 volt output so we can compensate for supply voltage variations.

Using these taps, we usually find the primary coil on the outside because we can easily change the connections on the big one. Skid-mounted transformers also have tap changes, but the coils are immersed in oil for cooling, so we can't access them. Therefore, the coils are connected to different points on the tap changer and the dial changes which parts of the coil are connected. smaller commercial buildings powered by transformers mounted on three poles connected in Delta and this provides 208 volts three phase or 120 volts single phase or it could be 480 volts and 277 volts are sometimes connected Delta Delta usually provides 480 volts three phase only sometimes only two are used transformers.

Forming an open Delta, it normally provides 2403-phase volts and 120 volts single-phase with a high leg of 28 volts. However, this design is missing a coil, so it has a reduced capacity. I'll explain why I've made these three in a moment. Phase transformer cups with the formulas and diagrams to make it very easy to remember and you can also grab my PDF sheets, links below, if you want one for and connections, we have the three coils joined in the center at this point. We normally ground the transformer and run a neutral wire from here, so we have a three phase four wire system which gives us a line voltage or line to line voltage and also our phase voltage which is often called voltage line to neutral in this For example, we have a 208 120 volt secondary, so we have 208 volts between any two lines or we have 120 volts between any line and the neutral.

Now it is the coil that produces the 120 volts and that depends on how many turns the coil has and the primary voltage applied, the simple reason we get 208 volts is because we are connecting two coils, but these coils are not in phase, one is in the positive cycle while the other is in the negative at the widest point, approximately 120° rotation, the a phase will be 104 Vol RMS while the c phase will be negative 4 Vol RMS, so the difference is 208 Vol or we can solve using trigonometry. We have three coils that are all 120 volts and are separated by a 120° rotation so all we need to know is the length of the side of the trigonometry triangle we use this formula and lower our values ​​and that gives us 208 volts between Two phases.

Now no one wants to write that down every time, but look at the relationship between the two voltages, it's about 1732, etc., etc., this number. It goes on and on, so engineers just say 1732 or use the Ro T square of three because it's even easier to get it because if we square this ratio, which means we just multiply it by itself, we get three, so to undo that and finding the original number or the root that was used to make this number square, we just take the square root. the square root of three is 1732 Etc. so if we know the phase voltage we multiply it by the square root of three to find the line voltage and if we know the line voltage we divide it by sare < TK de 3 to find the phase voltage luckily the current in the Y configuration is very easy, the phase current is the same as the line current so if we had 100 amps on the line we would also read 100 amps across the coil in the Delta connection, each coil is joined end to end the phase wires are connected to the intersections between two coils, this gives us a three phase three wire system, there is no neutral so we only have voltages line voltage in this example we have 480 volts between any two phases, the line voltage is the same as the voltage across the coil or the phase voltage because we only have one coil between any two phases, but the current is divided between two coils and we know they flow at different times so the line current will be greater than the phase current if we know there are 43.3 amps flowing in the line then we divide it by the square < TK of 3 to get 25 amps in the coil and if we know that the coil current is 25 amps, then we multiply by the square < TK of 3 to get 43.3 amps. the line, a fairly quick way to see mathematically is to simply separate each of our coil currents by 120° to find line a, we have coil C to a and coil A to B connected to it, so we reverse the coil C a a, which sits 60 from the other two coil currents, then we slide that line to the end of the coil current line ab and we're actually just making a parallelogram, but we're going to skip that step, we know that this angle must be 180° total, so 180° minus the 60° we already have means that this remaining angle is 120°, so our current line will be the resulting line between these two points, so now we only have a triangle with one unknown side and we can use trigonometry to solve Then our line current is therefore 43.3 amps, which is 1732 times greater than the coil current, which is equivalent to the square < TK of 3, so If we had a Delta transformer and with 80 turns in the primary and 20 in the secondary, we have a ratio of 4:1 the primary side has 480 volts between two phases, so there are 480 volts in the coil.

We calculate that the voltage on our secondary coil is therefore 120 volts and that is our phase voltage, so we can easily find the line voltage of 208. volts, the secondary side has 100 amps on each phase, we find the primary winding current of 25 amperes through each Cor oil, the current in each line is therefore 43.3 amperes, so the primary side power is 36,000 vol amperes. The secondary side is also 36,000 amps, so the power transferred is the same but the voltage and current are transformed into the delta of the high leg. We have three coils connected in delta. In this case, the voltage between any two phases is 240 volts.

The voltage across the Ross coil is also 240 vol, but one of the coils has a wire connected to the center of the coil, this is also connected to ground, the wire acts as a neutral, so we can use the half of the 240 volt coil to obtain 120 volts between phase a and neutral or between phase C and neutral, however, between phase b a. At neutral we get 208 volts because we are using a coil and a half, we can find that using trigonometry that is why phase B will usually be orange on the panel to warn that it is a higher voltage and that is because it will probably destroy any 120 volts .

The appliance, if connected incorrectly, the open Delta uses only two transformer coils, so it is missing one of the coils, we get the same voltage across the coil, so it is the same voltage between lines and the same voltage between line and neutral, however, the capacity is reduced. If we had three transformers each with a rated capacity of 20 KVA, giving us 60 KVA in total, then the maximum coil current or phase current would be 83.33% 4.6 kba, i.e. 57.7 % compared or a reduction of 42.3% compared to the three Delta transformers of our rated capacity. For this two transformer design there is now only 40 KVA, but we can only handle 34.6 KVA, which means we can only use 86.6% of the listed capacity, so this design is cheaper to install, but It is not always practical, however, it is easy to increase it in the future.

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