If I rotate this magnet past this coil of wire, we can see it produces a sine wave. The magnetic field interacts with the electrons in the wire and forces them to move. Imagine the north pole is pushing them away and the south pole is pulling them back. The electrons are alternating their direction forward and backwards.
To prove this, we can use LED''s. Because, led''s only allow current to flow in one direction. So, by connecting two led''s in opposite directions, we can tell which direction current is flowing. At normal speed it''s a little hard to see, but in slow motion, we can clearly see only one LED illuminates at a time so the current is definitely flowing forwards and backwards in the sine wave.
The magnet in our generator rotates and pushes the electrons forwards, then pulls them backwards. This will create a single phase, alternating current with a sine wave which repeats every time the magnet makes a full rotation past the coil.
So, we have a single phase generator. The voltage will start at zero, then increase up to the peak positive value and then decrease back to zero. Then on the negative half the value will increase to the peak negative value and again decrease back to zero. This is what the sine wave is representing. Notice, this value changes but the voltage at the outlet is constant, I''ll explain why later in the article.
As a side note, if you use the slow-motion feature on your smart phone, you can see an incandescent lamp flicker because of the AC current. But It''s too fast for the human eye to see. However, most lights are now LED which are usually constant so you probably won''t see those flicker.
The current is flowing back and forth in each phase, we can prove that with this small 3 phase generator and some LED''s. We arrange the LED''s in pairs of opposite polarities, so that only one will illuminate at any time depending on the direction of current in the wire. We can see they are illuminating, and in slow motion we can clearly see the current is alternating direction.
The coils in the generator are placed 120 degrees apart simply because that gives us even spacing of the sine waves that are produced. We can move the coil to any angle, but the sine wave will also move and we won''t have equal spacing.
You can download my excel sheet and see how the angle changes, as well as the instantaneous phase voltages HERE.
We could add a 4th phase, a 5th phase or 6th phase, but the generator becomes more and more complex and expensive. We also need more cables, more control and protection equipment, complex transmission and distribution infrastructure, more complex transformers and motors etc. It''s then harder to balance the network and it''s very hard to synchronise generators to work together.
So, we instead settled on 3 phase for generators and equipment. Perfect. Oh, but unfortunately each country decided to us a different voltage, frequency and distribution design. Great.
This means we could join the ends of the coils and the ends of the loads together. That allows us to use just 3 wires, which is a lot cheaper. The current will flow back and forth on which ever phase happens to be going that way. We can see on the 3 phase current wave form, at for example 180 degrees, phase A has zero amps flowing. Phase B has positive current and phase C has equal negative current flowing.
This works great for equal 3 phase loads. But with this design we can only connect across two phases, the voltage will be very high. We can''t use this to power our outlets, because it will destroy our appliances.
But, if we reconfigure this into a wye connection. Then we can run a neutral wire from the centre point back to the centre point of the generator. We can also connect this point to ground, meaning this point in the system is zero volts.
If the current is balanced on all phases then no current will flow on the neutral. However, if one phase increases to say 30Amps, then 20Amps will flow on the neutral. The neutral will carry the difference back to the generator or transformer to keep the system balanced.
Because we now have a neutral, we can connect across just one phase and neutral, this gives us single phase. We''re basically just connected across one coil of the generator or transformer. We can do that on each phase.
We can either connect the three phases in wye or delta configuration. There are different reasons, but basically if the loads were the same resistance or impedance, and the phase to phase voltage was also the same, the current would be larger in a delta configuration, because the loads connect across two phases, whereas the wye connected loads connect to a zero point, so they experience different voltages. The delta can deliver much more power, but it can only power balanced three phase loads. If you need a neutral, we need a wye configuration.
Each power station generates 3 phases, a transformer increases the voltage to hundreds of thousands of volts, this keeps the current and energy loses low over the long transmission distance. When it reaches a city, it enters another transformer which reduces the voltage and distributes this on the sub transmission lines which might feed large industrial or commercial customers, but it otherwise continues to a distribution substation where the voltage is again reduced and distributed along the streets to the properties.
Typically, residential properties are provided single phase connections and commercial properties have three phase connections. Although some parts of the world do provide three phases to homes.
Homes generally need less power, because they have less stuff to power. So, a single phase connection is usually fine. We can also convert single phase into 3 phase using a rotary converter.
If we connect too many appliances to a single phase, we will overload the circuit and trip the breaker. Three phase lets us distribute the power so we can connect more appliances.
A three phase heater will use more energy than a single phase version, but it produces more heat so it does more work. The heat is also consistent unlike the pulsating single phase version.
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