3 Phase & 1 Phase AC Power Generation explained with a demo

What is AC power?

It is an alternating current power. Alternating means current flows in forward and reverse directions alternatively through the conductor. If the power frequency is 50Hz then the current alters its direction 100 times,and 120 times for 60Hz power supply.

How single-phase power is generated?

We know that when a conductor is moved in a magnetic field, an EMF is generated in that conductor. Similarly, when a conductor is placed in a moving magnetic field, an EMF is generated in that conductor. A magnetic field is created in the generator using DC power. DC supply is given to the field windings of the generator, forming the north and south poles. This field winding is mounted in the rotor of the generator.  Since field winding is mounted on the rotor, the magnetic field is rotating,  and conductors in which EMF is generated, are stationary. This generated EMF causes current to flow in the conductors. Now let’s see the nature of this current. The direction and the magnitude of the current vary with the position of the coil with respect to the poles.

Now let’s see a single rotation of the rotor and the respective current behavior in a single-phase generator:

When rotor-mounted field winding rotates. EMF gets generated in the conductors. This EMF causes current to flow through these conductors. When concentrated magnetic lines of force that is the magnetic field are cut across by these conductors, the current value reaches its peak, which means electrons’ speed is maximum through the conductors. And when a relatively weaker part of the magnetic field starts to cut across these conductors current value starts decreasing and eventually falls to zero. It means the electrons’ speed through the conductors decreases. Similarly, the current reaches its peak value on the negative side, when another part of the magnetic field is cut across by the conductors. This means the speed of electrons reaches its maximum value but in opposite direction through the conductors. This is how alternating current is sinusoidal in nature. This happens in one complete rotation of the field winding. This one rotation of the rotor field winding is called one cycle. This rotor field winding rotates 50 times in one second, for 50Hz frequency. And 60 times in one second for 60Hz frequency. Hence the number of cycles of the rotor per second is called the frequency of power supply.

How Phase difference is created?

As we know this complete peripheral distance is 360 degrees. This 360-degree is angular distance. If we accommodate three coils at an equal distance, each coil gets set at 120 degrees apart from the other coils. This is nothing but the phase difference between the phases. This peripheral distance is the cause of this time difference between these two peaks of the three-phase current. 

Now let’s see another interesting fact about three-phase power. Vector addition of the current of all the three phases at any given instant is zero. for example at a 90-degree instant. The value of one of the phase’s current is ‘X’ units while the value of the other two phases’ current is ‘-X/2’ and ‘-X/2’ respectively. 

Hence, X + (-X/2) + (-X/2) = 0 

In a balanced three-phase system, the current through neutral is zero.

How Three Phase Power is Generated?

Why not 6 Phase or 12 Phase Power?

Now, if we can generate three-phase power just by using additional windings in the generator, then why not produce six-phase or twelve-phase power?

As we know, we have three poles circuit breakers, three conductors transmission line, three-phase transformer and all the other equipment suitable for a three-phase power system. These three-phase power installations increase the complexity and cost of the system compared to the single-phase power system. And this complexity and cost of power installations increase very drastically for more than three phases that is six phases or twelve phases. Hence after thorough study and analysis, it is found that only a three-phase system offers an optimally complex & economic solution. Similarly using less number of phases for power that is two phases or a single phase is not desirable.

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