Why power is generated and transmitted in 3 phases only?

As we know that, power is generated and transmitted in 3 phases and we call it 3 phase system. Why don’t we use other phase system like single phase, 2-phase, 4-phase, 5-phase, 6-phase or more system? Why power is generated and transmitted in 3 phases only?

So, it is very clear that 3 phase system must have some significant advantages over other phase system, that’s why scientists and engineers found it suitable for generation and transmission purpose. In this particular section, we are going to discuss those advantages of 3 phase system over other phase system.

Why 3 phase system is better than 1-phase system?

  • A three-phase generator generates 3 times more power than single phase generator as shown below,

Single phase power = VIcosɸ

3-phase power = 3VIcosɸ

  • Rotating magnetic field is not present in single phase because of no phase shift. But, in 3-phase system there is a phase shift of 120°, hence it has rotating magnetic field.

Rotating magnetic field (RMF) helps the motor to start own its own (self-start) without any external devices.

Due to absence of RMF in single phase system, the single-phase motor cannot start own its own, that’s why we put capacitor in our ceiling fan motor to create a phase difference between its auxiliary winding and running winding.

  • Single-phase power is pulsating in nature and hence, the torque produced by it is also pulsating, which creates noise and vibrations in the Motor.

Whereas, power quality of 3 phase system is very smooth as shown in the figure below. We get constant power and constant Torque which gives less noise and vibration in the motor.

single phase waveform
Single Phase Waveform
three phase waveform
Three Phase Power Waveform
  • We cannot supply heavy loads from single phase system but we can directly supply the heavy loads from three-phase system.
  • In three-phase system, there is no need of neutral wire to transmit the power. But single-phase system needs a neutral wire to provide return path. We can understand it with the help of waveform.

At any instant, when we add the instantaneous values of current of all three phases then the result comes to be zero. It means that current is flowing between these three phases only.

                   Ir + Iy + Ib = 0                    

Let’s try to understand it more clearly. In the given waveform there are three phases R-phase, Y-phase and B-phase.

When current in R-phase is at its peak value (say 1A), then at same instant the current in other two phases (Y-phase and B-phase) is just half and in opposite direction (-0.5A).

It means that when R-phase is taking the current in forward direction then at the same time the other two phase are providing the return path to the same amount of current. Hence there is no need of neutral.

In simple words, we can say that whenever the sum of Instantaneous values of current in all phases comes zero, there is no need of neutral wire.

  • In single phase system, we need six wires (3 phase wires and 3 neutral wires) to transmit the power. Whereas, In three phase system we need only three phase wires to transmit the power as we don’t the neutral wire in three phase system.

So, the copper losses are reduced by 50% and voltage drop also reduces by 50% and voltage regulation becomes good in 3-phase system when compared to single phase system.

Hence, we have seen that 3-phase system is better than single phase system in every aspect. Now, we will compare 2-phase system.

Why three phase system is better than 2-phase system?

In two-phase system although the power (2VIcosɸ) is less than three-phase system (3VIcosɸ), but many things are same like it has RMF with a phase shift of 90° between its phases. Also, it delivers constant power and torque. Still, we do not prefer 2-phase system over 3-phase system, why?

When we add the instantaneous values of current in both phases then the result not comes equals to zero, it means that we need a neutral wire to provide return path for current.

Two phase Waveform
Two Phase Waveform

Ultimately, we are using three wires (2 phase and 1 neutral) in 2-phase system, then why don’t we use 3-phase system which is also using 3 wires for transmission purpose. Also, 3 phase system is transmitting more power than 2 phase system.

Why we don’t use 4-phase, 6-phase, 12-phase system over 3-phase system?

Phase systems higher than 3-phase system are having all qualities which are present in a 3-phase system also they provide more power than 3-phase.

Let’s take 6-phase system to compare.

  • A six-phase system has more power (6VIcosɸ).
  • 6-phase system has rotating magnetic field which makes the motor self-start just like 3-phase system.
  • Six-phase system also provides constant power and torque.
  • 6-phase system can supply heavy loads directly.

So, we see that 6-phase system or any other phase system higher than the 3-phase system are better than 3 phase system, but still, we prefer 3-phase system because of the reason discussed below.

By increasing the number of phases, the system becomes more costly and complex.

To generate 6-phase power we need a six-phase generator which will take large area and becomes more complex.

To step-up or step-down the voltage level, a 6-phase transformer is needed, which is again not cost effective and becomes more complex.

We need 6 phase wires to transmit the power through transmission lines. To provide sufficient ground clearance, the height of transmission tower has to increase. Also, to handle 6 phase wires we need transmission towers to be mechanically very strong.

At substations, we need very large area to Increase the Switchgear and protection devices such as of circuit breakers, relays, bus-bars, isolators, current and potential transformers and many such equipment are needed which highly increases the cost and complexity of the system.

Scientists and Engineers did not find any significant profit of using 4-phase or 6-phase or higher phase system over 3-phase system.

We can also understand it with the help of Tripod analogy, when 3 legs of a tripod are giving sufficient stability for photography then why do we need more legs which will increase the cost also.

Hence, 3-phase system is best among all.

More to read,

What is skin effect?

Why Transformers are rated in kVA and not in kW?

What are those colourful balls on transmission lines?