We know that the most common supply systems for power transmission, distribution and utilization are Single Phase and Three Phase systems. There is a big difference between Single Phase and Three Phase supply systems where a three phase supply system has some advantages over a single phase supply system.
Keep in mind that there are multiple applications of 6-phase, 12-phase etc. for rectifier circuits, VFD and other uses in power electronics which help to reduce the ripple and pulsating DC. In addition, it is easy to get a different number of phases (like 6, 9 and 12 etc.) with the help of phase shifting and a motor generator set which was very complex somehow in the past , but it is not an economical system for power distribution and transmission over long distances.
Why 3-Phase Instead of 1-Phase Supply System?
The main advantage of three phase over a single phase or two phase system is that we can transmit more (constant and uniform) power.
Power in Single Phase System
- P = V . I . CosФ
Power in Three Phase System
- P = √3 . VL . IL . CosФ … Or
- P = 3 x. VPH . IPH . CosФ
Where:
- P = Power in Watts
- VL = Line Voltage
- IL = Line Current
- VPH = Phase Voltage
- IPH = Phase Current
- CosФ = Power factor
It clearly shows that the value of electric power in a three phase system is 1.732 (value of √3) times bigger than the power transmitted in a single phase supply system. Where two-phase supply tranmsit 1.141 time extra power than single phase power supply ststem.
In addition, there is RMF (rotating magnetic field) which helps in self-stating the three phase motors with constant instantaneous power and torque. On the other hand, these features and characteristics are not available in single phase supply systems i.e. no RMF and pulsating power
Also, there is less transmission loss and voltage drop in 3-phase. For example, in a typical resistive circuit;
Single Phase System
- Power loss in transmission line = 18I2r … (P = I2R)
- Voltage drop in transmission line = I.6r … (V = IR)
Three Phase System
- Power loss in transmission line = 9I2r … (P = I2R)
- Voltage drop in transmission line = I.3r … (V = IR)
It shows a 50% reduction in voltage drop and power loss in a three phase system as compared to a single phase system.
Now, There is also content power, produced RMF and constant torque in two phase supply same as three phase supply. But there is more power in 3-phase as compared to 2-Phase due to one extra number of phase. So the question arises here why not more phrases like 6, 9 12, 24, 48 etc. OK we will discuss in detail and also show how it is possible to transmit extra power in a three phase as compared to two phase with the same number of wires.
Good to Know: There is 120° of phase angle between each phase in a three phase system while the phase angle between single phase (lagging or leading) and two phase are 90° (degrees).
Why Not 2-Phase? Why Only 3-Phase?
Well, If there is RMF, constant power and torque in both 2-phase and 3-phase systems then why use 3-phase instead of 2-phase system? Well, the reason is extra phase in a 3-phase system which carries extra power as compared to 2-phase system.
In addition, there are four wires needed to provide the supply from the generator to the load points (both phase pairs as Phase and neutral to complete the circuit). To eliminate the extra wire in two-phase four wire system, one common neutral wire is used which makes it a two-phase, three wire system but the returning amount of current from two phases is larger for common neutral, hence thicker conductor size (e.g. copper material) is needed for neutral.
Now, for the same reasons, the three wires (as phases in a 3-phase system) do the desired job with extra power as compared to a 2-phase system. In symmetrical and balanced load systems or delta configuration, three-phase, three wire system are used while in asymmetrical and unbalanced load or star configuration, three-phase, four wire system are used to achieve the desired features smoothly which are needed for power transmission and distribution.
Why Not 6-Φ, 9-Φ or 12-Φ instead of 3-Φ?
Well, one may ask if the increase in number of phases is advantageous, then why not to use more poly-phase systems like 6-phase, 9-phase, 12-phase or even more number of phases? As there is RMF, constant power and torque in the polyphase systems while the transmission loss and voltage are reduced by 50% by addition extra phase e.g. 4-phase, 5-phase, 7-phase, 24-phase, 48-phase or any other “n” number of phases. Why then only a 3-phase system among them?
The reason is simple. The least number among all of them is 3 which means we will be needing the least number conductor (3 in three phase as compared to 12 wires in a 12-phase system) while the overcome is almost the same. Also, the configuation of phase angle of 120 degrees between all phases in three phase system cancel out and elominate the third-harmonic and its multiples, if we opt out for 6, 9, 12 or more phases will only lead to extra cost of the mateials, complex design and additional maintinaces of the transmission & distibution lines as well as related switchgeas and potective equipoment.
Not only the conductors, in polyphase systems (like, 6, 9 or 12) we will redesign the overall systems (which leads to more complex systems) like each of 12 conductors, busbars, circuit breakers, power & distribution T/F, current (CT) and potential transformers etc. and increase in height of transmission towers, the space occupied by power and substation. That’s why we are restricting it to the three phase system instead of any other phase system.
FAQ
Why is 3-phase power not a higher number of phases?
The three conductors under three phase all carry the same current: they are balanced. For all these reasons, three phases represents an optimum. If it is a given that electricity is used for induction motors, more than three phases is wasteful, and so is fewer than three
Why is three-phase used for high power transmission?
A three-phase power supply can transmit three times as much power as a single-phase power supply, while only needing one additional wire (that is, three wires instead of two)
Why do we use single-phase and three-phase power rather than 4.5-6 power?
This is because three-phase power provides a balanced load distribution, which minimizes the amount of current flowing through each phase conductor, reducing I^2R losses (where I is the current and R is the resistance)
What is the advantage of 6 phase over 3 phase?
Compared to three-phase machines, six-phase machines are more compact in size, have superior fault tolerant capability, reduced torque ripple and more power density. So, six-phase machines are used where high power density is required like, hybrid electrical vehicles
Why 3-phase power, why not 6 12 or more?
6 and 12 are multiples of 3. one of the advantage of 3 phase ac is that 3rd harmonic and its multiples cancels out. so going for 6 or12 phase will only lead to extra cost and maintenance in the transmission and distribution lines
Why don’t we use 6 phase?
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
Why is 3-phase more powerful?
Three-phase supplies have properties that make them desirable in electric power distribution systems: The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load, which allows a reduction of the size of the neutral conductor because it carries little or no current
Why does 3-phase not need a neutral?
A three-phase motor does not require a neutral wire because it operates on three-phase power, which provides three separate alternating currents that are 120 degrees out of phase with each other. This means that at any given moment, one of the three phases is always providing power to the motor
Is 3-phase 415V or 440V?
440 Volts or 480 Volts are used for large loads such as three phase electric motors, etc. 415V is the voltage between two phases
