Difference Between Single Phase and Three Phase Power Supply

Electrical Technology

Main Difference between Single Phase & Three Phase Power Supply

The power generation and distribution started in mid-1880 and since then the AC power supply has been used for powering common household items, office equipment and industrial machines. The AC power exists and used in two forms i.e. it is either single phase power or three phase power supply.

There is no two-phase power supply. In either form, it is used for powering AC appliances and machine. Generally, three phase power is used to accommodate heavy loads in industries while single phase is used for powering small loads in homes and small businesses. However, there are various differences between single phase and three phase power supply and this article explains it in detail with a chart of comparison.

Before going into the differences between single phase and three phase AC power, let’s discuss the basics of phase in electricity.

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Phase in Electricity

Unlike Direct Current DC, the Alternating current AC swings between zero and its peak value. The current constantly varies to form a sinusoidal waveform as shown in the figure below.

In electrical signal, the frequency and phase of a periodic signal are defined as; the frequency of a signal is the number of cycles per second. While the phase is the point or position in time of the waveform and it is always mentioned in degrees, ° or radians. In electrical system, the phase refers to the number of distinct sinusoidal AC waveforms having same frequency but having a common phase difference.

AC power supply usually has 50/60 Hz frequency depending on the region. While the phase of the system depends on the type of electrical system being used. There are 360^° in one cycle. A single phase power supply has only one AC wave.

While the three-phase supply has three equally separated waveforms having same frequency. The 360° of phase per-cycle is evenly divided into three parts. Therefore, the phase difference between any two phases is 120°.

Single Phase Power Supply

The type of AC supply where all the voltages in the system varies in a same sinusoidal pattern is called single phase power supply. It is delivered using only two wires i.e. the power wire also known as hot wire or live wire or line or phase wire and a neutral wire.

The power flows between the phase and neutral wire through the load. The phase wire is used for supplying the power to the load while the neutral wire provides a return path for the current flowing through the load. Sometimes, it is also called as split phase.

The given figure shows the AC voltage and current waveform for a single phase supply. The voltage fluctuates between zero and peak points. Where the maximum peak point occurs at 90° and the minimum peak point occurs at 270°. The voltage does go to zero some times. That is why single phase power is not very consistent as compared to a three phase power and cannot be used to power high rated machines. It is mostly used for appliances below 1000 Watt.

Since it only require two wires, single phase power supply is quite simple and the cost of designing is very cheap. But it is not as reliable as three-phase because a power failure can occur due to a fault in the line. It has a wide range of applications from every household item to offices and small equipment in industries. It is efficient for powering small appliances below 1000W.

The downside of a single phase supply is that it cannot run motors on its own as it cannot generate rotating magnetic field (RMF) in an induction motor. it requires extra motor starting circuitry for proper operation.

Three Phase Power Supply

Three phase power contains three voltage that are 120° apart individual sinusoidal waveform. The three phase power supply consist of three individual power conductor i.e. 3 different color coded wires for individual phases. While the voltage and current between any two power conductors is 120° phase apart. Three phase power supply is also known as Poly-phase power system.

Three phase power supply used for powering a load can be connected in either of the two configuration i.e. delta configuration and Star or Wye configuration (Star and Delta represented by the symbols of Y and Δ respectively). In delta configuration, there is no neutral cable. It only uses 3 phase cable or power cable to supply power to the load.

While in Star or Wye configuration, there is a fourth neutral cable. The neutral wire is extended from the common point of combination of three phase windings in a transformer or generator. Therefore, wye or star configuration requires four cables to supply three-phase power including three power cables and one neutral cable.

The neutral cable is beneficial for detecting fault currents in the system that is why AC power transmission is done using three-phase star or wye configuration. The wye connected supply can be used to power 3 individual single phase circuits. A single phase supply can be taken between a phase cable and neutral cable which is mostly done in EU and Asian countries. The houses are supplied with a single phase power from an existing three phase power line that is in wye configuration.

3 phase power supply contains three individual AC waveform with 120° difference between them. As shown on the figure, the instantaneous voltage at any time does not reach 0 point. Therefore, the power delivered by three phase is much more consistent and supply more power than a 3 single-phase power lines. It is used for powering heavy loads greater than 1000 Watts. It is impervious to power failure in case of a fault in one phase because the other two phases still delivers power.

Key Differences between Single Phase & Three Phase Power Supply

Following are the main differences between single phase and poly phase (3-phase) power supply.

Single Phase Power Supply	Three Phase Power Supply
The AC power where all the voltages has same sinusoidal pattern.	The AC power where there are 3 sinusoidal voltages having 120° phase difference.
It requires only two wires to complete the circuit.	It require either 3 or 4 conductors depending on the configuration.
It is also known as Split Phase System.	It is also known as Poly Phase System
Single phase is very simple.	It is complex than single phase.
It can power small loads less than 1000 Watts.	It can power heavy loads above 1000 Watts.
Power failure occurs due to a fault in power line.	Impervious to power failure due fault in signal power line.
It is less reliable and efficient as three phase.	It more reliable and efficient than single phase power.
The voltage and current goes to zero in a cycle.	The voltage and current never goes to zero.
It cannot generate rotational magnetic field in induction motor. it uses extra circuitry.	It can generate RMF without any extra circuitry.
The supplied voltage is equal to the voltage difference between phase and neutral.	The phase to phase voltage is equal to √3 times the phase voltage. While phase to neutral voltage is equal to single phase voltage.
It carries less power than equivalent three phase supply.	It can carry high power as compared to 3 single phase AC power supplies.
It is used for power distribution over short distances.	It is used for power transmission over long distances because of lower copper losses.
The power losses in single phase is relatively higher.	The power losses in three phase is relatively lower.
It needs expensive converters to convert into three phase power supply.	Wye arranged three phase supply can offer 3 single-phase using either phase wire and neutral wire.
It is less economical as compared to poly phase system.	It is more economical as it uses only 4 wires instead of 6 to supply the same power.
It is used for powering home appliance and office equipment.	It is used for powering heavy loads in industries.

The conclusion of this topic is that the single phase power supply being simple and cheaper to design is used for small load such as in homes ad offices. While the three phase load being much more reliable and consistent in heavy power delivering capability is used for heavy loads in industries to utilize less copper.