2-Phase vs 3-Phase

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A 3D rendering of a modern building exterior
Courtesy: CyberPower

2-phase and 3-phase electricity are common configurations of transmitting power from one location to another. Although they are both alternating current systems, each of them have features that are suitable to certain applications. In this article, we will provide an overview of each phase type, compare 2-phase vs 3-phase power, and review commonly asked questions.

2-Phase Supply

2-phase electricity is a power distribution setup developed early in the 20th century as an improvement on the single-phase system. Generally, this system delivers power using two circuits with a 90° voltage phase difference between them. As a result, it provides constant shaft power to an ideal load and a good starting torque for induction motor operation. This minimizes the disruptive pulsating power and necessity for additional starting means that are common with single-phase power systems. At the onset, 2-phase power used two wires per phase (one live, one neutral) making a total of four wires. In order to conserve wiring, it evolved to a three-wire system with a single neutral for returning current from the load. However, this neutral wire has to be larger as it carries over 50% of the current flowing in each of the phase conductors.

2-phase four wire system, which may seem like 3-phase
Courtesy: EEP

This limitation made the 2-phase system less popular especially with the advent of the 3-phase power supply.

3-Phase Power

3-phase supply is the system with the highest level of adoption for industrial use because of several desirable features. First, three conductors carry alternating current at the same voltage and frequency but with a phase difference of 120°. As a result, it can deliver more power than the 2-phase configuration to equipment with higher demands such as 3-phase motors. In addition, it also has the advantage of a smooth power output and good starting torque for motors. Notably, the 3-phase system is the only supply configuration that can operate without a neutral. For precautionary measures, most 3-phase systems use four wires– including a neutral, rather than the three-wire configuration without a neutral.

480V 3-phase four-wire configuration.
Courtesy: Hyperphysics

2-phase vs 3-phase Electricity

The 2-phase and 3-phase electricity set-ups both offer good starting torque for motors and smoother power output for motors in comparison to single phase systems. Both systems could also have either three or four wires in their setup. However, there are some differences between both configurations as the table below highlights.

Has only two conductors, so power transmission is limited usually below 10 kW.Able to transmit more power as three conductors are present. Generally, it is installed when electricity consumption of a facility is beyond 10 kW.
Because of the conductors are out of phase by 90°, the system is unbalanced. So, there is always significant current in the neutral.The 120° phase difference results in a balanced system. As a result, no current is in the neutral when operating as it should.
Although 2-phase electricity provides smoother power output than single phase, there remains possibility of power pulsations.3-phase electricity eliminates possibility of power pulsations completely. Thus, it is the go-to option for running motors.
Generally, more expensive due to cost of meter and conductor material.It is cheaper when using 3-phase to transmit the same power with the same voltage drops as in a 2-phase system.
It is impossible to use a 2-phase current to serve 3-phase power distribution system.A Scott connection between two transformers can convert 3-phase current to a 2-phase system.
2-phase vs 3-phase

Commonly Asked Questions on 2-phase vs 3-phase

Because of the similarities between 2-phase and 3-phase electricity, there are often questions about them, which this section addresses.

What is the difference between a 2-phase and 3-phase motor?

The main difference between a 2 and 3-phase motor is that the alternator of a 2-phase motor has two windings seated 90° from each other. While the positioning of the three windings of a 3-phase motor is at a 120° interval. Because of this, the 2-phase motor remains prone to power pulsations, whereas the output from 3-phase is much smoother.

Can a 3-phase motor run on 2-phases?

In some situations, yes. If one phase gets bad when a 3-phase motor is in operation, it will still run with a 2-phase supply, but with lower efficiency. However, using a 2-phase supply to start a 3-phase motor will not work because the stator circuit will not achieve the rotating magnetic field.

Why is there no 2-phase motor?

Although both 2-phase and 3-phase system have a constant combined power for an ideal load, 2-phase motors experience power pulsations. This is due to the imbalance in the magnetic field. As a result, there is an increase in mechanical noise and torsional vibration in the motor. Generally, 2-phase systems have been replaced with 3-phase systems in most applications.