Three Phase AC Circuits

Unlike a DC circuit which only has one type of circuit, an AC circuit can be differentiated to single phase AC circuits and three phase AC circuits. The fundamentals between these two stay the same but we need to build and analyze three phase AC circuits differently.

There are advantages and disadvantages when comparing these two. A single phase AC circuit is easy to build and to analyze, but it has lower efficiency compared to three phase. A three phase AC circuit is known to have great power transfer efficiency but it is costly to make and more difficult to analyze.

What is Three Phase AC Circuits

A simple answer to the question “what is a 3 phase AC circuit?” is an electrical circuit which has three oscillating single phase voltage and current waveforms. In an ideal condition, all of the three oscillating waveforms have identical magnitude and frequency, only with their phase shifted one another.

In order to answer why a three phase AC circuit has better power transfer, you can see from the waveforms above. When phase 1 is 0, the other two are not. This tells us the power never reaches zero at any point. We will get a continuous supply of electric power.

Since we have three electric voltages and electric currents, we need three or four wires to draw electric energy from the supply. Why is it not three wires but also four wires? It depends on our three phase AC circuit configuration. There are delta configuration and star configurations.

We will learn this further for a complete understanding.

A three phase circuit belongs to the polyphase system.

Polyphase system is a circuit or system where AC sources are operating at the same frequency but different phase. It can be two or three phases. Just like stated above, both two phase and three phase circuits have identical magnitude and frequency for each line but other one or two have different phases.

Where:

VP = rms magnitude of source voltage

∠𝝓 = phase angle

ZL = load impedance

Below is the circuit of single phase two wire system

And this is a two phase AC circuit which has two identical sources (both magnitude frequency).

And our main topic, a three phase AC circuit

We will skip the single phase since it is already covered in another post, and we won’t learn the two phase circuit further since it is not commonly used compared to the three phase circuit.

We will be learning a three phase circuit.

Below is the three phase AC supply diagram where the AC circuit is supplied by three sources with the identical amplitude and frequency but out of phase with each other by 120°.

Importance of Three Phase AC Circuit

We need to accept the superiority of a three phase system to make it highly used in our life, especially in an industrial scale and electrical motor. It has three important reasons:

First, almost every electric power generation and distribution is using a three phase system with 50Hz or 60Hz frequency.

When single phase or two phase sources are required, we can make it by taking it from a three phase system rather than generating them independently.

Second, the instantaneous power in a three-phase system can be constant (not pulsating).

This results in uniform power transmission and less vibration of three-phase machines.

Third, for the same amount of power, the three-phase system is more economical than the single- phase.

Configuration of Three Phase System

Since we are dealing with three sources rather than one, there may be some cases where the three voltages are not identical. They may have different magnitude or phase angles from each other. This is what we call the Unbalanced Three Phase System.

We will now talk about a balanced three phase voltage and balanced three phase power to learn all the four configurations of three phase AC circuits.

Just as stated above, whether we use three or four wires, it depends on our configuration. We will not talk about it deeply here since they have been covered in their respective dedicated posts.

A balanced three phase circuit is when all the sources have identical magnitude and phase angle from each other. In this case, 120° from each other.

But we will summarize them here.

Balanced wye-wye connection is a three phase system that has balanced wye connected sources and balanced wye connected loads.

Balanced wye-delta connection is a three phase system that has balanced wye connected sources and balanced delta connected loads.

Balanced delta-delta connection is a three phase system that has balanced delta connected sources and balanced delta connected loads.

Balanced delta-wye connection is a three phase system that has balanced delta connected sources and balanced wye connected loads.

The three phase power measurement depends on our configuration, and each configuration has their own unique equations. Make sure to understand them all.

Conclusion

Three phase AC circuits use three single phase alternating current shifted 120 degrees from each other. This brings us continuous power transfer and better efficiency compared to the single phase AC circuit.

Three phase AC circuit utilizes three independent single phase sources that have identical magnitude and frequency but have equal phase shift to each other.

It is mainly used to deliver more constant power in an industrial aspect.

The advantages of three phase AC circuits are:

• Deliver constant power and better power transfer efficiency.
• Minimum power loss on transmitting over a long distance.
• Require less space compared to three single phase circuits for delivering the exact same power.
• Since there is no zero power at any point, the noise will also be minimal compared to a single phase system.
• A single phase system can be derived from a three phase system, no need to build it independently.
• Almost every electric motor in industrial scale is operated in a three phase system, especially the induction motor.

The configuration of the three phase system consists of wye or delta for sources and loads.

Frequently Asked Questions