Definition of Balanced Three-Phase Voltages in AC Circuits

balanced three phase voltages

Three-phase voltage may have different voltages in each phase. There will be some cases where we need the balanced three-phase voltages.

There are four types of three phase circuit:

  1. Balanced three phase voltage
  2. Balanced three phase power
  3. Unbalanced three phase power
  4. Three phase power measurement

Three-phase voltages are often produced with a three-phase ac generator (or alternator) whose cross-sectional view is shown in Figure.(1). The generator basically consists of a rotating magnet (called rotor) surrounded by a stationary winding (called the stator).

Balanced Three-Phase Voltages
Figure 1. A three-phase generator

Make sure to read what is three-phase ac circuit first.

Balanced Three-Phase Voltages
Figure 2. The generated voltages are 120o apart from each other

Balanced Three-Phase Voltages

Three separate windings or coils with terminals a-a’, b-b’, and c-c’ are physically placed 120o apart around the stator.

Terminals a and a’, for example, stand for one of the ends of coils going into and the other end coming out of the page.

As the rotor rotates, its magnetic field “cuts” the flux from the three coils and induces voltages in the coils.

Because the coils are placed 120o apart, the induced voltage in the coils are equal in magnitude but out of phase by 120o like shown in Figure.(2).

Since each coil can be regarded as a single-phase generator by itself, the three-phase generator can supply power to both single-phase and three-phase loads.

Balanced Three-Phase Voltages
Figure 3. Three-phase voltage sources: (a) Y-connected source, (b) delta-connected source

A typical three-phase system consists of three voltage sources connected to loads by three or four wires (or transmission lines). (Three-phase current sources are very scarce.)

A three-phase system is equivalent to three single-phase circuits. The voltage sources can be either wye-connected as shown in Figure.(3a) or delta-connected as in Figure.(3b).

Let us consider the wye-connected voltages in Figure.(3a) for now. The voltages VanVbn, and Vcn are respectively between lines a, b, and c, and the neutral line n.

These voltages are called phase voltages.

If the voltage sources have the same amplitude and frequency ω and are out of phase with each other by 120o, the voltages are said to be balanced. This implies that

Balanced Three-Phase Voltages
(1)
Balanced Three-Phase Voltages
(2)

Thus,

Balanced phase voltages are equal in magnitude and are out of phase with each other by 120o.

Since the three-phase voltages are 120o out of phase with each other, there are two possible combinations. One possibility is shown in Figure.(4a)

Balanced Three-Phase Voltages
Figure 4. Phase sequences: (a) abc or positive sequence, (b) acb or negative sequence

and expressed mathematically as

Balanced Three-Phase Voltages
(3)

where Vp is the effective or rms value. This is known as the abc sequence or positive sequence.

In this phase sequence, Van leads Vbn, which in turn leads Vcn.

This sequence is produced when the rotor in Figure.(1) rotates counterclockwise. The other possibility is shown in Figure.(4b)

and is given by

Balanced Three-Phase Voltages
(4)

This is called the acb sequence or negative sequence. For this phase sequence, Van leads Vcn, which in turn leads Vbn.

The acb sequence is produced when the rotor in Figure.(1) rotates in the clockwise direction.

It is easy to show that the voltages in Equations.(3) or (4) satisfy Equations.(1) and (2). For example, from Equation.(3),

Balanced Three-Phase Voltages
(5)

The phase sequence is the time order in which the voltages pass through their respective maximum values.

The phase sequence is determined by the order in which the phasors pass through a fixed point in the phase diagram.

In Figure.(4a), as the phasors rotate in the counterclockwise direction with frequency ω, they pass through the horizontal axis in a sequence abcabca . . . .

Thus, the sequence is abc or bca or cab. Similarly, for the phasors in Figure.(4b), as they rotate in the counterclockwise direction, they pass the horizontal axis in a sequence acbacba . . . .

This describes the acb sequence. The phase sequence is important in three-phase power distribution.

It determines the direction of the rotation of a motor connected to the power source, for example.

Like the generator connections, a three-phase load can be either wye-connected or delta-connected, depending on the end application.

Figure.(5a) shows a wye-connected load, and Figure.(5b) shows a delta connected load.

Balanced Three-Phase Voltages
Figure 5. Two possible three-phase load configurations: (a) a Y-connected load, (b) a Δ-connected load

The neutral line in Figure.(5a) may or may not be there, depending on whether the system is four- or three-wire. (And, of course, a neutral connection is topologically impossible for a delta connection.)

A wye- or delta-connected load is said to be unbalanced if the phase impedances are not equal in magnitude or phase.

A balanced load is one in which the phase impedances are equal in magnitude and in phase.

For a balanced wye-connected load,

Balanced Three-Phase Voltages
(6)

where ZY is the load impedance per phase. For a balanced delta-connected load,

Balanced Three-Phase Voltages
(7)

where ZΔ is the load impedance per phase in this case.

Balanced Three-Phase Voltages
(8)

so we know that a wye-connected load can be transformed into a delta-connected load, or vice versa, using Equation.(8).

The table below shows us the summary of phase and line voltages/ currents for a balanced three-phase system:

Balanced Three-Phase Voltages

Since both the three-phase source and the three-phase load can be either wye- or delta-connected, we have four possible connection :

In subsequent sections, we will consider each of these possible configurations.

It is appropriate to mention here that a balanced delta-connected load is more common than a balanced wye-connected load.

This is due to the ease with which loads may be added or removed from each phase of a delta-connected load.

This is very difficult with a wye-connected load because the neutral may not be accessible.

On the other hand, delta-connected sources are not common in practice because of the circulating current that will result in the delta-mesh if the three-phase voltages are slightly unbalanced.

Balanced Three-Phase Voltages Example

Determine the phase sequence of the set of voltages

Balanced Three-Phase Voltages

Solution :
The voltages can be expressed in phasor form as

Balanced Three-Phase Voltages

We notice that Van leads Vcn by 120o and Vcn, in turn, leads Vbn by 120o. Hence, we have an acb sequence.