In general three phase loads are considered either “balanced” or “unbalanced”. A three phase circuit is considered balanced if the voltages, currents and power factors in all 3 phases are identical. Conversely, when any of these parameters are not identical the circuit is classified as unbalanced. The computations of electrical properties of balanced loads are relatively straightforward and may be performed by simple computations. On the other hand, the calculations of the electrical properties of unbalanced three phase circuits become somewhat more complicated.
Types Of Load
- Balanced Load
- Unbalanced Load
Balanced Load Three Phase Circuits
A load which results in equal amount of current flowing through all the phases displaced by 120 degrees in the electric circuit is known as a balanced load.
In the balance circuit the voltage of each phase should be in same magnitude and the equal phase difference the we can say that circuit is balanced.
If Ir , Iy and Iz are the currents flowing through the 3 phases of a system and In is the current flowing through the neutral, if the vector sum of
Ir + Iy + Iz = 0 , i.e. In = 0
the load is a balanced load.
Voltages in Three Phase Circuits – General
It is fair to say a three phase circuit consists of three separate, single phase voltages. A trace of the three voltages of a three phase circuit with time would be similar to that represented in Fig. 1 where the sequence is assumed to be A-B-C, i.e. Vab –Vbc –Vca which is the more popular USA sequence.
Determination of a 3 phase voltage sequence is of particular importance when large motors are involved. In any installation, a motor is intended to rotate in a predetermined direction. If the sequence of the voltage connected to the motor has been inadvertently reversed, the motor will rotate in an unintended direction. Depending on the application, considerable property damage could result when a motor is started and it rotates in reverse to the intended direction. A voltage sequence meter is especially valuable in the field to ascertain voltage sequence.
A typical three wire three phase circuit with a delta load is represented in Fig. 2. Of course, single phase loads may be taken from any two of the conductors of a three phase circuit and this is often the case. If there are a large number of single phase users, then a four wire three phase service is more practical. A typical three phase wye circuit is shown in Fig. 3. Three phase wye circuits could be three wire or four wire. If the circuit is a four wire circuit, point “d” of Fig. 3 would be connected to ground. A 480 VAC, four wire – three phase service is especially suited for commercial building as the single phase circuits can be taken for fluorescent lighting without imposing a need for a transformer. The lighting circuits would be at 277 VAC. If a separate 120/240 VAC single phase circuit is not brought to the building an in-house transformer would be needed to provide a 120/240 VAC single phase service.
Phasor Diagrams of Three Phase Circuits
Construction of a phasor diagram for a three phase circuit essentially follows the rules that are applicable to single phase circuits. The voltage vectors for the three phase circuit of Fig. 1, Fig. 2 and Fig. 3 are shown in Fig. 4. As is the case with the single phase circuit, projections of the three vectors on the ordinate describe the voltages of the three phases. The vectors of Fig. 4, in accordance with the rules for the construction of phasor diagrams.
Unbalanced Three Phase Circuits
The unbalanced circuit has at least one phase current that is not equal to the other phase currents. Of course, all three phase currents could be of unequal magnitude.
when one or two phases circuit’s the current and voltage will not be in equal in magnitude as well as phase difference condition said to be unbalanced. Generally, unbalance condition may come to the system due to fault.
Once a understands balanced three phase circuits and the use of phasor diagrams to visualize the voltages and currents in those circuits, it is an easy transition into the realm of unbalanced three phase circuits. As with balanced circuits, phasor diagrams can be used to present a clear picture of the voltages and currents that are involved. Below, delta circuits are considered first. The wye circuits are considered after the delta circuits. Because of the very large number of possible combinations of loads and phase angles, all of these combinations cannot be treated individually. Rather, the method for calculating line currents is described. With that methodology, a person may then readily calculate currents in any possible, specific combination of unbalanced delta and wye circuits.
By definition, an unbalanced circuit has at least one phase current that is not equal to the other phase currents. Of course, all three phase currents could be of unequal magnitude. In all cases, line voltages are assumed to be of equal magnitude, separated by 120º of rotation and in the sequence A-B, B-C, C-A. In wye circuits all three phase voltages are assumed to be equal and separated by 120º of rotation.