Three-Phase Circuits

From our past lessons, we always dealt with single-phase circuits. A single-phase ac power system is consists of a generator connected to a pair of wires to a load. These pair of wires is what we called transmission line. Polyphase are circuits or systems which the ac sources operate at the same frequency but different phases. A two-phase system is produced by a generator consisting of two coils placed perpendicular to each other so that thee voltage generated by one lags the other by 900 while a three-phase system is produced by a generator consisting of three sources having the same amplitude and frequency but out of phase with each other by 1200.
See the difference of the said three systems above by means of figures:

Single-phase two wire system

Two-phase three wire system

Three-phase four wire system

In our household, we single-phase three wire system because the terminal voltages have the same magnitude and the same phase. Also, whether it is single-phase, two-phase or three-phase system, we can’t put load as many as we can. We must also consider the power supplied by the source if it is enough to accommodate certain number of loads. Example of this is a power amplifier we’ve made; we have 70V and 8A a total of 560W power. Our power amplifier is 100W so; I can say that there must only 10 100-W power amplifier that must be put in the circuit

BALANCED THREE-PHASE VOLTAGES


Three-phase voltage is often produced by a three-phase ac generator. Balanced phase voltages are equal in magnitude but different in phase angle because they are out of phase with each other by 1200. A three-phase system is equivalent o three single-phase circuits. A voltage sources can be connected either wye-connected or delta-connected

DELTA CONNECTION

WYE CONNECTION

There are also two types of phase sequence, one is positive or abc sequence in which Van leads Vbn which in turn leads Vcn. The other one is negative or acb sequence in which Van leads Vcn which in turn leads Vbn. This phase sequence is the time order in which the voltages pass through their respective maximum values.

POSITIVE SEQUENCE

NEGATIVE SEQUENCE

NOTE:


A balanced load is one in which the phase impedances are equal in magnitude and in phase. Wye connected load has impedances connected in neutral node while delta connected has impedances connected around a loop.


DEFINITION OF IMPORTANT TERMS:


Line current - current flowing from the generator to the load in each
transmission line
line voltage - the voltage between in each pair of lines
phase current – current flowing through each phase
phase voltage – voltage of each phase


BALANCED WYE-WYE CONNECTION


A balanced Wye-Wye system is a three-phase system with a balanced connected source and a balance connected load. We can solve for its line current by getting a single-phase loop from the circuit and applying KVL.

Summary: 

BALANCED DELTA-DELTA CONNECTION


A balanced delta-delta system is one in which both the balanced source and balanced load are delta-connected. For easy analyzing the delta-delta connected circuit, transform both the source and the load to their wye equivalent.

Summary: 

BALANCED DELTA-WYE CONNECTION


A balanced delta-wye system consists of a balanced delta-connected source and a balanced wye-connected load. There are also the line voltages as well as the phase voltages. We can generate an equation through getting a loop from the circuit and this may help us solved for the line currents. Like in other connections, we may also transform delta-connected source to wye-connected source.

Summary: 

LEARNINGS:


After the partial discussion of three-phase system, I’ve learned that in a wye-connected balanced source, line currents and phase currents are equal while in a delta-connected balanced source, line voltage and phase voltage is equal. For solving, its easy to solve the required parameters if the system is a wye-wye connection because it has a single-phase equivalent circuit that is easy to analyze. Also, I’ve found out that the electric power is generated and distributed in three-phase at the standard operating frequency.