FAQ on three-phase AC power part 2: implementation

We’re familiar with single-phase AC at the outlet, but multiphase AC offers advantages and is necessary for higher-power installations.

The first part of this article examined the need for three-phase AC. This part looks at the implementation.

Power sourcing and wiring

Q: How is the three-phase AC power generated?
A: It does not use three separate generators that are somehow synchronized. Instead, it is relatively simple in principle: the generator is wound with three independent coils, one for each phase, and spaced 120° apart around the shaft. The coils are then wired to each other in one of two arrangements (more below).

Q: How is the three-phase wiring labeled compared to the hot and neutral designations of single-phase power?
A: The wires are often labeled simply as phase 1, 2, and 3 or as L1, L2, and L3. Depending on the configuration, there may also be a neutral wire.

Q: What is the power-path flow from source to load?
A: Power is generated at the three coils or poles (some generators use 6, 9, or even 12 physical poles for better balance, but they are wired to look like three poles). The output is stepped up to thousands of volts via a three-phase transformer for transmission, sent along the three-wire distribution poles, and then stepped down via a complementary transformer to serve local needs.

Q: Can you have both single-phase and three-phase power in the same distribution path?
A: Yes, that is the normal flow of power from the source to various classes of users (Figure 1).

Figure 1. The electricity supply chain uses step-up and step-down transformers, supplying users with 3-phase and single-phase AC power. (Image: U.S. Federal Energy Regulatory Commission and U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability)

Q: What do the transformer windings and three-phase motor windings look like? What are the available wiring configurations?
A: Here’s where things get both interesting and complicated. There are two configurations: the delta and the wye (star), and the phase-to-phase voltages will differ for each. These names are derived from the way each of their stylized schematic representations looks (Figure 2).

Figure 2. The three phases can be connected in a delta or wye arrangement; the names are derived from their topology. (Image: Pumps & Systems/Cahaba Media Group, Inc.)

The delta is also known as π (pi) network because it resembles the letter after rearranging the branches, while the star is also known as the “T” connected network due to its shape after rearranging the network branches (Figure 3).

Figure 3. The delta triangle can be redrawn in a more conventional x-y arrangement, which is easier for some engineers to comprehend. Similarly, the wye (star) can be redrawn in x-y form as a “T.” (Image: Electrical Technology)

Dolivo-Dobrovolsky also patented these two types of 3-phase transformers. With these transformers, the three phases can be safely transformed to high voltages and low current for long-distance transmission without much loss and then transformed back to lower voltages to be safely used.

Q: Can you explain wye and delta further?
A: Figure 3 also shows a wye-connected, three-phase transformer’s secondary (output) side. The green line is a center tap that leads to ground. The individual phases are 120 V, each producing 120 volts when connected to the center tap.

When connected phase to phase, the voltage is only 208 — not the 240 volts we might expect. The reason is that wye connections produce a different phase angle among the phases, and the phase angle determines the phase-to-phase voltage. Using math or phasor diagrams makes this clear. The benefit is that a constant allows you to compute the phase-to-phase voltage produced by a Wye connection. The phase-to-phase voltage will always be 1.732 times the phase voltage.

Figure 4. The wye-connected three-phase transformer has a ground connection, and its output can be used as lower-voltage single-phase lines. (Image: Pumps & Systems/Cahaba Media Group, Inc.)

Q: What about the delta?
A: Figure 4 shows a delta-connected, three-phase transformer’s secondary (output) side. As in the wye example, the individual phases produce 120 volts. The phase-to-phase voltages are twice the individual phase voltages or 240 volts. It may appear that the delta is a more efficient design, but phase angle also has a role here.

The phase-to-phase current in a delta circuit is only 1.732 times the phase current, but it is two times the phase current in a Wye circuit. This is why the constant of 1.732 appears in the equations used to calculate wattage and other values in three-phase circuits. It accounts for the phase angle’s effect on voltage and current in the two connections.

Q: Do you have to use wye or star windings exclusively in a transformer?
A: No, the primary side can be either wye or delta, and the secondary side can be either as well (Figure 5). That means there are four possible pairings, each with their own attributes. The decision of which one to use is made based on the power level, application, and other factors.

Figure 5. The delta-connected three-phase transformer has a general functionality similar to the wye version but differs in some specifics. (Image: Pumps & Systems/Cahaba Media Group, Inc.)

Q: Which arrangement is better to use?
A: There are pros and cons to using either star or delta 3-phase transformer wiring systems. When choosing the right system for your applications, it is necessary to understand the phase and line currents and voltages. Phase currents and voltages are measured over one component, whereas line parameters are measured over two terminals. Figure 6 summarizes the relationships between these characteristics:

Figure 6. Either side of the transformer can be wye or delta, and they don’t have to be the same; this shows a transformer with a delta arrangement on the primary side and a wye arrangement on the secondary side. (Image: Tameson)

Q: When doing analysis, can you transform the impedance of one transformer arrangement to the other?
A: Yes. To solve a complex electrical network or simplify it, use the star-delta conversion technique. It replaces any star-connected network with its equivalent delta-connected network and vice versa (Figure 7).

Figure 7. This table summarizes key attributes of the wye and delta arrangements and some measurement factors. (Image: Tameson)

Q: What do the equations look like?
A: The derivation is complicated, but the results are relatively simple (Figure 8).

Figure 8. A reconfiguration of the wye and delta with suitable equations allows the impedance specifications of one version to be transformed into the other. (Image: Electrical Technology)

Q: Is there an overview of single-phase attributes versus those of the three-phase approach?
A: Yes, as shown in Figure 9.

Figure 9. The circuit analysis results in some basic transformation equations. (Image: Electrical Technology)

Q: What about circuit protection?
A: A three-phase system needs a circuit breaker for each phase (Figure 10).

Figure 10. This table summarizes the attributes of single-phase versus three-phase systems (numbers are for a nominal 240-VAC system). (Image: Electronics Hub)

Figure 11. A circuit-protection scheme for a three-phase system needs a breaker for each phase. (Image: Cyber Power Systems)

Conclusion

Three-phase power is an innovative solution to the problem of providing, transmitting, and using large amounts of AC power. While it has been refined and updated, it is a relatively old concept dating from the early days of the generation of electric power and its use primarily with motors. Like many other advances, it is conceptually fairly straightforward, but in-depth understanding requires considerable mathematical analysis and insight.

Related EE World Online content

Scope-based diagnosis of three-phase motor drives
Can a three-phase power supply operate from wye and delta ac inputs?
Three-phase testing basics – Mitigating harmonic current
Power supply operates from 400/440/480 Vac Delta and Wye inputs
How do I choose an electric motor, and how do I test it? Part 4

References

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