Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Define single-phase and three-phase systems.
• Explain the advantages of three-phase power.
• Describe how three-phase voltages are generated.
• Differentiate between Star (Y) and Delta (Δ) connections.
• Calculate voltages in a balanced star-connected system.

Lesson summary

Welcome, Grade 11 learners, to our introduction to three-phase electrical systems! This is a crucial topic in Electrical Technology. You may not realize it, but three-phase power is everywhere around you. From the power grid that lights up your homes and schools to the large industrial motors that drive pumps in water treatment plants providing clean water, and even the air conditioners keeping supermarkets running smoothly to preserve food supplies, three-phase electricity plays a vital role.

Lesson notes

2.1 Single-Phase vs.

Three-Phase Single-Phase: This is the type of electricity typically found in our homes. It consists of a single alternating voltage source. Think of it as a single "wave" of power. It's simple and suitable for low-power applications like lighting, small appliances, and electronics.

Three-Phase: This system uses three alternating voltage sources, each offset by 120 electrical degrees. Imagine three "waves" of power, each arriving at a different time. This provides a smoother and more consistent power delivery, making it ideal for high-power applications like powering large motors, running factories, and distributing electricity over long distances.

Think of it like this: imagine trying to push a heavy box. Single-phase is like one person pushing and then pausing. Three-phase is like three people pushing in sequence, so there's almost always someone pushing, resulting in smoother and more powerful movement. 2.2 Advantages of Three-Phase Power Higher Power Capacity: For the same size and weight of equipment, a three-phase system can deliver significantly more power than a single-phase system. This is critical for industries that require large amounts of energy.

Smoother Torque Production: In electric motors, three-phase power produces a more constant and uniform torque, leading to smoother operation and less vibration. This is important for applications where precise control is required, such as in manufacturing processes.

Higher Efficiency: Three-phase systems are generally more efficient than single-phase systems, meaning less energy is wasted in transmission and distribution. This is particularly important in South Africa, where energy conservation is a national priority.

Reduced Conductor Size: For the same amount of power delivered, three-phase systems require smaller conductors than single-phase systems. This reduces the cost of materials and installation. Consider Eskom’s power plants: They generate three-phase power because it's the most efficient way to transmit large amounts of electricity across the country. 2.3 Generation of Three-Phase Voltages Three-phase voltages are generated using an AC generator (alternator). Inside the generator, there are three separate sets of windings (coils of wire) arranged 120 degrees apart. As the rotor (the rotating part) spins, it induces a voltage in each of the three windings. Because the windings are physically offset, the voltages generated are also offset by 120 electrical degrees. Imagine three magnets equally spaced around a circle. As you spin a metal coil past them, each magnet induces a voltage, but at slightly different times because of their spacing. This is essentially how a three-phase generator works. 2.4 Star (Y) and Delta (Δ) Connections These are the two common ways to connect the three phases of a three-phase system.

Star (Y)

Connection: In a star connection, one end of each of the three windings is connected to a common point, called the neutral point (N). The other ends of the windings are connected to the three lines (L1, L2, L3) that supply power to the load.

Line Voltage (V L ): The voltage between any two lines.

Phase Voltage (V P ): The voltage between a line and the neutral point.

Line Current (I L ): The current flowing through a line.

Phase Current (I P ): The current flowing through a phase winding.

In a balanced star connection: V L = √3 V P I L = I P Delta (Δ)

Connection: In a delta connection, the three windings are connected in a closed loop, forming a triangle. Each corner of the triangle is connected to a line (L1, L2, L3).

Line Voltage (V L ): The voltage between any two lines.

Phase Voltage (V P ): The voltage across a phase winding.

Line Current (I L ): The current flowing through a line.

Phase Current (I P ): The current flowing through a phase winding.

In a balanced delta connection: V L = V P I L = √3 I P Think of a star connection like a "Y" shape. All the legs of the "Y" meet at a central point (the neutral). Think of a delta connection like a triangle; each corner is a connection point. 2.5 Calculations in a Balanced Star-Connected System A balanced system means each phase has the same impedance and voltage.

Example 1: A balanced star-connected three-phase generator has a phase voltage of 230

V. Calculate the line voltage.

Solution: V P = 230 V V L = √3 * V P V L = √3 * 230 V V L ≈ 398.4 V Therefore, the line voltage is approximately 398.4

V. Example 2: A balanced star-connected three-phase motor is connected to a 400 V line voltage. Calculate the phase voltage.

Solution: V L = 400 V V L = √3 * V P V P = V L / √3 V P = 400 V / √3 V P ≈ 230.9 V Therefore, the phase voltage is approximately 230.9

V. Example 3: A three-phase, star-connected load has a line current of 10A. What is the phase current?