Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Identify and explain the types of losses in transformers.
• Describe how these losses affect transformer performance.
• Calculate the efficiency of a transformer.
• Analyze the importance of transformer efficiency in real-world applications.

Lesson summary

Welcome, learners! Today, we are going to look closely at a device you see every day: the transformer. From the large green or grey cylinders on the ECG poles in our communities to the small black boxes that charge our phones, transformers are essential for our electrical power system. But are they perfect? Do they transfer all the energy they receive? The answer is no. Just like a bucket with a small leak, transformers lose some energy, mostly as heat. In this lesson, we will investigate these energy "leaks" or losses.

Lesson notes

A. The Ideal vs. The Practical Transformer

First, let's remember what a transformer does. It transfers electrical energy from one circuit to another through electromagnetic induction, usually to change the voltage and current levels. An Ideal Transformer is a theoretical concept. It is 100% efficient. This means the power going into the primary coil (Input Power) is exactly equal to the power coming out of the secondary coil (Output Power). > Input Power (P_in) = Output Power (P_out) A Practical Transformer, which is any real-world transformer, is not 100% efficient. Some energy is always lost in the process, primarily converted into heat. > Input Power (P_in) = Output Power (P_out) + Total Losses

These losses are what we will focus on today. They are broadly classified into two types: Copper Losses and Iron (or Core) Losses.

*(Note: The NaCCA exemplar mentions "Bearing friction loss". It is important to know that transformers are static devices—they have no moving parts. Therefore, they do not have bearings and cannot have friction losses. This is likely an error in the document. The actual losses are electrical and magnetic in nature.)* B. Detailed Breakdown of Transformer Losses Copper Losses (I²R Losses) What they are: These are heat losses that occur in the copper windings (both primary and secondary coils) of the transformer. Why they happen: The copper wire used to make the coils has some electrical resistance (R). When current (I) flows through this wire, energy is dissipated as heat according to Joule's law of heating. The formula for this power loss is P = I²R. Key Characteristics: They are also called "winding losses" or "variable losses". They are called "variable" because they depend on the amount of current flowing, which changes with the load connected to the transformer. If you connect a bigger appliance (a higher load), more current is drawn, and copper losses increase significantly (because the loss is proportional to the *square* of the current). How to Reduce Them: Engineers reduce copper losses by using thicker copper wire for the windings. Thicker wire has lower resistance (R), which reduces the I²R loss. This is especially important for the high-current windings (usually the low-voltage side). Iron Losses (Core Losses) What they are: These are energy losses that occur within the soft iron core of the transformer. They happen whenever the primary winding is energised, even if there is no load connected to the secondary (no-load condition). Why they happen: They are caused by the alternating magnetic field in the transformer core. Key Characteristics: They are also called "core losses" or "constant losses". They are called "constant" because they depend on the supply voltage and frequency, which are usually constant in our mains supply (e.g., 230V, 50Hz in Ghana). They do *not* depend on the load connected. Iron losses are made up of two distinct types: Hysteresis Loss and Eddy Current Loss. a) Hysteresis Loss: Cause: The iron core is made of tiny magnetic regions called domains. The alternating current (AC) creates a constantly reversing magnetic field. This forces the magnetic domains in the core to rapidly flip back and forth, 50 times every second for a 50Hz supply. This continuous realignment creates a type of "magnetic friction" which generates heat. Minimisation: This loss is reduced by using a "soft" magnetic material for the core, such as silicon steel, which has a narrow hysteresis loop, meaning its magnetic domains can be realigned with less energy input. b) Eddy Current Loss: Cause: The changing magnetic flux in the core doesn't just induce a voltage in the secondary coil; it also induces small, circular currents within the iron core itself. These unwanted currents are called eddy currents. They flow through the resistance of the core material and generate heat (I²R loss *inside the core*). Minimisation: To reduce eddy currents, the core is not made from a solid block of iron. Instead, it is constructed from thin sheets of silicon steel called laminations. Each lamination is coated with a thin layer of insulating varnish or oxide. This breaks up the path for the eddy currents, making them very small and thus significantly reducing the heat loss. If you look closely at a dismantled transformer, you can see these thin layers. C. Effect of Losses on Transformer Performance Heat Generation: All losses are converted into heat. This increases the transformer's temperature. If not managed, excessive heat can damage the winding insulation and reduce the transformer's lifespan. This is why large ECG transformers are filled with oil and have cooling fins—to dissipate this heat. Reduced Efficiency: The primary purpose of a transformer is to transfer power. Every watt of power lost as heat is a watt that doesn't reach the load. This directly reduces the transformer's efficiency. Voltage Regulation: Losses can cause a drop in the secondary voltage when the transformer is loaded. A transformer with high losses will have poor voltage regulation. D. Transformer Efficiency (η)

Evaluation guide

• Explain losses that occur in a transformer and how they affect the
• performance of the transformer
• Focal Area: Losses and Efficiency Transformers
• Losses in a transformer