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# E283C2 - EEEB344 Electromechanical Devices Chapter 2...

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EEEB344 Electromechanical Devices Chapter 2 CHAPTER 2 – TRANSFORMERS Summary: 1. Types and Construction of Transformers 2. The Ideal Transformer Power in an Ideal Transformer Impedance transformation through a transformer Analysis of circuits containing ideal transformer 3. Theory of operation of real single-phase transformers. The voltage ratio across a transformer The magnetization current in a Real Transformer The current ratio on a transformer and the Dot Convention 4. The Equivalent Circuit of a Transformer. Exact equivalent circuit Approximate equivalent circuit Determining the values pf components in the transformer model 5. The Per-Unit System of Measurement 6. Transformer voltage regulation and efficiency The transformer phasor diagram Transformer efficiency 7. Three phase transformers 1

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EEEB344 Electromechanical Devices Chapter 2 1. Types and Construction of Transformers Types of cores for power transformer (both types are constructed from thin laminations electrically isolated from each other – minimize eddy currents) i) Core Form : a simple rectangular laminated piece of steel with the transformer windings wrapped around two sides of the rectangle. ii) Shell Form : a three legged laminated core with the windings wrapped around the centre leg. The primary and secondary windings are wrapped one on top of the other with the low-voltage winding innermost, due to 2 purposes: i) It simplifies the problem of insulating the high-voltage winding from the core. ii) It results in much less leakage flux Types of transformers: i) Step up/Unit transformers – Usually located at the output of a generator. Its function is to step up the voltage level so that transmission of power is possible. ii) Step down/Substation transformers – Located at main distribution or secondary level transmission substations. Its function is to lower the voltage levels for distribution 1 st level purposes. iii) Distribution Transformers – located at small distribution substation. It lowers the voltage levels for 2 nd level distribution purposes. iv) Special Purpose Transformers - E.g. Potential Transformer (PT) , Current Transformer (CT) 2
( 29 ( 29 a N N t v t v s p s p = = ( 29 ( 29 a t i t i s p 1 = EEEB344 Electromechanical Devices Chapter 2 2. The Ideal Transformer 1. Definition – a lossless device with an input winding and an output winding. 2. Figures below show an ideal transformer and schematic symbols of a transformer. 3. The transformer has N p turns of wire on its primary side and N s turns of wire on its secondary sides. The relationship between the primary and secondary voltage is as follows: where a is the turns ratio of the transformer. 4. The relationship between primary and secondary current is: N p i p (t) = N s i s (t) 5. Note that since both type of relations gives a constant ratio, hence the transformer only changes ONLY the magnitude value of current and voltage. Phase angles are not affected.

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E283C2 - EEEB344 Electromechanical Devices Chapter 2...

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