SINGLE PHASE TRANSFORMER MAGNETIC CIRCUIT AND ELECTRIC TRANSFORMER PART1: FAMILIARIZATION

Transcription

1 Islamic University of Gaza Faculty of Engineering Electrical Engineering department Electric Machine Lab Eng. Omar A. Qarmout Eng. Amani S. Abu Reyala Experiment 3 SINGLE PHASE TRANSFORMER MAGNETIC CIRCUIT AND ELECTRIC TRANSFORMER PART1: FAMILIARIZATION OBJECTIVE: When you have completed this assignment, you will know the principle of transformer operation. INTRODUCTION: Transformers are magnetically-operated devices that can change voltage, current, and impedance values in ac circuits. In its simplest form, a transformer consists of two coils of wire wound on a common core of ferromagnetic material, such as iron. Figure 3-1: Single-Phase Transformer - The first coil is called primary winding, used for the input power, and has number of turns. - The second coil is called the secondary winding, connected to the load, and has number of turns. - The ratio of the number of turns of wire in the primary winding ( ) to the number of turns of wire in the secondary winding (.) is called the turns ratio which set the relation between the input and the output values of a transformer. Transformers are probably the most universal pieces of equipment in the electrical industry, and range in size from tiny units in transistor radios to extremely large units weighing several tons in power distribution stations. However, all transformers have basically the same operating principles and characteristics. Figure 3-2: Single-Phase Transformer Connected to a Resistive Load.

2 PRACTICAL 2.1.A: DETERMINING THE TURNS RATIO USING VOLTAGE RATIO WITH NO LOAD: 1. Ensure that the power supply (unit ) is switched off. 2. Make the appropriate connections either for virtual or conventional instrumentation shown in figure 3-3 using unit In the virtual instrumentation set the range of ammeters in low mode. Figure 3-3: Determining the turns-ratio using voltage ratio 4. Ensure the dial on the power supply is set to zero position. 5. Connect E2 between 3&4 6. Switch on the power supply unit and using the output voltage dial set the phase voltage to 220 V as shown on the virtual or conventional voltmeter E1. 7. Read the measurement from the computer and calculate the turns ratio,,,,,, 8. Connect E2 between 5&6 and 5&9 then repeat 6 and 7. PRACTICAL 2.1.B: DETERMINING THE TURNS RATIO USING CURRENT RATIO: 1. Ensure that the power supply (unit ) is switched off. 2. Make the appropriate connections either for virtual or conventional instrumentation shown in figure 3-4 using unit In the virtual instrumentation set the range of ammeters in low mode. Figure 3-4: Determining the turns-ratio using current ratio 4. Ensure the dial on the power supply is set to zero position. 5. Connect I2 between 3&4 6. Switch on the power supply unit and using the output voltage dial set the current I2 0.1A. 7. Read the measurement from the computer and calculate the turns ratio,,,,

3 ,, 8. Connect I2 between 5&6 and 5&9 then repeat 6 and 7 but set I2 0.2A. Exercise 3-1: Compare between the results on the two parts, write your notes. Practical aspects: 1. Most transformers are used to provide either a set up or a step down of secondary voltage. 2. Large electricity supply transformers are used to change the relatively low voltage produced by the power station generators to much higher level for transmission of power over long distance; e.g. power may be generated at 11kV and transmitted at 27.5kV. 3. At sub-station near to a town or city where the electricity will be used several step-down transformers will be connected between the transmission lines and the distribution system. 4. Further step-down transformers reduce the main supply to the nominal value specified by the supply authorities; for example 240 volt in domestic and light industry applications. 5. In many types of electrical equipments such as industrial electronics instruments, step-down transformer are used to provide the low voltage power supplies required for transistors and integrated circuits 6. Stepped ratio transformers enable a small motor or possibly heating element to be powered from a range of different supply voltages. In these cases, a mains voltage selector switch may by connected to tapping points on the primary winding for 100, 120, 200 or 240V. PART2: THE RATED VOLTAGE AND SATURATION ELEMENTS OBJECTIVE: When you have completed this assignment, you will be familiarized with the definition of the rated voltage and the saturation case of the transformer. INTRODUCTION: An exciting current is needed in order to generate alternating magnetic flux across the transformer windings. The exciting current, which is directly related to the alternating magnetic flux, increases in direct proportion to the applied voltage until core saturation sets in. This occurs when the applied voltage exceeds the rated value of the primary, and then the linear relationship between the primary voltage and the exciting current breaks down. The curve of primary voltage versus exciting current flattens and smaller increases in primary voltage lead to larger increases in exciting current as shown in Figure 3-5. The exciting current is only a few milliamperes in the EMS Single- Phase Transformer module, and generally its value is a small percentage of the nominal current of a transformer. Figure 3-5: Saturation Curve of a Transformer

4 PRACTICAL 3.2.A: THE RATED VOLTAGE AND SATURATION 1. Ensure that the power supply (unit ) switched off. 2. Make the appropriate connections either for virtual or conventional instrumentation shown in figure 3-6. Figure 3-6: Effect of Core Saturation on Exiting Current 3. In the virtual instrumentation set the range of ammeters in low mode. 4. Ensure the dial on the power supply is set to zero position. 5. Switch on the power supply unit and using the output voltage dial, take the results after reaching 10% from the indicator. 6. Repeat 5 after increasing another 10% (Use data table on the virtual program to record the results) and continue repeating until reaching 100% (Table 3-1). 7. Switch off supplies. 8. Observe the curve of primary voltage versus exciting current represented by E1. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Table 3-1 Exercise 3.2: - Does the exciting current increase more rapidly after the rated voltage is exceeded? Why? - Does the curve illustrate that the transformer core becomes saturated? - Review the measured data to determine how the primary-to-secondary voltage ratio was affected when the transformer core became saturated. Practical aspects: Practically, it is necessary to use the transformer at its rated values or fewer than it in order to preserve the transformer from damaging.

5 PART3: THREE PHASE SEQUENCE DETERMINATION OBJECTIVES When you have completed this exercise, you will be able to determine and use transformer polarities to properly connect separate windings so that the voltages add (series-aiding) or subtract (series-opposing). INTRODUCTION: When the primary winding of a transformer is energized by an ac source, an alternating magnetic flux is established in the iron core. This alternating flux links, or couples, the turns of each winding on the transformer and induces ac voltages in the windings. However, when two or more windings are connected together, their relative instantaneous polarities have a significant effect on the resulting net voltage. If the voltage in one winding is at its maximum positive peak when the voltage in another winding is at its maximum negative peak, i.e. they are 180 out of phase, they will oppose each other and the resulting voltage will be the difference between the two. For this reason, standards have been adopted for marking the polarity of transformer leads. When we speak of the polarity of transformer windings, we are identifying all terminals that have the same polarity, either negative or positive, at any instant in time. The dots used in Figure 3-7 indicate that at a given instant in time, when terminal 1 is positive with respect to terminal 2, then terminal 5 is positive with respect to terminal 6. Figure 3-7: Dot indicator to specify the Transformer Polarity This means that if the terminals 1 and 5 are connected to gather then the voltage between the terminals 2 and 6 is equal to the deference between, and,, and if the terminals 1 and 6 are connected to gather then the voltage between the terminals 2 and 5 is equal to the total of, and,. PRACTICAL 3.3.A: TRANSFORMER POLARITY DETERMINATION 1- Ensure that the power supply (unit ) switched off. 2- Connect the power supply to the primary terminal and connect the terminals 1&5. 3- Ensure the dial on the power supply is set to zero position. 4- Set the power supply voltage at 100V then measure the voltage between the terminals 2&6. 5- Turn off the power supply then connect the terminals 1&6. 6- Repeat 4 but measure the voltage between 2&5. 7- Use your result to determine the polarity of the transformer. a b Figure 3-8: Determining transformer polarity

6 PRACTICAL ASPECTS 1. In most transformer applications, it is not necessary to know the polarity of the windings. However, when two transformers are required to be connected in parallel, it is essential to establish correct polarity; otherwise destructively large currents would circulate in the secondary windings. This is of particular importance in power distribution systems. 2. In most transformers supplying industrial loads, e.g. motors, heaters etc., two or more secondary windings may be connected in series or in parallel to provide a range of voltage and current levels. Again, the polarity of the secondary winding must be known