ATE1120: Electrical Fundamental-II



Electrical MachinesModule 1: Transformer Types and ApplicationsPREPARED BYIAT Curriculum UnitApril 2012? Institute of Applied Technology, 2012Module 1: Transformer Types and ApplicationsModule ObjectivesUpon successful completion of this module, students should be able to:Describe the function of the transformer and give its application.Describe the operation of transformer and give its schematic symbol.Describe how to calculate the output voltage of a transformer.Describe how to calculate the efficiency using input and output powers.Discuss the basic categories of the transformers.Discuss the function of the control transformer. Design a control transformer circuit to provide a given output voltage.Module Contents:TopicPage No.1.1Transformer Construction and Principle of Operation31.2Transformer Types 101.3Control Transformers 121.4Lab Activity 1161.5Lab Activity 2191.6Lab Activity 3231.7Lab Activity 4261.8Review Exercises32Transformer Construction and Principle of Operation Definition: A Transformer is an electrical device that converts AC electricity from one voltage level to another. A typical small size transformer is shown in Figure 1.1.Figure 1.1: A typical Transformer.Operation of Transformer and its Schematic Symbol A Transformer consists of three basic components:Primary Coil- An electrical wire wrapped around the core, as shown in Figure 1.2.Secondary Coil- An electrical wire wrapped around the core, as shown in Figure 1.2.Core- A ferromagnetic material that is capable of conducting a magnetic field. Iron is a common material.Figure 1.2: Construction of a Typical TransformerA transformer uses the principle of mutual inductance to create an AC electrical voltage in the secondary coil from the alternating electrical current flowing through the primary coil.The principle of mutual inductance says that when two electrical coils are placed near to each other, AC electrical current flowing in one coil induces an AC voltage in the other coil. This occurs because electrical current creates a magnetic field around the first coil which in turn induces a voltage in second coil, as shown in Figure 1.3.Figure 1.3: Mutual Inductance ConceptsThe transformer improves the efficiency of the transfer of energy from one coil to another by using a core to concentrate the magnetic field. As shown in Figure 1.4, the primary coil creates a magnetic field that is concentrated by the core and induces a voltage in the secondary coil. The voltage induced in the secondary coil can be used to drive a load.Figure 1.4: Operation of a TransformerWhat makes the transformer useful is its ability to make the voltage at the secondary coil different from the voltage at the primary. This occurs when there is a difference between the numbers of times each of the coils is wound around the core.Figure 1.5 shows the schematic symbol of a transformer.Figure 1.5: Schematic Symbol of a TransformerConduct Lab Activity 1 on Page Number 16Output Voltage of a TransformerTo determine the voltage output generated in the secondary coil of a transformer we only need to know two things: the input voltage and the turns ratio.The turn’s ratio is the number of turns (times the wire is looped around the core) in the primary coil compared to the number of turns in the secondary coil. This ratio is determined by dividing the number of turns in the primary by the number of turns in the secondary as follows:The voltage output of a transformer is then calculated using the following formula:This is an important relationship because it allows us to decide, which transformer to select for an application and troubleshoot one to decide if it is good.Examples :A transformer has a primary voltage of 230v and turns ratio of 5:1. Calculate the secondary voltage .VP / VS = TP / TS 230/ VS = 5 /1VS = 46 VA transformer has 200 turns in the primary, 50 turns in the secondary, and 120 volts applied to the primary (Vp). What is the voltage across the secondary (V s)?VP / VS = TP / TS120/ VS = 200/50VS = 120 X 50 / 200 = 30 VThere are 400 turns of wire in an iron-core coil. If this coil is to be used as the primary of a transformer, how many turns must be wound on the coil to form the secondary winding of the transformer to have a secondary voltage of one volt if the primary voltage is five volts? VP / VS = TP / TS5 / 1 = 400 / TS TS = 1 x 400 /5 = 80 turns A 12 volts transformer has 20 turns in the primary, 5 turns in the secondary. What is the voltage across the primary side (VP)?VP / VS = TP / TSVP / 12 = 20/ 5VP = 12 x 20 / 5 = 48 VConduct Lab Activity 2 on Page Number 19Input and Output Power of a TransformerIdeally, the output power of a transformer should be the same as the input power. However, because of the construction of a transformer, there is usually some amount of power loss between the primary and the secondary. The “Power In” equals the “Power Out” plus any “Losses”, as shown in Figure 1.6.This power lost by a transformer is actually converted into heat, which must be dissipated by the transformer. Large transformers used by utility company substations actually have cooling systems that use pumps to circulate oil around the transformer core to help dissipate this heat.Figure 1.6: Power through a TransformerThe heat generated by a transformer can be found by calculating the efficiency of the transformer. This can be calculated with the following formula:Conduct Lab Activity 3 on Page Number 23Transformer Types Most transformers fall into one of two categories:Isolation transformersAutotransformersIsolation TransformersAn isolation transformer, such as the one shown in Figure 1.7, has primary and secondary windings that are physically and electrically isolated. The isolation transformer is magnetically coupled, not electrically coupled.Figure 1.7: An Isolation Transformer SchematicThis characteristic is very important. Since there is no electrical connection between the primary and secondary, the transformer is basically a filter between the two. Voltage spikes that might occur on the primary are greatly reduced or eliminated in the secondary. If the primary is shorted somehow, any load connected to the secondary is not damaged. Most electrical devices or machines that operate on AC use an isolation transformer to provide the desired operating voltage, as well as protection. Some isolation transformers produce an output voltage that is equal to the primary voltage. Traditional TV monitors make use of this type of isolation transformer to protect the picture tube from voltage spikes on the main power lines.AutotransformersAn autotransformer as shown in Figure 1.8 uses only one coil for the primary and secondary. It uses special connections on the coil called taps to produce the different ratios and voltages.Autotransformers are used frequently by power companies to provide voltage regulation to large power lines by providing a small increase or decrease to the line voltage as required.Figure 1.8: Single phase tapped Autotransformer TransformerThe Control Transformer A control transformer is a type of transformer that is often used to reduce voltage from the main power line in the plant to a lower voltage that operates a machine’s electrical control system. The most common type of control transformer has two primary coils and one secondary coil, as shown in Figure 1.9. Figure 1.9: A Typical Control Transformer ConfigurationIn most cases, the control transformer is used to reduce the main or line voltage of 240 VAC or 480 VAC to a control voltage of 120 VAC. Notice that the primary windings are crossed. This is done so that metal links can be used to connect the primaries for either 240 or 480 VAC operation, as shown in Figure 1.10.To get a control voltage of 120 VAC from a line voltage of 240 VAC, the primaries are connected differently than if the line voltage is 480 VAC. If the supplied line voltage is 240 VAC, the two primaries must be connected in parallel, as shown in Figure 1.10.Figure 1.10: Connecting a Control Transformer Primary for 240 VAC OperationFigure 1.11 is actually the same as Figure 1.10, except that it has been redrawn to allow you to more easily see that the primary coils are connected in parallel.Figure 1.11: Primaries Connected in ParallelIf the supplied line voltage is 480 VAC, the primaries are connected in series, as shown in Figure 1.12.Figure 1.12: Connecting a Control Transformer Primary for 480 VAC Operation.Figure 1.13 is actually the same as Figure 1.12, except that it has been redrawn to allow you to more easily see that the primary coils are connected in series.Figure 1.13: Primaries Connected in SeriesThe reason the primaries are connected in either parallel or series is to create a different turns ratio. To understand this, let’s assume that each primary has 200 turns and the secondary has 100 turns.If these coils are connected in parallel, as shown in Figures 1.10 and 1.11, the effective turns of the two primaries is still 200 turns, the same as if there was only one primary. If the secondary is 100 turns, the turns ratio is then 2:1. This means an input voltage of 240 VAC will create an output voltage of 120 VAC.In contrast, if the primary coils are connected in series, as shown in Figures 1.12 and 1.13, the effective turns of the two primary coils in series will be 400, making the turns ratio 4:1. This would cause an input voltage of 480 VAC to create an output voltage of 120 VAC.Conduct Lab Activity 4 on Page Number 26Lab Activity 1 Objective: To Connect and Operate a Transformer.Procedure: In this procedure, we will connect and operate a transformer. Then we will use a DMM to show that a voltage is induced in the secondary.Perform the following sub steps to connect and supply power to the transformer. Connect the transformer to the power supply as shown in Figures 1.14 and 1.15. This transformer has two primaries and two secondaries. In this application, we will connect the two primary windings in parallel.Place the AC/DC selector switch in the AC position.Turn on the T7017 power supply.Figure 1.14: Operating a TransformerFigure 1.15: Schematic of Transformer Connections2. Set the DMM to measure AC volts and measure the voltage across the primary, as shown in Figure 1.16.Figure 1.16: Measurement of Primary VoltagePrimary voltage = ___________________________ (VAC)Now measure the voltage across one of the secondaries, as shown in Figure 1.17.Secondary Voltage = _________________________ (VAC)Figure 1.17: Measurement of Secondary VoltageMeasure the voltage across the other secondary.Secondary voltage = _________________________ (VAC).It should be similar to the value in step 3.Turn off the power supply.Disconnect the circuit.Store all components.Summarise your findings…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………Lab Activity 2Objective: To Calculate the Secondary Coil Voltage of a Transformer.Procedure: In this procedure, we will determine the voltage output of each secondary coil given the number of turns of the primary, the number of turns of each secondary and the primary coil voltage.Perform the following sub steps to determine the secondary voltage of the transformer shown in Figure 1.18.First, calculate the turn’s ratio of the transformer.TR = ________________________________Figure 1.18: Calculating Secondary VoltageNext, calculate the secondary voltage of the transformer.The schematic shows that the input voltage is 120 VAC.VS ________________________________ (VAC)Calculate the turns ratio and the secondary voltage of the transformer shown in Figure 1.19.TR _______________________________VS ________________________________ (VAC)Figure 1.19: Transformer CircuitCalculate the turns ratio and the secondary voltage for the circuit shown in Figure 1.19, if the primary has 400 turns and the secondary has 100 turns.TR _______________________________ VS ________________________________ (VAC)Calculate the secondary voltage for the circuit shown in Figure 1.19 and described in step 3 if the input voltage is 480 VAC.VS ________________________________ (VAC)A transformer can also have more than one secondary coil, as shown in Figure 1.20. The amount of voltage induced in each secondary is calculated using the same formula used for a transformer with a single secondary.Figure 1.20: A Step-Down TransformerPerform the following sub steps to calculate the voltage output of each secondary in Figure 1.20.Calculate the turns ratio for each secondary.Calculate the voltages for each secondary.Perform the following sub steps to calculate the voltage output of each secondary in Figure 1.21.Calculate the turns ratio for each of the secondary in Figure 1.21.Calculate the voltage of each secondary. Figure 1.21: Voltage Output of Each Secondary CalculationSummarise your findings and differentiate between a step-down and a step-up transformer.…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………Lab Activity 3Objective: To Calculate the Transformer Power Loss.Procedure: In this procedure, you will load a transformer and take measurements which will allow you to calculate the power in and out of the transformer. You will then use these actual power values to calculate the efficiency of the transformer. This will show that the transformer does in fact lose power.Connect the circuit shown in Figure 1.22. Set the DMM to measure AC current (you will want to use the mA input jack).This circuit uses the resistor module which has two 220 ohm resistors. These resistors are connected in series to create a load of 440 ohms.Figure 1.22: Measuring the Current in the primary circuitPlace the AC/DC selector switch on the power supply in the AC position.Turn on the power supply and record the reading displayed by the DMM in the space provided. This is the transformer primary current.Transformer primary current is equal to = ______________(mA)Use the analog voltmeter on the T7017 to measure the voltage across the primary and record this value below.Transformer primary voltage = __________(VAC)Turn off the power supply.Now move the DMM to measure the current in the secondary, as shown in Figure 1.23.Figure 1.23: Measure the Secondary CurrentTurn on the power supply and record the reading displayed by the DMM in the space provided. This is the transformer secondary current.Transformer secondary current =________(mA)Use the analog voltmeter on the T7017 to measure the voltage across the secondary and record this value below.Transformer secondary voltage _________ (VAC)Turn off the power supply.Use the data you obtained in steps 3 and 4 to calculate the power being used by the primary.Primary Power ________________________(Watts)Use the data you obtained in steps 7 and 8 to calculate the power being used by the load on the secondary. Secondary Power__________ ______________(Watts) Use the actual power values that you calculated in steps 10 and 11 to calculate the efficiency of the transformer. Record your answer in the space provided. Transformer Efficiency =________________________ %Calculate the input-output voltage ratio:……………………………….Calculate the input-output current ratio:……………………………….Compare the two ratios obtained in steps 13 and 14. Explain the relationship between current, voltage and truns ratios?………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………..Lab Activity 4Objective: To design a control transformer circuit to provide a given output voltage.Procedure OverviewIn this procedure, you will determine how the primaries of a control transformer should be connected to produce a desired output given the line voltage and number of turns of each coil. You will then connect a transformer to deliver their desired output.Redraw the control transformer schematic shown in Figure 1.24 in the space provided below. Make the connections so that the output voltage is 120 VAC.Figure 1.24: Control Transformer Schematic2. Redraw the schematic shown in Figure 1.25 with the transformer connected to produce a turns ratio of 1:1. Use both primary and secondary coils.Figure 1.25: Transformer on the Transformer ModuleDraw your circuit below.Figure 1.25 above shows the transformer on the T7017 transformer module. Notice that the transformer has two primaries and two secondaries. Also, the primary coils are not crossed.3. Connect the circuit you drew in step 2 on the T7017. Use the 24 VAC jacks to supply power to the transformer.4. Turn on the power supply and use the DMM to measure the input and output voltages. Record these values below:Input Voltage ____________________________________ (VAC)Output Voltage ___________________________________ (VAC)You should find an input voltage of approximately 26.5 VAC and an output voltage of approximately 35 VAC. The output voltage is actually greater than the input voltage. This is because transformers of the size used on the T7017 are rated for the output voltage at a specific output current.In the next four steps, you will add a load to the secondary of the transformer. This load is sized to draw rated current from the transformer. You will then re-check the input and output voltages.5. Turn off the power supply.6. Add a 440 ohm load to the secondary of the transformer, as shown in Figure 1.26.Figure 1.26: Load for Transformer Secondary7. Turn the power supply back on.8. Use the DMM to measure the input and output voltages. Record these values below:Input Voltage ________________________________________ (VAC)Output Voltage _______________________________________ (VAC)Calculate the input-output voltage ratio:.........................................9. Turn off the power supply.10. Solve the following design problem.With the primary connected in parallel, determine how the transformersecondary should be connected to produce a turns ratio of 2:1.Draw the circuit below.11. Connect your circuit on the T7017.12. Add a 110 ohm load to the secondary of the transformer, as shown in Figure1.27. This will properly load the transformer for this ratio.Figure 1.27. Load for Transformer Secondary13.Turn the power supply back on.14. Use the DMM to measure the input and output voltages, record these values below:Input Voltage ________________________________________ (VAC)Output Voltage _______________________________________ (VAC)Calculate the input-output voltage ratio:.........................................15. Turn off the power supply.16. Disconnect and store all components.Review Exercises___________________is the ability of one coil to induce voltage in another coil.A(n) ___________________ is an electrical device that converts AC electricity from one voltage level to another.The _______________coil of the transformer creates a magnetic field that is concentrated by the core.The voltage induced in the _________________coil can be used to drive load.The _______________ of a transformer determines the relationship between the primary coil and the secondary coil.Many machines require a(n) _________________ to step down theline voltage to the machine’s operating voltage.Because of the construction of a transformer, there is usually asmall amount of ____________ between the primary power and the secondary power.If a transformer is operated at something other than rated load andvoltage, the _____________ will be lower.A(n) ______________ transformer is commonly used to reduce theline voltage from 480 VAC or 240 VAC to 120 VAC.Determine the voltage across the load resistor.Determine the current in the secondary.A 230 V to 12 V bell transformer is constructed with 800 turns on the primary winding. Calculate the number of secondary turns and the primary and secondary currents when the transformer supplies a 12 V 12 W alarm bell. ................
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