ATE1120: Electrical Fundamental-II · Web viewA transformer uses the principle of mutual inductance to create an AC electrical voltage in the secondary coil from the alternating electrical

Electrical MachinesModule 1: Transformer Types and

Applications

PREPARED BY

IAT Curriculum UnitApril 2012

© Institute of Applied Technology, 2012

ATE1230: Electrical Machines

Module 1: Transformer Types and Applications

Module ObjectivesUpon successful completion of this module, students should be able to:

1. Describe the function of the transformer and give its application.2. Describe the operation of transformer and give its schematic

symbol.3. Describe how to calculate the output voltage of a transformer.4. Describe how to calculate the efficiency using input and output

powers.5. Discuss the basic categories of the transformers.6. Discuss the function of the control transformer. 7. Design a control transformer circuit to provide a given output

voltage.

Module Contents:

Module 1: Transformer Types and ApplicationsPage 2


1.2 Transformer Types

Most transformers fall into one of two categories: Isolation transformers Autotransformers

Isolation Transformers

An 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 Schematic

This 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.

Autotransformers

An 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 Transformer

1.3 The 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 Configuration

In 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 Operation

Figure 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 Parallel

If 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 Series

The 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 26

1.4 Lab 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.1. Perform the following sub steps to connect and supply power to the

transformer.

A. 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.B. Place the AC/DC selector switch in the AC position.C. Turn on the T7017 power supply.



Figure 1.14: Operating a Transformer

Figure 1.15: Schematic of Transformer Connections

2. 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 Voltage

Primary voltage = ___________________________ (VAC)3. Now measure the voltage across one of the secondaries, as shown in Figure 1.17.

Secondary Voltage = _________________________ (VAC)



Figure 1.17: Measurement of Secondary Voltage4. Measure the voltage across the other secondary.

Secondary voltage = _________________________ (VAC).It should be similar to the value in step 3.

5. Turn off the power supply.6. Disconnect the circuit.7. Store all components.8. Summarise your findings

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1.5 Lab Activity 2



Objective: 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.1. Perform the following sub steps to determine the secondary voltage of the

transformer shown in Figure 1.18.

A. First, calculate the turn’s ratio of the transformer.

TR = ________________________________

Figure 1.18: Calculating Secondary Voltage

B. Next, calculate the secondary voltage of the transformer.The schematic shows that the input voltage is 120 VAC.VS ________________________________ (VAC)

2. Calculate the turns ratio and the secondary voltage of the transformer shown in Figure 1.19.

TR _______________________________VS ________________________________ (VAC)



Figure 1.19: Transformer Circuit3. Calculate 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)

4. 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 Transformer5. Perform the following sub steps to calculate the voltage output of each secondary

in Figure 1.20.



A. Calculate the turns ratio for each secondary.

B. Calculate the voltages for each secondary.

6. Perform the following sub steps to calculate the voltage output of each secondary in Figure 1.21.A. Calculate the turns ratio for each of the secondary in Figure 1.21.

B. Calculate the voltage of each secondary.



Figure 1.21: Voltage Output of Each Secondary Calculation

7. Summarise your findings and differentiate between a step-down and a step-up transformer.

…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

1.6 Lab Activity 3

Objective: 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.

1. 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 circuit

2. Place the AC/DC selector switch on the power supply in the AC position.

3. 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)4. Use the analog voltmeter on the T7017 to measure the voltage across the

primary and record this value below.Transformer primary voltage = __________(VAC)

5. Turn off the power supply.6. Now move the DMM to measure the current in the secondary, as shown in

Figure 1.23.



Figure 1.23: Measure the Secondary Current

7. Turn 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)8. Use the analog voltmeter on the T7017 to measure the voltage across the

secondary and record this value below.Transformer secondary voltage _________ (VAC)

9. Turn off the power supply.10.Use the data you obtained in steps 3 and 4 to calculate the power being used by

the primary.Primary Power ________________________(Watts)

11.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)

12. 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 =________________________ %13. Calculate the input-output voltage ratio:……………………………….14. Calculate the input-output current ratio:……………………………….15. Compare the two ratios obtained in steps 13 and 14. Explain the relationship

between current, voltage and truns ratios?

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………………………………………………………………………………………………………………………………………………………………………………………………………………………..

1.7 Lab Activity 4

Objective: To design a control transformer circuit to provide a given output voltage.

Procedure Overview

In 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.

1. 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 Schematic



2. 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 Module

Draw 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 Secondary

7. 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 Secondary

13.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.

1.8 Review Exercises

1. ___________________is the ability of one coil to induce voltage in another coil.

2. A(n) ___________________ is an electrical device that converts AC electricity from one voltage level to another.

3. The _______________coil of the transformer creates a magnetic field that is concentrated by the core.

4. The voltage induced in the _________________coil can be used to drive load.

5. The _______________ of a transformer determines the relationship between the primary coil and the secondary coil.

6. Many machines require a(n) _________________ to step down theline voltage to the machine’s operating voltage.

7. Because of the construction of a transformer, there is usually asmall amount of ____________ between the primary power and the secondary power.

8. If a transformer is operated at something other than rated load andvoltage, the _____________ will be lower.



9. A(n) ______________ transformer is commonly used to reduce theline voltage from 480 VAC or 240 VAC to 120 VAC.



10. Determine the voltage across the load resistor.

11. Determine the current in the secondary.

12. 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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ATE1120: Electrical Fundamental-II · Web viewA transformer uses the principle of mutual inductance to create an AC electrical voltage in the secondary coil from the alternating electrical