ac/dc electrical systems learning activity packetengineering.richmondcc.edu/courses/eus...
TRANSCRIPT
LEARNINGACTIVITYPACKET
AC/DCELECTRICALSYSTEMS
BB227-BC06XEN
TRANSFORMERS
BB227-BC06XEN TRANSFORMERSCopyright © 2011 Amatrol, Inc.
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LEARNING ACTIVITY PACKET 6
TRANSFORMERS
INTRODUCTIONA previous LAP discussed the principle of inductance and its effects in AC circuits.
This LAP will explore how inductance is used to change AC voltages from one level to another. The device that produces this change is called a transformer.
The transformer is one of the most commonly used inductive devices in electric power distribution. This LAP will explain how transformers work and how to select, connect, and troubleshoot them.
ITEMS NEEDED Amatrol Supplied 1 T7017 AC/DC Electrical Learning System
FIRST EDITION, LAP 6, REV. AAmatrol, AMNET, CIMSOFT, MCL, MINI-CIM, IST, ITC, VEST, and Technovate are trademarks or registered trademarks of Amatrol, Inc. All other brand and product names are trademarks or registered trademarks of their respective companies.Copyright © 2011 by AMATROL, INC.All rights Reserved. No part of this publication may be reproduced, translated, or transmitted in any form or by any means, electronic, optical, mechanical, or magnetic, including but not limited to photographing, photocopying, recording or any information storage and retrieval system, without written permission of the copyright owner.Amatrol,Inc., 2400 Centennial Blvd., Jeffersonville, IN 47130 USA, Ph 812-288-8285, FAX 812-283-1584 www.amatrol.com
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TABLE OF CONTENTS
SEGMENT 1 INTRODUCTION TO TRANSFORMERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4OBJECTIVE 1 Describe the function of a transformer and give an application OBJECTIVE 2 Describe the operation of a transformer and give its schematic symbol
SKILL 1 Connect and operate a transformer OBJECTIVE 3 Describe how to calculate the output voltage of a transformer
SKILL 2 Calculate the secondary coil voltage of a transformer OBJECTIVE 4 Describe how to troubleshoot a transformer
SKILL 3 Troubleshoot a transformer by measuring continuity
SEGMENT 2 SIZING A TRANSFORMER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22OBJECTIVE 5 Describe how to size a transformer
SKILL 4 Size a transformerOBJECTIVE 6 Describe a transformer’s input and output power relationship and explain its importance
Activity 1 Transformer power lossOBJECTIVE 7 Describe how to calculate the current load of a transformer
SKILL 5 Calculate the current load on a transformer
SEGMENT 3 TRANSFORMER TYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35OBJECTIVE 8 Describe the function of two basic categories of transformers OBJECTIVE 9 Describe the function of a control transformer
SKILL 6 Design a control transformer circuit to provide a given output voltage OBJECTIVE 10 Describe the function of a tap on the secondary of a transformer and give an application
Activity 2 The distribution transformer
BB227-BC06XEN TRANSFORMERSCopyright © 2011 Amatrol, Inc.
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SEGMENT 1INTRODUCTION TO TRANSFORMERS
OBJECTIVE 1 DESCRIBE THE FUNCTION OF A TRANSFORMER AND GIVE AN APPLICATION
A transformer is an electrical device that converts AC electricity from one voltage level to another. A typical small-size transformer is shown in fi gure 1.
Figure 1. A Typical Transformer
Transformers are often used to transform the high levels of voltage transmitted by the power company into lower level voltages that can be more easily and safely used by homes, businesses and manufacturing facilities. You will fi nd transformers on the electric lines leading to your house. Transformers are also used in the power supplies of many devices which operate on DC electricity but use an AC source. A computer is a common example.
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OBJECTIVE 2 DESCRIBE THE OPERATION OF A TRANSFORMER AND GIVE ITS SCHEMATIC SYMBOL
A transformer consists of three basic components:• Primary coil - An electrical wire wrapped around the core, as shown in fi gure 2.• Secondary coil - An electrical wire wrapped around the core, as shown in fi gure 2.• Core - A ferromagnetic material that is capable of conducting a magnetic fi eld. Iron is a common material.
Figure 2. Construction of a Typical Transformer
A transformer uses the principle of mutual inductance to create an AC elec-trical voltage in the secondary coil from alternating electrical current fl owing through the primary coil.
The principle of mutual inductance says that when two electrical coils are placed near each other, AC electrical current fl owing in one coil induces an AC voltage in the other coil. This occurs because the electrical current creates a magnetic fi eld around the fi rst coil which in turn induces a voltage in the second coil, as shown in fi gure 3.
Figure 3. Mutual Inductance Concept
IRON CORE
PRIMARYWINDING
SECONDARYWINDING
ACVOLTAGE
LOAD
INDUCEDMAGNETIC
FIELD
COIL2
COIL1APPLIED
AC CURRENT
INDUCEDAC VOLTAGE
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A transformer improves the effi ciency of the transfer of energy from one coil to another by using a core to concentrate the magnetic fi eld. As shown in fi gure 4, the primary coil creates a magnetic fi eld that is concentrated by the core and induces a voltage in the secondary coil. The voltage induced in the secondary coil can then be used to drive a load.
Figure 4. Operation of a Transformer
What 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 number of times each of the coils is wound around the core.
Figure 5 shows the schematic symbol for a transformer.
Figure 5. Schematic Symbol for a Transformer
MAGNETICFIELD
INDUCEDVOLTAGE
LOAD
APPLIEDAC CURRENT
ACSOURCE
SCHEMATIC SYMBOL
PRIMARY SECONDARY
IRON CORE
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SKILL 1 CONNECT AND OPERATE A TRANSFORMER
Procedure Overview
In this procedure, you will connect and operate a transformer. Then you will use a DMM to show that a voltage is induced in the secondary.
1. Perform the following substeps to connect and supply power to the transformer.
A. Connect the transformer to the power supply as shown in fi gures 6 and 7.
This transformer has two primaries and two secondaries. In this applica-tion, you will connect the two primary windings in parallel (the different connection possibilities will be covered later in this LAP).
B. Place the AC/DC selector switch in the AC position.
C. Turn on the T7017 power supply.
Figure 6. Operating a Transformer
SOURCE SELECT
AC DC
12V 12V
24V
TRANSFORMERMODULE
PRIMARY 1
PRIMARY 2
SECONDARY 1 SECONDARY 2
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Figure 7. Schematic of Transformer Connections
2. Set the DMM to measure AC volts and measure the voltage across the primary, as shown in fi gure 8.
Figure 8. Measurement of Primary Voltage
Primary voltage = ________________________________________ (VAC)
It should be approximately 26.5 VAC.
PRIMARY1
SECONDARY1
PRIMARY2
SECONDARY2
24 VAC
MAX600V600V
200mAMAX
FUSED
10A MAXFUSED
CAT 600VCAT 300V
HOLD
30XR
MIN MAXNON
CONTACTVOLTAGE
VBATT 9V
BATT 1.5V
COM
10A
mA
V 600600200200
200m200m
20M
2M
20k
200k
2k200 200m
20m
202022
V
A
10 A
2002m
BATT
OFF
2m
200m
200
20m
A
10 A
1.5V 9V
SOURCE SELECT
AC DC
12V 12V
24V
TRANSFORMERMODULE
PRIMARY
SECONDARY
ACVOLTS
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3. Now measure the voltage across one of the secondaries, as shown in fi gure 9.
Secondary voltage = ______________________________________ (VAC)
The secondary voltage should be approximately 17.5 VAC. This voltage is being induced in the secondary winding by the magnetic fi eld from the primary.
Figure 9. Measurement of Secondary Voltage
4. Measure the voltage across the other secondary.
Secondary voltage = ______________________________________ (VAC)
It should be similar to the value in step 3. You will learn later why this is so. 5. Turn off the power supply. 6. Disconnect the circuit. 7. Store all components.
MAX600V600V
200mAMAX
FUSED
10A MAXFUSED
CAT 600VCAT 300V
HOLD
30XR
MIN MAXNON
CONTACTVOLTAGE
VBATT 9V
BATT 1.5V
COM
10A
mA
V 600600200200
200m200m
20M
2M
20k
200k
2k200 200m
20m
202022
V
A
10 A
2002m
BATT
OFF
2m
200m
200
20m
A
10 A
1.5V 9V
SOURCE SELECT
AC DC
12V 12V
24V
TRANSFORMERMODULE
PRIMARY
SECONDARY
ACVOLTS
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OBJECTIVE 3 DESCRIBE HOW TO CALCULATE THE OUTPUT VOLTAGE OF A TRANSFORMER
As mentioned, the most common use of a transformer is to change the voltage output to a circuit. To determine the voltage output generated in the secondary coil of a transformer you only need to know two things: the input voltage and the turns ratio.
The turns 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 you to decide what trans-former to select for an application and troubleshoot one to decide if it is good.
TURNS RATIO FORMULA
Where NP = number of turns in the primary NS = number of turns in the secondary
P
S
NTR
N=
TRANSFORMER OUTPUT VOLTAGE FORMULA
Where VS = secondary voltage (Volts) VP = primary voltage (Volts) TR = turns ratio
PS
VV
TR=
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SKILL 2 CALCULATE THE SECONDARY COIL VOLTAGEOF A TRANSFORMER
Procedure Overview
In this procedure, you 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 substeps to determine the secondary voltage of the transformer shown in fi gure 10.
A. First, calculate the turns ratio of the transformer.
TR = ______________________________________________________
Figure 10. Calculating Secondary Voltage
The solution is as follows:
TR = NP / NS
TR = (100 / 50)
The turns ratio is 2/1.
This ratio is usually written as 2:1.
V120 VAC
PRIMARY SECONDARY
VS100
TURNS50
TURNS
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B. Next, calculate the secondary voltage of the transformer.
The schematic shows that the input voltage is 120 VAC.
VS _______________________________________________________________________________________
(VAC)
The solution is as follows:
The secondary voltage should be 60 VAC. Since the secondary voltage is less than the primary voltage, the transformer
is called a step-down transformer. 2. Calculate the turns ratio and the secondary voltage of the transformer shown
in fi gure 11.
TR = ________________________________________________________
VS = ___________________________________________________ (VAC)
The secondary voltage in this case is 360 VAC, which is 3 times higher than the primary voltage. When a transformer has a higher secondary voltage than the primary voltage, it is called a step-up transformer.
Figure 11. Transformer Circuit
3. Calculate the turns ratio and the secondary voltage for the circuit shown in fi gure 11 if the primary has 400 turns and the secondary has 100 turns.
TR = ________________________________________________________
VS = _________________________________________________________
The turns ration is 4:1 and the secondary voltage is 30 VAC.
P
S
S
VV
TR
V 120 / 2
=
=
V120 VAC
PRIMARY SECONDARY
VS60
TURNS180
TURNS
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4. Calculate the secondary voltage for the circuit shown in fi gure 11 and described in step 3 if the input voltage is 480 VAC.
VS ____________________________________________________________________________________________________
The secondary voltage is 120 VAC. A transformer can also have more than one secondary coil, as shown in fi gure
12. The amount of voltage induced in each secondary is calculated using the same formula used for a transformer with a single secondary.
Figure 12. A Step-Down Transformer
5. Perform the following substeps to calculate the voltage output of each secondary in fi gure 12.
A. Calculate the turns ratio for each secondary.
TR S1 = ___________________________________________________
TR S2 = ___________________________________________________
TR S3 = ___________________________________________________
The turns ratio for the secondary 1 is 2:1. The turns ratio for secondary 2 is 3:1. The turns ratio for secondary 3 is 10:1.
B. Calculate the voltages for each secondary.
VS1
= _______________________________________________ (VAC)
VS2
= _______________________________________________ (VAC)
VS3
= _______________________________________________ (VAC)
The voltage of secondary 1 should be 60 VAC. The voltage of secondary 2 should be 40 VAC. The voltage of secondary 3 should be 12 VAC.
V
120 VAC
PRIMARY SECONDARY
VS1
30TURNS
300TURNS
100TURNS
150TURNS
VVS2
VVS3
SECONDARY 1
SECONDARY 2
SECONDARY 3
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6. Perform the following substeps to calculate the voltage output of each secondary in fi gure 13.
A. Calculate the turns ratio for each secondary.
TR S1 = ___________________________________________________
TR S2 = ___________________________________________________
TR S3 = ___________________________________________________
TR S4 = ___________________________________________________
The turns ratio for secondary 1 is 10:1, secondary 2 is 5:1, secondary 3 is 2:1 and secondary 4 is 100:1.
B. Calculate the voltage of each secondary.
VS1
= ________________________________________________ (VAC)
VS2
= ________________________________________________ (VAC)
VS3
= ________________________________________________ (VAC)
VS4
= ________________________________________________ (VAC)
The voltage of secondary 1 is 24 VAC, secondary 2 is 48 VAC, secondary 3 is 120 VAC and secondary 4 is 2.4 VAC.
Figure 13. Voltage Output of Each Secondary Calculation
240 VAC
5TURNS
500TURNS
250TURNS
100TURNS SECONDARY 2
SECONDARY 3
SECONDARY 4
50TURNS SECONDARY 1
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OBJECTIVE 4 DESCRIBE HOW TO TROUBLESHOOT A TRANSFORMER
Once a transformer is installed in a circuit, it will probably operate without fail for a long time. One reason for this is there are no moving parts.
If the transformer does fail, it will appear as either a short or an open in one of the coils. The two ways you can determine if a transformer has failed are:
• Measure the input and output voltages • Check the transformer with an ohmmeter
Measure the Input and Output Voltages
If the transformer is already connected in a circuit, the transformer can be tested by measuring the input and output voltages. If they are reasonably close to the theoretical values, the transformer is good.
If the voltage does not stay constant, you may want to also test the current levels. Although the initial voltage may appear normal, it may not hold up when the transformer is fully loaded.
CAUTION
It is important to remember that a transformer usually handles high AC voltage. Therefore, it can be very dangerous. Use extreme caution when working around a transformer.
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Checking the Transformer with an Ohmmeter
An ohmmeter can be used to check for opens in coils, shorts between coils or coils shorted to the core without any power applied to the transformer. The tests for these three conditions can be done as follows:
1. Opens in Coils - The resistance of each coil should be checked, the actual DC resistance of the wire will be indicated by the ohmmeter reading. If any of the coils show infi nity, the winding is open and the transformer is bad.
NOTE
Very low resistance readings do not indicate a short. Remember, this is just the resistance of the wire. An ideal transformer would actually have zero resistance! Inductive reactance is what limits the current in an AC circuit, not resistance.
2. Shorts Between the Primary and Secondary Coils - A check for shorts should be made between the primary and secondary coils of the trans-former. You should measure infi nite resistance between the primary and secondary coils.
NOTE
An exception to this is the autotransformer which will be discussed later in this LAP.
3. Coils Shorted to the Core - A resistance check should be made from each transformer coil to the core of the transformer, all coils should show infi nite resistance. If a resistance is shown between any coil and the core, the transformer should not be used.
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SKILL 3 TROUBLESHOOT A TRANSFORMER BY MEASURING CONTINUITY
Procedure Overview
In this procedure, you will troubleshoot a transformer for opens and shorts on the primary and secondary to determine if it is good.
1. Place the transformer module from the T7017 on the work surface of the trainer.
2. Set the DMM to measure resistance. 3. Check the primary coils for an open by measuring the resistance across each
as shown in fi gure 14. If the ohmmeter reads infi nite resistance, the coil is open and the transformer is bad. Record the resistance of the primary coils below.
Primary 1 _______________________________________________ (ohms)
Primary 2 _______________________________________________ (ohms)
You should fi nd that both primaries have resistance. This is the DC resistance of the wire used for the primary coil. The transformer module used on the T7017 should display a resistance of approximately 80 to 110 ohms. Larger control transformers may only read a fraction of an ohm to a few ohms.
Figure 14. Measuring Resistance Across the Primary
TRANSFORMERMODULE
PRIMARYTERMINALS
SECONDARYTERMINALS
PRIMARY 1 PRIMARY 2
SECONDARY 1 SECONDARY 2
MAX600V600V
200mAMAX
FUSED
10A MAXFUSED
CAT 600VCAT 300V
HOLD
30XR
MIN MAXNON
CONTACTVOLTAGE
VBATT 9V
BATT 1.5V
COM
10A
mA
V 600600200200
200m200m
20M
2M
20k
200k
2k200 200m
20m
202022
V
A
10 A
2002m
BATT
OFF
2m
200m
200
20m
A
10 A
1.5V 9V
OHMS
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4. Check the secondary coils for an open by measuring the resistance across each. If the ohmmeter reads infi nite resistance, the coil is open and the transformer is bad. Record the resistance of the secondary coils below.
Secondary 1 _____________________________________________ (ohms)
Secondary 2 _____________________________________________ (ohms)
You should fi nd that both secondaries have resistance. This is the DC resistance of the wire used for the secondary coil. The transformer module used on the T7017 should display a resistance of approximately 30 to 50 ohms. Larger control transformers may only read a fraction of an ohm to a few ohms.
In the next 5 steps, you will be checking for shorts between the coils of the transformer. It does not matter from which side of the coils you take these measurements since anything other than infi nite resistance between the coils indicates a bad transformer.
5. Connect one lead from your ohmmeter to one terminal of the Primary 1 coil. 6. Touch the second lead of your ohmmeter to one terminal of the other primary
coil. Observe the reading on the ohmmeter.
Resistance ______________________________________________ (ohms)
If the reading on the ohmmeter is anything other than infi nite resistance, the transformer is bad and should not be used.
7. Move the second lead of the ohmmeter to one terminal of each of the secondary coils. Record the resistance for each measurement.
Resistance ______________________________________________ (ohms)
Resistance ______________________________________________ (ohms)
Both measurements should be infi nite if the transformer is good.
8. Connect one lead from your ohmmeter to the Primary 2 coil. 9. Repeat step 7 with the second lead of the ohmmeter connected to one terminal
of each of the secondary coils.
Resistance ______________________________________________ (ohms)
Resistance ______________________________________________ (ohms)
Again, the resistances should both be infi nite if the transformer is good. 10. Now place one lead of the ohmmeter on each secondary coil and record the
resistance below.
Resistance ______________________________________________ (ohms)
Again, the resistance should be infi nite if the transformer is good.
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You have now verifi ed that no shorts exist between any of the transformer coils.
In the next step, you will test for any shorts between the coils of the transformer and the core of the transformer. The core of the transformer is normally connected to earth ground. This is done to ensure that the core is never above ground potential and will not expose people to a shock hazard if they come in contact with the transformer core.
NOTE
The transformer used on the T7017 is a small circuit board mount unit which when used with the 24VAC on the trainer does not present a shock hazard. If this transformer were larger or being used with higher voltages, the core would need to be grounded.
NOTE
The core of the transformer on the T7017 has a coating of shellac to protect it from oxidizing. To make contact with the core itself you will need to place the leads of the ohmmeter inside the tapped holes of the transformer core.
11. Place the leads of the ohmmeter inside the two tapped holes of the transformer core as shown in fi gure 15. This allows you to make contact with the core itself.
Figure 15. Tapped Holes in Core for Ohmmeter Lead Placement
MAX600V600V
200mAMAX
FUSED
10A MAXFUSED
CAT 600VCAT 300V
HOLD
30XR
MIN MAXNON
CONTACTVOLTAGE
VBATT 9V
BATT 1.5V
COM
10A
mA
V 600600200200
200m200m
20M
2M
20k
200k
2k200 200m
20m
202022
V
A
10 A
2002m
BATT
OFF
2m
200m
200
20m
A
10 A
1.5V 9V
SECONDARY
PRIMARY
STANCOR
OHMS
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12. Press the leads of the ohmmeter against the sides of the tapped holes and observe the ohmmeter. You should be reading a very low resistance (less than 2 ohms). If the ohmmeter does not indicate a connection, use the leads of the ohmmeter to scrape any shellac off the inside of the tapped holes until the ohmmeter shows a connection.
Resistance ______________________________________________ (ohms)
13. Now, leave one lead in contact with the core and use the second lead to touch one terminal of each of the transformer coils. The ohmmeter should indicate infi nite resistance between the core and all coils. If it does not, the transformer is bad and should not be used.
Resistance ______________________________________________ (ohms)
You have now verifi ed that none of the transformer coils are shorted to the transformer core.
14. Store all components.
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SEGMENT 1 SELF REVIEW
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 fi eld that is concentrated by the core.
4. The voltage induced in the ____________ coil can be used to drive a load.
5. The ____________________________ of a transformer determines the relationship between the primary coil and the secondary coil.
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SEGMENT 2SIZING A TRANSFORMER
OBJECTIVE 5 DESCRIBE HOW TO SIZE A TRANSFORMER
You need the following information to size a transformer is:• Input voltage available• Output voltage desired• Output current required, both in-rush and steady state
With this information, you can use a catalog specifi cation sheet, like the one shown in fi gure 16, to select the proper transformer.
TRANSFORMER ELECTRICAL SPECIFICATIONS ANDORDERING DATA (SUPPLY VOLTAGE 220 VAC)
VAMax.
InrushVA†
Temp.Rise
DimensionsModel636-A B C
110-120 V Secondary Voltage Rating
50 75 100 150 250 5001000
180 218 273 660136019644014
55°C 55 55 55 55 115 115
3-5/16”3-9/16”3-3/44-5/1655-1/26-3/4
3”3-3/83-3/84-1/24-1/24-1/25-1/4
2-1/22-7/82-7/83-13/163-13/163-3/44-3/8
1111112111311141116111911211
22-24V Secondary Voltage Rating
50 100 150
180 273 660
55 55 55
3-5/163-3/44-5/16
33-3/84-1/2
2-1/22-7/83-13/16
1111-824
1131-824
1141-824
(*) Terminal Type (†) Capability VA. Refers to maximum inrush VA after calculations are made.
Figure 16. Transformer Electrical Specifi cations
The most important guideline to use when sizing a transformer is to select a transformer that safely and effi ciently provides the maximum current that can be drawn by a load. A common example of sizing a transformer occurs in selecting a transformer to operate a machine.
Many machines require a transformer to step down the line voltage (480 VAC or 240 VAC) to the operating voltage of 120 VAC. Machines with motors or other high inrush devices draw their maximum current when they are fi rst started. For these machines, this inrush current is the critical value that must be considered when selecting the transformer.
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If the machine does not have devices like motors with high in-rush character-istics, in-rush current is not a consideration. In this case, the steady-state current is more important. Most machines list both their maximum in-rush and steady-state current requirements.
Transformer specifi cation sheets, like the one shown in fi gure 16, usually list a rating value known as steady-state Volt Amperes (VA). This is simply the secondary voltage multiplied by the secondary current (V
S × I
S) during steady-state current.
Another listed value is the maximum in-rush VA. This is the secondary voltage multiplied by the secondary current during in-rush.
To select a transformer size, pick a transformer model that has a higher VA rating than is needed for your application. However, you do not want to pick a transformer that has a VA rating that is much higher than the rating you need. The higher you go above the needed rating, the less effi cient the transformer will be. You should pick the one that is above but closest to the required value.
CAUTION
NEVER UNDERSIZE A TRANSFORMER! ALWAYS SELECT A TRANSFORMER THAT IS RATED ABOVE THE VALUE REQUIRED FOR AN APPLICATION. UNDERSIZING A TRANSFORMER CAN RESULT IN SERIOUS DAMAGE TO MACHINERY OR EVEN FIRE!
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SKILL 4 SIZE A TRANSFORMER
Procedure Overview
In this procedure, you will use the table in fi gure 16 to size a transformer for several applications. This is a typical catalog specifi cation.
1. Perform the following substeps to size a transformer for the following application:
A machine needs a transformer to step down the line voltage from 240 VAC to 120 VAC. The maximum in-rush current the machine’s motor draws is 4 amps.
A. Calculate the maximum in-rush VA that will be required.
Maximum in-rush VA = ___________________________________(VA)
This can be found by multiplying the maximum in-rush current by the operating voltage of the machine.
Maximum in-rush VA should be 480 VA (120 V × 4A).
B. Choose a transformer from the table in fi gure 16 that has a greater maximum in-rush VA rating than the machine requires.
Maximum In-rush VA Rating = _____________________________(VA)
In this case, the fourth transformer from the top with a maximum in-rush VA of 660 VA would be the correct choice.
2. Select a transformer from the table in fi gure 16 given the following information:
Maximum in-rush current drawn by the machine’s motor is 10A. The line voltage is 240 VAC from a 25A circuit breaker. The operating voltage of the machine is 120 VAC.
Maximum in-rush VA = _____________________________________(VA)
Maximum in-rush VA of selected transformer = __________________(VA)
The selection should be the transformer with a rating of 1360. 3. Select a transformer from the table in fi gure 16 given the following
information: Maximum in-rush for the machine is 15A. Operating voltage = 120 VAC.
Maximum in-rush VA = _____________________________________(VA)
Maximum in-rush VA of selected transformer = __________________(VA)
The transformer with a VA rating of 1964 should be selected.
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This skill has taught how to select a transformer to operate at rated load and voltage. A transformer can be operated at something other than rated load and voltage. However, the effi ciency will be lower. In the next objective, you will learn more about this.
OBJECTIVE 6 DESCRIBE A TRANSFORMER’S INPUT AND OUTPUT POWER RELATIONSHIP AND EXPLAIN ITS IMPORTANCE
Ideally, 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 fi gure 17.
This power lost by a transformer is actually converted into heat, which must be dissipated by the transformer. Large transformers used by utility company substa-tions actually have cooling systems that use pumps to circulate oil around the transformer core to help dissipate this heat.
Figure 17. Power Through a Transformer
It is important to know how much heat is generated when designing a circuit that uses a transformer. Heat buildup reduces the life and eventually destroys the transformer. You should always follow the manufacturer’s recommendations for proper ventilation. Proper ventilation ensures maximum service life and operating effi ciency from the transformers.
TRANSFORMER
POWERIN
POWEROUT
POWERLOSS
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The heat generated by a transformer can be found by calculating the effi ciency of the transformer. This can be calculated with the following formula.
A transformer operates at its highest effi ciency when you use a load that is equal to the rated load shown in the specifi cations. If you use a load that is either larger or smaller than the rated load, the effi ciency decreases. The difference is in the amount of heat generated. A larger-than-rated load creates more heat and could cause the transformer to burn out.
NOTE
The rule for applying transformers is to operate at rated voltage and rated load and follow the manufacturer’s recommendations for ventilation.
Also, if you apply a primary voltage that is different than its rating, the trans-former’s effi ciency will be lower.
TRANSFORMER EFFICIENCY FORMULA
Where Power Out= output power (Watts or VA) Power In = input power (Watts or VA)
Power OutTransformer Efficiency % 100
Power In= ×
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Activity 1. Transformer Power Loss
Procedure Overview
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 effi ciency of the transformer. This will show that the transformer does in fact lose power.
1. Connect the circuit shown in fi gure 18. 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 18. Test Circuit for Activity 1
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 = _______________________________ (mA)
Primary current should be approximately 78mA.
MAX600V600V
200mAMAX
FUSED
10A MAXFUSED
CAT 600VCAT 300V
HOLD
30XR
MIN MAXNON
CONTACTVOLTAGE
VBATT 9V
BATT 1.5V
COM
10A
mA
V 600600200200
200m200m
20M
2M
20k
200k
2k200 200m
20m
202022
V
A
10 A
2002m
BATT
OFF
2m
200m
200
20m
A
10 A
1.5V 9V
220
220
SOURCE SELECT
AC DC
12V 12V
24V
TRANSFORMERMODULE
PRIMARY
SECONDARY
2
3
5
6
4
1
7
8
24VAC
A
R220
1
R220
2
1234
8 7 6 5
R1
R2TRANSFORMER LOAD
MODULE
ACCURRENT
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4. Use the analog voltmeter on the T7017 to measure the voltage across the primary and record this value below.
Transformer primary voltage = ______________________________ (VAC)
Primary voltage should be approximately 26.5 VAC. 5. Turn off the power supply. 6. Now move the DMM to measure the current in the secondary, as shown in
fi gure 19.
Figure 19. Move the AC Ammeter to 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)
Secondary current should be approximately 54mA. 8. Use the analog voltmeter on the T7017 to measure the voltage across the
secondary and record this value below.
Transformer secondary voltage = ____________________________ (VAC)
Since the transformer is connected in a 1:1 confi guration this value should be close to the value you recorded in step 4 (approximately 26.5VAC).
9. Turn off the power supply.
MAX600V600V
200mAMAX
FUSED
10A MAXFUSED
CAT 600VCAT 300V
HOLD
30XR
MIN MAXNON
CONTACTVOLTAGE
VBATT 9V
BATT 1.5V
COM
10A
mA
V 600600200200
200m200m
20M
2M
20k
200k
2k200 200m
20m
202022
V
A
10 A
2002m
BATT
OFF
2m
200m
200
20m
A
10 A
1.5V 9V
ACCURRENT
SOURCE SELECT
AC DC
12V 12V
24V
TRANSFORMERMODULE
PRIMARY
SECONDARY
2
3
5
6
4
1
7
8
24VAC
R220
1
R220
2
A
1234
567
220
220R1
R2
TRANSFORMER LOADMODULE
8
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10. Use the data you obtained in steps 3 and 4 to calculate the power being used by the primary.
Primary Power = _________________________________________ (Watts)
The solution is as follows:
P = I × E
P = 78mA × 26.5V
P = 2.06 Watts
The power used by the primary is approximately 2 Watts. Your computed value may vary slightly depending on actual values obtained in steps 3 and 4.
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)
The solution is as follows:
P = I × E
P = 54mA × 26.5V
P = 1.43 Watts
Power used by the secondary is approximately 1.43 Watts, your computed value may vary slightly depending on actual values obtained in steps 7 and 8.
12. Use the actual power values that you calculated in steps 10 and 11 to calculate the effi ciency of the transformer. Record your answer in the space provided.
Transformer Effi ciency = ______________________________________ %
The solution is as follows:
The actual transformer effi ciency is approximately 71%. Your computed value may vary slightly depending on actual values obtained in steps 10 and 11.
NOTE
This effi ciency is fairly low because of this transformer’s small size. Also we are using it at a voltage other than that for which it was designed (24 volts instead of 240 volts). A normally-loaded transformer usually has an effi ciency in the 90% range.
Power OutTransformer Efficiency % 100
Power In
1.43Transformer Efficiency % 100
2.0
Transformer Efficiency % 0.71 100
Transformer Efficiency % 71%
= ×
= ×
= ×
=
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OBJECTIVE 7 DESCRIBE HOW TO CALCULATE THE CURRENT LOADOF A TRANSFORMER
The current load of a transformer is the current drawn by the primary winding, I
P, as shown in fi gure 20. This is the amount of current that must be supplied to the
primary coil in order to power the circuit that is connected to the secondary coil.
It is very important to know the current load of a transformer so that you can size the circuit breaker or make sure the existing circuit breaker is large enough to handle the added load.
Figure 20. Current Load of a Transformer
The steps used to calculate the actual current load are as follows:
• Calculate the VA drawn by the secondary winding (IS × V
S = VA
S).
• Because the power in approximately equals the power out, the primary VA is the same as the secondary VA. This assumes that power losses are small enough to be ignored.• Calculate the actual current load of the primary winding, I
P, by dividing the
VA by the primary voltage (VAP ÷ V
P = I
P).
While the above steps give you the actual current load, the normal procedure for sizing a circuit breaker is to use the maximum VA of the transformer you selected. The maximum VA is then divided by the primary voltage to get the current load. This method ensures that you will not overload circuit if another device is added to the transformer circuit later.
CURRENTLOAD
DEVICE
IP IS
TRANSFORMER
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SKILL 5 CALCULATE THE CURRENT LOAD ON A TRANSFORMER
Procedure Overview
In this procedure, you will use the information given to size a transformer for the application. You will also determine if the machine will cause an overload on the feed circuit.
1. Perform the following substeps to calculate the current load on a transformer. The secondary voltage (V
S) is 120 volts and secondary current (IS) is 3 amps.
The primary voltage (VP) is 240 volts.
A. Calculate the VA.
VA = _________________________________________________ (Amps)
The solution is as follows:
VA = VS × IS
VA = 120 × 3
VA = 360
B. Select a transformer from the chart in fi gure 16.
Model No. _________________________________________________
You should pick one which has a VA larger than 360. This is model 636-1191.
C. Calculate the maximum current of the transformer.
Maximum Current ____________________________________ (Amps)
The solution is found by dividing the maximum VA of the transformer by the primary voltage. This is:
IP = 500 ÷ 240
IP = 2.08 Amps
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2. Calculate the maximum current load of a transformer given the circuit in fi gure 21. Then, select a transformer from the chart in fi gure 16.
Transformer Model _____________________________________________
Current load, IP = ________________________________________ (Amps)
The answer is transformer 636-1161 and 1.0 amp.
Figure 21. Minimum Current Load Calculation
3. Calculate the maximum current load for the transformer in fi gure 22 given the following information. Then, select a transformer from the chart in fi gure 16.
HINT
The wattage rating of the light bulb determines the VA of the secondary.
Transformer Model _____________________________________________
Current load, IP = _______________________________________________
The answer is transformer 636-1131 and .42 amps.
Figure 22. Maximum Current Load Calculation
DEVICE
IP 2 AMPS
240VOLTS 120
VOLTS
IP
240VOLTS
120 VOLTS100 WATT
LIGHT BULB
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4. Determine if a new machine can be added to a circuit given the following information:
Line voltage of 240 VAC is fed from a 30 A circuit breaker to three other machines which are drawing a total of 25 A. You need to know whether or not adding a fourth machine will overload the circuit.
The current required for the fourth machine is 4 A and its operating voltage is 120 VAC. Select the correct transformer from fi gure 16 and use its maximum VA to calculate the current load.
Machine #4 Current Load, IP = _____________________________ (Amps)
Machine can be added ___________________________________ (Yes/No)
The current load is 2.08 Amps. The machine can be added to the existing circuit.
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SEGMENT 2 SELF REVIEW
1. Many machines require a(n) _________________ to step down the line voltage to the machine’s operating voltage.
2. Transformers are usually rated in _______________.
3. Because of the construction of a transformer, there is usually a small amount of ____________ between the primary power and the secondary power.
4. If a transformer is operated at something other than rated load and voltage, the _____________ will be lower.
5. The current load of a transformer is the current drawn by the ________________ winding, I
P.
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SEGMENT 3TRANSFORMER TYPES
OBJECTIVE 8 DESCRIBE THE FUNCTION OF TWO BASIC CATEGORIESOF TRANSFORMERS
Most transformers fall into one of two categories:• Isolation transformers• Autotransformers
Isolation Transformer
An isolation transformer, such as the one shown in fi gure 23, has primary and secondary windings that are physically and electrically isolated. The isolation transformer is magnetically coupled, not electrically coupled.
Figure 23. 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 fi lter 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.
120VAC LOADSECONDARYPRIMARY
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Some isolation transformers produce an output voltage that is equal to the primary voltage. 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 uses only one coil for the primary and secondary. It uses special connections on the coil called taps to produce the different ratios and volt-ages. You will learn more about taps later in this LAP.
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.
OBJECTIVE 9 DESCRIBE THE FUNCTION OF A 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 fi gure 24.
Figure 24. A Typical Control Transformer Confi guration
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 fi gure 25 shows.
H1
H2
H3
H4
X1
X2
PRIMARYCOIL
1
PRIMARYCOIL
2
SECONDARYCOIL
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To get a control voltage of 120 VAC from a line voltage of 240 VAC, the prima-ries 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 fi gure 25.
Figure 25. Connecting a Control Transformer Primary for 240 VAC Operation
Figure 26 is actually the same as fi gure 25, except that it has been redrawn to allow you to more easily see that the primary coils are connected in parallel.
Figure 26. Primaries Connected in Parallel
H3
H2
X1
X2
CONNECTHERE
240 VAC 120 VAC
CONNECTHERE
200TURNS
200TURNS
100TURNS
H1
H4
METALLINK
METALLINK
V
240 VAC 120 VAC200TURNS
200TURNS
100TURNS V
H1
H3
H2
X1
X2
H4
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If the supplied line voltage is 480 VAC, the primaries are connected in series, as shown in fi gure 27.
Figure 27. Connecting a Control Transformer Primary for 240 VAC Operation
Figure 28 is actually the same as fi gure 27, except that it has been redrawn to allow you to more easily see that the primary coils are connected in series.
Figure 28. 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.
H3
H2
X1
X2
CONNECTHERE
480 VAC
200 TURNS
200 TURNS
100TURNS
H1
H4
METALLINK
V 120 VAC
H3
H2480 VAC
H1
H4
120 VAC100TURNS
200TURNS
200TURNS
V
X1
X2
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If these coils are connected in parallel, as shown in fi gures 25 and 26, the effec-tive turns of the two primaries is still 200 turns, the same as if there were 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 fi gure 27 and 28, 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.
NOTE
In either case, the output current will be higher than the input current because the power in equals the power out and the output voltage is lower.
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SKILL 6 DESIGN A CONTROL TRANSFORMER CIRCUIT TO PROVIDEA 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 fi gure 29 on a separate piece of paper. Make the connections so that the output voltage is 120 VAC.
Figure 29. Control Transformer Schematic
H3
H2
X1
X2
240 VAC
100TURNS
100TURNS
100TURNS
H1
H4
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2. Redraw the schematic shown in fi gure 30 with the transformer connected to produce a turns ratio of 1:1. Use both primary and secondary coils.
Draw your circuit on a separate piece of paper. This is the transformer on the T7017 transformer module. Notice that the
transformer has two primaries and two secondaries. Also, the primary coils are not crossed.
Figure 30. Transformer on the Transformer Module
NOTE
The number of turns listed for the transformer coils in fi gure 30 is used simply to show the turns ratio concept. The actual number of turns in the coils is proprietary information and not available from the manufacturer.
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 fi nd 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 specifi c output current.
2
3
5
6
4
1
7
8
100TURNS
50TURNS
100TURNS
50TURNS
PRIMARY SECONDARY
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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.
NOTE
This is the same reason you actually read 26.5 VAC at the T7017 power supply jacks. It is rated for 24 VAC at a specifi c load current.
5. Turn off the power supply. 6. Add a 440 ohm load to the secondary of the transformer, as shown in fi gure
31.
NOTE
The 440 ohm load is created by connecting the two 220 ohm resistors in series.
Figure 31. 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)
You should fi nd that the input voltage is still approximately 26.5 VAC but the output voltage is now also approximately 26.5. This is the 1:1 ratio when the transformer is loaded.
220
220
TRANSFORMERMODULE
SECONDARY
8
7
6
5
R220
2
R220
1
58
R1
R2
TRANSFORMER LOADMODULE
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9. Turn off the power supply. 10. Solve the following design problem. With the primary connected in parallel, determine how the transformer
secondary should be connected to produce a turns ratio of 2:1. Draw your circuit on a separate piece of paper. 11. Connect your circuit on the T7017. 12. Add a 110 ohm load to the secondary of the transformer, as shown in fi gure
32. This will properly load the transformer for this ratio.
NOTE
The 110 ohm load is attained by connecting the two 220 ohm resistors in parallel.
Figure 32. 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)
You should fi nd an input voltage is still approximately 26.5 VAC but the output voltage is approximately 13.2. This is the 2:1 step down ratio.
15. Turn off the power supply. 16. Disconnect and store all components.
220
220
TRANSFORMERMODULE
TRANSFORMER LOADMODULE
SECONDARY
8
7
6
5
R220
2R220
1
8 5
R1
R2
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OBJECTIVE 10 DESCRIBE THE FUNCTION OF A TAP ON THE SECONDARYOF A TRANSFORMER AND GIVE AN APPLICATION
Many transformers have a secondary coil that has an extra lead attached to it, such as the one shown in fi gure 33. This extra lead is known as a tap.
Figure 33. A Secondary with a Tap
The tap is connected at a point between the ends of the coil so that you can get a different output voltage from the transformer. The tap allows a different voltage to be produced because it uses a reduced number of turns in the secondary coil.
For example, in fi gure 33, the output voltage between leads 1 and 2 is 120 VAC because the turns ratio is 1:1 (100 to 100). If you measured between the tap and lead 1, the output voltage would be 24 VAC. In this case, the turns ratio is 5:1 (100 to 20).
A tap that splits a secondary in half is called a center tap. A common applica-tion of a transformer with a center tap is a distribution transformer. This type of transformer is used for homes and businesses to change the high voltage of power company distribution lines to the common 240/120 VAC supply.
120VAC
100TURNS
80TURNS
20TURNS
LEAD 1
24VAC
120VAC
LEAD 2
TAP
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In this case, the center tap is connected to earth ground and becomes a common conductor, as shown in fi gure 34. The voltage across the output lines will be 240 VAC. However, if you measure between either output line and the center tap, the voltage will be 120 VAC.
Figure 34. A Distribution Transformer
The circuit shown in fi gure 34 is a typical circuit used by the power company to deliver power to a house. The 240 VAC power is used to supply devices in the home that require a large amount of operating power. Some examples are the central air conditioner, water heater, clothes dryer and the cooking range. These high power devices all run on 240 VAC to allow smaller conductor wires to deliver power to them (current is the limiting factor when sizing wire).
The 120 VAC power is wired to the electrical outlets and lighting system. This provides a much safer level of voltage which can be used by people in the house on smaller electrical devices.
120 VAC loads should always be balanced as closely as possible between the two legs of the transformer. This prevents overloading one side of the transformer and allows the transformer to operate at its maximum effi ciency. In fact, with a perfectly balanced load, the current in the grounded conductor (the tap) is zero because the common currents returning from each side of the transformer actually cancel each other.
480VAC
120 VAC
240VAC
120 VAC
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Activity 2. The Distribution Transformer
Procedure Overview
In this procedure, you will connect the transformer module on the T7017 in a confi guration which will simulate a distribution transformer. You will then take measurements to verify the available voltages and currents present in the transformer. This will show you how important it is to have balanced loads on a distribution transformer.
1. Connect the circuit shown in fi gure 35. This circuit simulates a distribution transformer. Since each secondary has an
equal number of turns, the connection of the two secondaries is equivalent to a center tap.
NOTE
The transformer is actually set up for a 1:1 ratio. In an actual distribution transformer, this would be a much greater ratio. The secondary confi guration is what we will be testing in this procedure.
Figure 35. Distribution Transformer Confi guration
220
22024VAC
PRIMARY SECONDARY
CENTERTAP
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2. Turn on the power supply. 3. Use the DMM to measure the voltage across the secondary as shown in fi gure
36. Record this reading below.
Voltage across secondary __________________________________ (VAC)
You should measure approximately 25 to 26 VAC. This is where the high power requirement loads would be attached to the distribution transformer.
Figure 36. Measuring the Secondary Voltage
220
22024VAC V
PRIMARY SECONDARY
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4. Use the DMM to measure the voltage across each of the two loads. Record below the voltage across load #1 and load #2 in the spaces provided.
Figure 37 shows the connection for measuring the voltage across load 1. The two 220 ohm resistors represent the 120 VAC loads that would be
balanced across the secondary of the transformer.
Voltage across load #1 = _____________________________________VAC
Voltage across load #2 = _____________________________________VAC
You should measure approximately 12.5 to 13 VAC across the loads. This is how the user accessible outlets and lighting circuits would be attached to the distribution transformer.
Figure 37. Measuring the Voltage Across the Loads
220
220
24VAC
V
PRIMARY SECONDARY
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5. Perform the following substeps to measure the current fl owing through load #1.
A. Turn off the power supply.
B. Confi gure the DMM to measure AC current, use the mA input jack.
C. Place the meter in series with load #1 as shown in fi gure 38.
Figure 38. Measuring Load #1 Current
D. Turn on the power supply. 6. Record below the current fl owing through load #1.
Load #1 current = _________________________________________ (mA)
You should measure approximately 55 to 65 mA of current through load #1. 7. Repeat step 5 to obtain the current through load #2. Record this value below.
Load #2 current = _________________________________________ (mA)
You should measure approximately 55 to 65 mA of current through load #2. In the next steps, you will calculate the unbalanced current that should be
fl owing in the common conductor. You will then measure the common current to verify that only the unbalanced current is fl owing in the common.
MAX600V600V
200mAMAX
FUSED
10A MAXFUSED
CAT 600VCAT 300V
HOLD
30XR
MIN MAXNON
CONTACTVOLTAGE
VBATT 9V
BATT 1.5V
COM
10A
mA
V 600600200200
200m200m
20M
2M
20k
200k
2k200 200m
20m
202022
V
A
10 A
2002m
BATT
OFF
2m
200m
200
20m
A
10 A
1.5V 9V
220
220
TRANSFORMERMODULE
LOAD #1
LOAD #2
5
6
7
8
R220
1
R220
2
A
SECONDARY
8 7 6 5
R1
R2
TRANSFORMER LOADMODULE
ACCURRENT
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8. Subtract the lower of the two values you recorded in steps 6 and 7 from the higher of the two values. This gives you the approximate current that is fl owing in the common conductor. Record this value below.
Unbalanced current = ______________________________________ (mA)
Typical values will range from 2 to 7 mA.
NOTE
It is possible to have these two exactly equal each other. If they match exactly, you should read zero current in the common line. However, actual values will usually not match exactly and will leave a small amount of unbalanced current.
9. Move the DMM to now measure current in the common as shown in fi gure 39.
Figure 39. Measuring Current in the Common
10. Turn on the power supply and record below the current fl owing in the common.
Current in the common = ___________________________________ (mA)
Typical values will range from 2 to 7 mA depending on actual resistance and voltage values.
MAX600V600V
200mAMAX
FUSED
10A MAXFUSED
CAT 600VCAT 300V
HOLD
30XR
MIN MAXNON
CONTACTVOLTAGE
VBATT 9V
BATT 1.5V
COM
10A
mA
V 600600200200
200m200m
20M
2M
20k
200k
2k200 200m
20m
202022
V
A
10 A
2002m
BATT
OFF
2m
200m
200
20m
A
10 A
1.5V 9V 220
220
TRANSFORMERMODULE
LOAD #1
LOAD #2
5
6
7
8
R220
1
R220
2
SECONDARY
A
8 7 56
R1
R2
TRANSFORMER LOADMODULE
ACCURRENT
BB227-BC06XEN TRANSFORMERSCopyright © 2011 Amatrol, Inc.
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11. Compare the value you measured in step 10 with the value you calculated in step 8.
Are they close? ________________________________________ (Yes/No)
You should fi nd that these values match exactly, any differences will be in the meter or reading accuracy.
As you can see, if the loads on a distribution transformer are balanced, the common current is almost zero.
12. Turn off the power supply. 13. Disconnect and store all components.
BB227-BC06XEN TRANSFORMERSCopyright © 2011 Amatrol, Inc.
52
SEGMENT 3 SELF REVIEW
1. In a(n) ______________ transformer, the primary and secondary windings are physically and electrically isolated.
2. A(n) _______________ splits the secondary voltage in half.
3. A(n) ______________ transformer is commonly used to reduce the line voltage from 480 VAC or 240 VAC to 120 VAC.
4. The ______________ transformer is used for homes and businesses to change the high voltage of power company distribution lines to common 240/120 VAC supply.
5. A(n) ____________ transformer uses the same coil for the primary and secondary.