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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


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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 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


26

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= ×


27

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


28

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


29

10. Use the data you obtained in steps 3 and 4 to calculate the power being used by the primary.

Primary Power = _________________________________________ (Watts)


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)


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 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%

= ×

= ×

= ×

=


30

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


31

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)


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


32

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


33

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.


34


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.


35

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


36

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


37

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


38

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


39

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.


40

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


41

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


42

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:



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



43

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:



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


SECONDARY

8

7

6

5

R220

2R220

1

8 5

R1

R2


44

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


45

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


46

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


47

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


48

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


49

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


ACCURRENT


50

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


ACCURRENT


51

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.


52


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.

ac/dc electrical systems learning activity packetengineering.richmondcc.edu/courses/eus...

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