exercise 10 transformers - lab-volt · exercise 10 – transformers discussion © festo didactic...

© Festo Didactic 89688-00 205

When you have completed this exercise, you will be familiar with the basic operating principles of transformers, as well as with the different ratios of transformers: turns ratio, voltage ratio, and current ratio. You will know the difference between step-up and step-down transformers. You will also be introduced to the voltage regulation of a transformer. You will be familiar with three basic types of transformers: control transformers, power transformers, and isolation transformers.

The Discussion of this exercise covers the following points:

Introduction to transformers

Transformer operation

Transformer turns, voltage, and current ratios

Step-up and step-down transformersStep-up transformers. Step-down transformers.

Transformer voltage regulation

Magnetizing current

Types of transformersControl transformers. Power transformers. Isolation transformers.

Training system moduleControl Transformer module.

Introduction to transformers

Transformers basically consist of two inductors (i.e., coils of wire) wound around a common core of ferromagnetic material such as iron. One coil is called the primary winding while the other is called the secondary winding. Each of these windings is electrically isolated one from the other (no current flows between the two windings). The primary winding is the one connected to the ac power source, while the secondary winding is the one connected to the load. Since transformers are bidirectional devices, any winding of a transformer can be used as the primary or secondary winding.

Transformers allow power to be transferred from the primary winding to the secondary winding through the magnetic field produced by each inductor. They also allow voltage and current to be modified from one winding to the other. Due to these properties, transformers are commonly found in ac circuits. Table 19 shows the electrical diagram symbol for a transformer. The symbol for the transformer shows two inductors separated by two lines. Those lines indicate that each winding is wrapped around an iron core.

Transformers

Exercise 10

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

Exercise 10 – Transformers Discussion

206 © Festo Didactic 89688-00

Table 19. Transformer symbol.

Component Symbol

Transformer operation

Consider the circuit shown in Figure 171. This circuit illustrates an ac power source connected to the primary winding of a transformer. A resistor is connected to the secondary winding of the transformer. Figure 172 shows the transformer in Figure 171 in more detail. Note that terminals A, B, C, and D in the circuit correspond to the same terminals in the detailed view.

Figure 171. Circuit of an ac power source connected to a resistor through a transformer.

Figure 172. Detailed view of the transformer in Figure 171.

When the ac power source in Figure 171 applies a voltage to the primary winding of the transformer, current flows in the circuit connected to the primary winding.

A

B

C

D

Primarywinding

100 turns

Secondary winding 100 turns

Magnetic field

AC power source24 V

Primary winding

100 turns


Resistor 60

A

B

C

D



This causes the primary winding to produce a magnetic field in the iron core of the transformer, as shown in Figure 172. This magnetic field then induces a voltage across the secondary winding of the transformer. This voltage is applied to the resistor, causing current to flow in the circuit connected to the secondary winding. Therefore, power has been transferred from the primary winding of the transformer to the secondary winding through electromagnetic induction, with no electric contact between the primary and secondary winding. This explains why transformers have the property to isolate the ac power source from the load.

Transformer turns, voltage, and current ratios

As mentioned previously, transformers have a primary winding and a secondary winding. The ratio between the number of turns of wire in the primary winding ( ) and the number of turns of wire in the secondary winding ( ) is called the turns ratio. This ratio determines the voltage and current ratios between the input and output of the transformer.

Consider, for example, the transformer in Figure 171. In the figure, the transformer primary winding is wound with the same number of turns as the secondary winding. Therefore, the turns ratio : of the transformer is 1:1. Knowing the turns ratio of a transformer allows for the calculation of its voltage ratio. This ratio is equal to the ratio between the primary voltage of the transformer and the secondary voltage. It is expressed as a ratio : and is calculated using the following equation:

(26)

where is the primary voltage of the transformer, expressed in volts (V) is the secondary voltage of the transformer, expressed in volts (V)

is the number of turns in the primary winding of the transformer is the number of turns in the secondary winding of the transformer

The secondary voltage of a transformer can thus be calculated using the following equation:

(27)

In this manual, the voltage

across the primary winding

of a transformer is referred

to as the primary voltage,

while the voltage across the

secondary winding of a

transformer is referred to as

the secondary voltage.



Knowing the turns ratio of a transformer also allows for the calculation of its current ratio. This ratio determines the ratio between the primary current of the transformer and the secondary current. It is expressed as a ratio : and is calculated using the following equation:

(28)

where is the primary current of the transformer, expressed in

amperes (A) is the secondary current of the transformer, expressed in

amperes (A)

The secondary current of a transformer can thus be calculated using the following equation:

(29)

Using these equations, it is possible to calculate both the primary and secondary

voltages and currents in the circuit of Figure 171. The secondary voltage is equal to:

The secondary current is equal to:

Finally, the primary current is equal to:

These values indicate that, when the turns ratio of a transformer is 1:1, the primary voltage and current are equal to the secondary voltage and current. Therefore, the voltage and current ratios of the transformer are both also 1:1.

Using the above values, it is also possible to calculate the power supplied to the transformer, as well as the power supplied by the transformer to the load. The

power at the primary winding is equal to:

The power at the secondary winding is equal to:

As you can see, the power at the primary winding is equal to the power at the secondary winding. This is an important property of transformers and is always

In this manual, the current

flowing in the primary wind-

ing of a transformer is re-

ferred to as the primary

current, while the current

flowing in the secondary

winding of a transformer is

referred to as the secondary

current.



true in theory, no matter the turns, voltage, and current ratios of the transformer. In practice, however, the power at the secondary winding of a transformer is always slightly lower than the power expected in theory. This is because transformers are never 100% efficient and certain power losses occur during the power transfer from the primary winding to the secondary winding.

Step-up and step-down transformers

In the previous section, you saw that a transformer with a turns ratio of 1:1 has the same number of turns in the primary winding as in the secondary winding. Therefore, the transformer voltage and current ratios are also equal to 1:1. However, most transformers have a turns ratio different than 1:1. These transformers are referred to as step-up transformers and step-down transformers.

Step-up transformers

In step-up transformers, the number of turns in the transformer primary winding is lower than the number of turns in the secondary winding, as illustrated in Figure 173. In the figure, the transformer has twice as many turns in the secondary winding as in the primary winding. Therefore, the transformer turns ratio is 1:2.

Figure 173. Step-up transformer with a turns ratio of 1:2.

If terminals A, B, C, and D of the transformer in Figure 173 are connected to the corresponding terminals of the circuit in Figure 171, it is possible to calculate both the primary and secondary voltages and currents in the circuit. The secondary voltage is equal to:


A

B

C

D

Primarywinding

100 turns


Magnetic field




Using the above values, it is also possible to calculate the power supplied to the transformer, as well as the power supplied by the transformer to the load. The power at the primary winding is equal to:


As you can see from these calculations, the secondary voltage of a step-up transformer is higher than the primary voltage by a ratio equal to the turns ratio. Conversely, the secondary current of a step-up transformer is lower than the primary current by a ratio equal to the turns ratio. Finally, the power at the primary winding is equal to the power at the secondary winding.

The characteristics of step-up transformers are summed up below.

Step-down transformers

In step-down transformers, the number of turns in the transformer primary winding is higher than the number of turns in the secondary winding, as illustrated in Figure 174. In the figure, the transformer has twice as many turns in the primary winding as in the secondary winding. Therefore, the transformer turns ratio is 2:1.

Figure 174. Step-down transformer with a turns ratio of 2:1.

A

B

C

D

Primarywinding

200 turns


Magnetic field



If terminals A, B, C, and D of the transformer in Figure 174 are connected to the corresponding terminals of the circuit in Figure 171, it is possible to calculate both the primary and secondary voltages and currents in the circuit. The secondary voltage is equal to:



Using the above values, it is also possible to calculate the power supplied to the transformer, as well as the power supplied by the transformer to the load. The power at the primary winding is equal to:


As you can see from these calculations, the secondary voltage of a step-down transformer is lower than the primary voltage by a ratio equal to the turns ratio. Conversely, the secondary current of a step-down transformer is higher than the primary current by a ratio equal to the turns ratio. Finally, the power at the primary winding is equal to the power at the secondary winding.

The characteristics of step-down transformers are summed up below.


In actual transformers, the higher the current flowing in the secondary winding, the more the secondary voltage decreases. The graph in Figure 175 shows the typical curve of the secondary voltage of a transformer as a function of its secondary current. This curve is the voltage regulation curve of the transformer. As you can see, the transformer secondary voltage decreases as the secondary current increases (i.e., as the load connected to the transformer becomes more important).



Figure 175. Typical voltage regulation curve of a transformer.

The extent of the decrease in the secondary voltage as the secondary current increases depends on the transformer voltage regulation. Therefore, the voltage regulation of a transformer is its ability to maintain the secondary voltage constant as the secondary current increases. The better the voltage regulation of a transformer, the less the secondary voltage decreases as the secondary current increases.

The voltage regulation of a transformer is calculated using Equation (30).

(30)

where is the no-load (when secondary current is null) secondary voltage

of the transformer, expressed in volts (V) is the full-load (when secondary current is equal to current rating)

secondary voltage of the transformer, expressed in volts (V)

In order to obtain the rated voltage at full load, the transformers are winded to produce a voltage higher than the rated voltage when the load is null (to compensate for the voltage drop caused by the current increase). This explains why you measure more than 24 V ac at the output of the transformer of the Control Transformer module.

Magnetizing current

Even with no load connected to the secondary winding of a transformer, current flows through the primary winding as soon as ac voltage is applied to the primary

0

10

20

30

40

50

60

70

80

90

100

110

120

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Secondary current (% of transformer current rating)

Se

co

nd

ary

vo

lta

ge

(%

of

tra

nsfo

rme

r vo

lta

ge

ra

tin

g)



winding. This current produces the magnetic field required for the operation of the transformer, and is commonly referred to as the magnetizing current, or exciting current. Consequently, some power is lost as heat in an actual transformer even when no load is connected to the secondary winding.

Types of transformers

There are many different types of transformers defined by both the function of the transformer and its characteristics. The most important types of transformers are covered in the following subsections.

Control transformers

Control transformers are generally used in electronic applications requiring a steady voltage or current. Control transformers usually have a low voltage and current rating, which means that they are used for applications requiring a relatively low power. They are often used to step-down the voltage. Figure 176 shows a typical control transformer; the transformer in the Control Transformer module is another example of this type of transformer.

Figure 176. Control transformer (© Siemens AG 2014, all rights reserved).

Power transformers

Power transformers are generally used to step-up or step-down the voltage of ac transmission lines. Because such transmission lines transfer large amounts of power, power transformers usually have high voltage, current, and power ratings. Also, since transformers have a better efficiency when they do not operate at their maximal ratings, power transformers often have a higher rating than necessary. This ensures that power transformers are highly efficient, an important property considering the amount of power that they transfer. Figure 177 shows a typical power transformer.



Figure 177. Power transformer.

Isolation transformers

Isolation transformers are used primarily for the electrical insulation provided by transformers. Although most transformers provide electrical insulation, the materials composing isolation transformers as well as their construction ensure a maximal electrical insulation. Because of this, isolation transformers prevent electric shocks, suppress electrical noise, and transfer power between two circuits which must not be connected. Isolation transformers are often used in circuits containing sensitive electrical loads, such as in a hospital. Isolation transformers usually have a turns ratio of 1:1 and thus have virtually no effect on the voltage and current in a circuit.

Training system module

Control Transformer module

(a) Control Transformer (120 V version).

Primary winding terminals (high voltage)

Secondary winding terminals (low voltage)

Control transformer

Fuse

IEC symbol for a transformer and a fuse



(b) Control Transformer (220-240 V version).

Figure 178. Control Transformer module.

The Control Transformer module is supplied with intermediate points of connection called "tap" on the primary winding to adjust the transformer voltage to the correct input voltage. Consider the control transformer of the Control Transformer module. This control transformer is winded to produce 24 V when a voltage of 480 V is applied to its primary winding. The transformer will also produce 24 V when voltages are applied to the corresponding intermediate terminals at the primary winding. The maximum voltage that should be applied to the primary winding of the transformer is 480 V (turns ratio of 20:1) and the maximal power that the transformer should transfer is 75 VA. Not respecting these ratings could damage the transformer.

Note that the power rating of transformers is usually given in VA, which stands for volt-amperes, instead of in watts. This value is equal to the product between current and voltage, just like for regular power values. The reasoning behind the use of volt-amperes instead of watts for the power rating of transformers is beyond the scope of this manual.

The connections to the primary winding of the control transformer are made through 4 mm terminals (high voltage connections), and the connections to the secondary winding are made through 2 mm terminals (low voltage connections). Intermediate points of connection to the primary winding as well as the connection to the secondary winding are fuse protected.

The Control Transformer module is also equipped with four fault switches and two ground terminals.

Primary winding terminals (high voltage)

Secondary winding terminals (low voltage)

Control transformer

Fuse

IEC symbol for a transformer and a fuse

Exercise 10 – Transformers Procedure Outline


The Procedure is divided into the following sections:

Setup

Calculating the ratios and ratings of a transformer

Troubleshooting a transformer

Measuring the ratios and ratings of a transformer


High voltages are present in this laboratory exercise. Do not make or modify any

banana jack connections with the power on unless otherwise specified.

Setup

In this section, you will install the training system modules in the workstation.

1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform this exercise.

Install the equipment required in the workstation.

Make sure that all fault switches are set to the O (off) position.

Calculating the ratios and ratings of a transformer

In this section, you will consider a circuit containing an ac power source connected to a resistor through a transformer. You will determine the turns, voltage, and current ratios of the transformer. Using these ratios, you will determine the transformer secondary voltage and current, as well as its primary current. Finally, knowing the transformer power rating, you will determine its current rating at the secondary winding.

2. Consider the circuit in Figure 179 of an ac power source connected to a resistor through a transformer.

Figure 179. AC power source connected to a resistor through a transformer.

PROCEDURE OUTLINE

PROCEDURE

AC power source120 V

Resistor300

Transformerprimary winding

50 turns

Transformer secondary winding

25 turns

L

N

Exercise 10 – Transformers Procedure


3. Knowing that the transformer in Figure 179 has 50 turns of wire in the

primary winding ( ) and 25 turns of wire in the secondary winding ( ), calculate the turns ratio of the transformer.

a The turns ratio of a transformer is given as follows:

:

Transformer turns ratio

4. Knowing the transformer turns ratio, determine the voltage ratio and current ratio of the transformer.

a The voltage ratio of a transformer is calculated using the following equation:

The current ratio of a transformer is calculated using the following equation:

Transformer voltage ratio

Transformer current ratio

5. Calculate the transformer secondary voltage in the circuit of Figure 179.

a The secondary voltage of a transformer is calculated using the following equation:

Transformer secondary voltage V

6. Knowing that the resistance of the resistor is equal to 300 and the transformer secondary voltage you calculated in the previous step,

calculate the secondary current in the circuit of Figure 179.

a Ohm’s law states that:

Transformer secondary current A



7. Knowing the transformer secondary current , calculate the primary current in the circuit of Figure 179.

a The primary current of a transformer is calculated using the following equation:

Transformer primary current A

8. Knowing that the power rating of the transformer in Figure 179

is 20 VA, calculate the current rating of the transformer secondary

winding.

a The current rating of the transformer can be calculated using the following equation:

Current rating of secondary winding A



Troubleshooting a transformer

In this section, you will use an ohmmeter to measure the resistance between all terminals of the control transformer. You will use the measured values to confirm whether continuity is present or not between the different pairs of terminals, and analyze the results.

9. Consider the circuit in Figure 180 showing the control transformer of the Control Transformer module.

(a) Control Transformer (for 120 V model)

(b) Control transformer (for 220-240 V model)

Figure 180. Control transformer of the Control Transformer module.

Primary winding Secondary winding

480 V 24 V

0 V

240 V

208 V

0 V

Primary winding Secondary winding

480 V 24 V

0 V

240 V

208 V

0 V

120 V



10. Using an ohmmeter, measure the resistance between the terminals of the control transformer indicated below. Record the resistance values in Table 20 (for 120 V) or Table 21 (for 220-240 V).

Table 20. Resistance between the terminals of the control transformer (120 V model).

Resistance between the terminals of the control transformer ( )

Secondary winding Primary winding

0 V 24 V 0 V 120 V 208 V 240 V 480 V

Secondary winding

0 V –

24 V –

Primary winding

0 V –

120 V –

208 V –

240 V –

480 V –

Table 21. Resistance between the terminals of the control transformer (for 220-240 V model).

Resistance between the terminals of the control transformer ( )

Secondary winding Primary winding

0 V 24 V 0 V 208 V 240 V 480 V

Secondary winding

0 V –

24 V –

Primary winding

0 V –

208 V –

240 V –

480 V –

11. From the resistance values you recorded in the previous step, what can you conclude regarding the continuity between the different terminals of the transformer?



12. Can you conclude that the transformer provides electrical insulation between the primary and secondary windings? Briefly explain.

Measuring the ratios and ratings of a transformer

In this section, you will connect an ac power source to a resistor through the control transformer of the Control Transformer module. You will measure the primary and secondary voltages and currents, and use these values to determine the transformer turns, voltage, and current ratios. You will calculate the power at the primary and at the secondary windings of the transformer. You will determine if the transformer is currently a step-up transformer, or a step-down transformer.

13. Make sure that the main power switch on the Power Source module is set to the O (off) position, then connect it to an ac power outlet.

Set up the circuit shown in Figure 181. Connect the two 50 resistors of the Resistors module in parallel to implement the 25 resistor.

(a) For 120 V model.

AC powersource

Primarywinding

Secondary winding

480 V

Resistor 25

Connect to the tap thatcorresponds to your

line voltage

240 V

208 V

0 V

24 V

0 V

L

N 120 V



(b) For 220-240 V model.

Figure 181. AC power source connected to a resistor through a transformer.

14. Turn the power source on.

Measure the transformer primary voltage and secondary voltage . Record the values below.

Transformer primary voltage V

Transformer secondary voltage V

15. Measure the transformer primary current and secondary current by successively connecting the ammeter at the two positions shown in Figure 181. Record the values below.

b Make sure to turn the power source off before making any change to the circuit connections. Also make sure to turn it back on before taking any measurements.

Transformer primary current A

Transformer secondary current A

16. Using the voltage and current values you measured in steps 14 and 15, determine the transformer turns, voltage, and current ratios.

Transformer turns ratio

Transformer voltage ratio

Transformer current ratio

AC powersource

Primarywinding

Secondary winding

480 V

Resistor 25

Connect to the tap thatcorresponds to your

line voltage

240 V

208 V

0 V

24 V

0 V

L

N



a Note that the transformer current ratio does not correspond to the expected ratio. The difference is caused by the magnetizing current and by losses in the transformer.

17. Using the primary and secondary voltages and currents you measured in

steps 14 and 15, calculate the power at the primary winding of the transformer and the power at the secondary winding.

a Power is calculated using the following equation:

Primary power VA

Secondary power VA

a Note that because of the magnetizing current and losses in the transformer, the power is significantly higher than the power for this type of transformer. For larger transformers such as power transformers, the efficiency is higher and the power at the secondary winding is more similar to that at the primary winding.

18. When connected as shown in Figure 181, does the control transformer operate as a step-up transformer, or as a step-down transformer? Briefly explain.

19. Turn the power source off.




In this section, you will vary the load connected to the control transformer. For each load change, you will measure the transformer secondary voltage and current. Using the measured values, you will determine the relationship between the transformer secondary voltage and current.

20. Using the transformer secondary voltage and current values you measured in steps 14 and 15, fill in the first row of Table 22.

Table 22. Transformer secondary current and voltage for different loads.

Resistance

( )

Secondary voltage

(V)

Secondary current

(A)

25

50

100

21. Modify the circuit in Figure 181 to obtain a resistance of 50 (by removing one of the 50 resistors connected in parallel).


Measure the control transformer secondary voltage and current . Record the values in the corresponding row of Table 22.


Modify the circuit in Figure 181 to obtain a resistance of 100 (by connecting the 50 resistors in series).


Measure the control transformer secondary voltage and current . Record the values in the corresponding row of Table 22.

25. Observe the transformer secondary current and voltage values you recorded in Table 22. What is the relationship between the secondary voltage and the secondary current of the transformer? Briefly explain in regard to the transformer voltage regulation.

Exercise 10 – Transformers Conclusion



Disconnect your circuit.

Return the leads and the multimeter(s) to their storage location.

In this exercise, you were introduced to transformers. You became familiar with the basic operating principles of transformers, as well as with the different ratios of transformers: turns ratio, voltage ratio, and current ratio. You learned the difference between step-up and step-down transformers. You were also introduced to the voltage regulation of a transformer. You became familiar with three basic types of transformers: control transformers, power transformers, and isolation transformers. Finally, you were introduced to the Control Transformer module.

1. Briefly define what a transformer is, as well as its basic functions.

2. What is the turns ratio of a transformer and which other ratios does it determine?

3. What are the main differences between a step-down transformer and a step-up transformer? Briefly explain.

CONCLUSION

REVIEW QUESTIONS

Exercise 10 – Transformers Review Questions


4. Name and briefly define two types of transformers.

5. A step-down transformer has 480 turns of wire in the primary winding ( ) and 120 turns of wire in the secondary winding ( ). Determine the

secondary voltage of the transformer, knowing that its primary voltage is equal to 100 V.

exercise 10 transformers - lab-volt · exercise 10 – transformers discussion © festo didactic...

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