Laboratory manual for Electronics: Transformers and Motors: Transformer Basics

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Objectives:

• Discuss the construction of an isolation transformer.

• Determine the winding configuration with an ohmmeter.

• Connect a transformer and make voltage measurements.

• Compute the turns-ratio of the windings.

A transformer is a magnetically operated machine that can change values of voltage, cur rent, and impedance without a change of frequency. Transformers are the most efficient machines known. Their efficiencies commonly range from 90% to 99% at full load.

Transformers can be divided into several classifications such as:

Isolation Auto Current

A basic law concerning transformers is that all values of a transformer are proportional to its turns-ratio. This does not mean that the exact number of turns of wire on each winding must be known to determine different values of voltage and current for a transformer.

What must be known is the ratio of turns. E.g., assume a transformer has two windings. One winding, the primary, has 1,000 turns of wire and the other, the secondary, has 250 turns of wire, as shown in Ill. I1. The turns-ratio of this transformer is 4 to 1 or 4:1 (1,000/250 = 4). This indicates there are four turns of wire on the primary for every one turn of wire on the secondary.

Helpful Hint: A basic law concerning transformers is that all values of a transformer are proportional to its turns-ratio.

Ill. I1 All values of a transformer are proportional to its turns-ratio.

Transformer Formulas

There are different formulas that can be used to find the values of voltage and current for a transformer. The following is a list of standard formulas: where: NP = Number of turns in the primary NS = Number of turns in the secondary EP = Voltage of the primary ES = Voltage of the secondary IP = Current in the primary IS = Current in the secondary

The primary winding of a transformer is the power input winding. It is the winding that's connected to the incoming power supply. The secondary winding is the load winding or output winding. It is the side of the transformer that's connected to the driven load, seen in Ill. I2. Any winding of a transformer can be used as a primary or secondary winding provided its voltage or current rating isn't exceeded. Transformers can also be operated at a lower voltage than their rating indicates, but they cannot be connected to a higher volt age. Assume the transformer shown in Ill. I2, for example, has a primary voltage rating of 480 volts and the secondary has a voltage rating of 240 volts. Now assume that the primary winding is connected to a 120 volt source. No damage would occur to the transformer, but the secondary winding would produce only 60 volts.

Isolation Transformers

Ill. I2 Isolation transformer.

Ill. I3 The current through an inductor rises at an exponential rate.

Ill. I4 Voltage spikes are generally of very short duration.

Ill. I5 The isolation transformer greatly reduces the voltage spike.

The transformers shown in Ill.s I1 and I2 are isolation transformers. This means that the secondary winding is physically and electrically isolated from the primary winding. There is no electrical connection between the primary and secondary winding. This transformer is magnetically coupled, not electrically coupled. This "line isolation" is often a very desirable characteristic. Since there is no electrical connection between the load and power supply, the transformer becomes a filter between the two. The isolation transformer will attenuate any voltage spikes that originate on the supply side before they are transferred to the load side. Some isolation transformers are built with a turns-ratio of 1:1. A transformer of this type will have the same input and output voltage and is used for the purpose of isolation only.

The reason that the transformer can greatly reduce any voltage spikes before they reach the secondary is because of the rise time of current through an inductor. The current in an inductor rises at an exponential rate, as shown in Ill. I3. As the current increases in value, the expanding magnetic field cuts through the conductors of the coil and induces a voltage that's opposed to the applied voltage. The amount of induced voltage is proportional to the rate of change of current. This simply means that the faster current attempts to increase, the greater the opposition to that increase will be. Spike voltages and currents are generally of very short duration, which means that they increase in value very rapidly ( Ill. I4). This rapid change of value causes the opposition to the change to increase just as rapidly. By the time the spike has been transferred to the secondary winding of the transformer, it has been eliminated or greatly reduced ( Ill. I5). Another purpose of isolation transformers is to remove or isolate some piece of electrical equipment from circuit ground. It is sometimes desirable that a piece of electrical equipment not be connected directly to circuit ground. This is often done as a safety precaution to eliminate the hazard of an accidental contact between a person at ground potential and the ungrounded conductor. If the case of the equipment should come in contact with the ungrounded conductor, the isolation transformer would prevent a circuit being completed to ground through a person touching the case of the equipment. Many alternating current circuits have one side connected to ground. A familiar example of this is the common 120 volt circuit with a grounded neutral conductor, as seen in Ill. I6. An isolation transformer can be used to remove or isolate a piece of equipment from circuit ground.

Ill. I6 Isolation transformer used to remove a piece of electrical equipment from ground. Equipment has no connection to ground.

Excitation Current

There will always be some amount of current flow in the primary of a transformer even if there is no load connected to the secondary. This is called the excitation current of the transformer. The excitation current is the amount of current required to magnetize the core of the transformer. The excitation current remains constant from no load to full load. As a general rule, the excitation current is such a small part of the full load current it's often omit ted when making calculations.

Transformer Calculations

In the following examples, values of voltage, current, and turns for different transformers will be computed.

Example #1: Assume the isolation transformer shown in Ill. I2 has 240 turns of wire on the primary and 60 turns of wire on the secondary. This is a ratio of 4:1 (240/60 = 4).

Now assume that 120 volts is connected to the primary winding. What is the voltage of the secondary winding?

The transformer in this example is known as a step-down transformer because it has a lower secondary voltage than primary voltage.

Now assume that the load connected to the secondary winding has an impedance of 5 ?. The next problem is to calculate the current flow in the secondary and primary windings. The current flow of the secondary can be computed using Ohm's law since the voltage and impedance are known.

Now that the amount of current flow in the secondary is known, the primary current can be computed using the formula:

Notice that the primary voltage is higher than the secondary voltage, but the primary cur rent is much less than the secondary current. A good rule for transformers is that power in must equal power out. If the primary voltage and current are multiplied together, the result should equal the product of the voltage and current of the secondary.

Primary 120 _ 1.5 = 180 volt-amps Secondary 30 _ 6 = 180 volt-amps

Helpful Hint: A good rule for transformers is that power in must equal power out.

Example #2: In the next example, assume that the primary winding contains 240 turns of wire and the secondary contains 1,200 turns of wire. This is a turns-ratio of 1:5 (1,200/240 = 5). Now assume that 120 volts is connected to the primary winding. Compute the voltage output of the secondary winding.

Notice that the secondary voltage of this transformer is higher than the primary voltage.

This type of transformer is known as a step-up transformer.

Now assume that the load connected to the secondary has an impedance of 2,400 ohm. Find the amount of current flow in the primary and secondary windings. The current flow in the secondary winding can be computed using Ohm's law.

Now that the amount of current flow in the secondary is known, the primary current can be computed using the formula:

Notice that the amount of power input equals the amount of power output.

Primary 120 _ 1.25 = 150 volt-amps

Secondary 600 _ 0.25 = 150 volt-amps

Calculating Transformer Values Using the Turns-Ratio

As illustrated in the previous examples, transformer values of voltage, current, and turns can be computed using formulas. It is also possible to compute these same values using the turns-ratio. There are several ways in which turns-ratios can be expressed. One method is to use a whole number value such as 13:5 or 6:21. The first ratio indicates that one winding has 13 turns of wire for every 5 turns of wire in the other winding. The second ratio indicates that there are 6 turns of wire in one winding for every 21 turns in the other.

A second method is to use the number 1 as a base. When using this method, the number 1 is always assigned to the winding with the lowest voltage rating. The ratio is found by dividing the higher voltage by the lower voltage. The number on the left side of the ratio rep resents the primary winding and the number on the right of the ratio represents the secondary winding. E.g., assume a transformer has a primary rated at 240 volts and a secondary rated at 96 volts, as shown in Ill. I7. The turns-ratio can be computed by dividing the higher voltage by the lower voltage.

RATIO: 240/96 RATIO: 2.5:1 Notice in this example that the primary winding has the higher voltage rating and the secondary has the lower. Therefore, the 2.5 is placed on the left and the base unit, 1, is placed on the right. This ratio indicates that there are 2.5 turns of wire in the primary winding for every 1 turn of wire in the secondary.

Now assume that there is a resistance of 24 ohm connected to the secondary winding. The amount of secondary current can be found using Ohm's law.

Ill. I7 Computing transformer values using the turns-ratio.

The primary current can be found using the turns-ratio. Recall that the volt-amps of the primary must equal the volt-amps of the secondary. Since the primary voltage is greater, the primary current will have to be less than the secondary current. Therefore, the secondary current will be divided by the turns-ratio.

To check the answer, find the volt-amps of the primary and secondary.

Primary = 240 _ 1.6 = 384

Secondary = 96 _ 4 = 384

Now assume that the secondary winding contains 150 turns of wire. The primary turns can be found by using the turns-ratio, also. Since the primary voltage is higher than the secondary voltage, the primary must have more turns of wire. Since the primary must contain more turns of wire, the secondary turns will be multiplied by the turns-ratio.

P S × =

P 150 2.5 × =

P 375 turns =

In the next example, assume a transformer has a primary voltage of 120 volts and a secondary voltage of 500 volts. The secondary has a load impedance of 1,200 ohm. The secondary contains 800 turns of wire ( Ill. I8). The turns-ratio can be found by dividing the higher voltage by the lower voltage.

Ill. I8 Calculating transformer values.

The secondary current can be found using Ohm's law.

In this example the primary voltage is lower than the secondary voltage. Therefore, the primary current must be higher. To find the primary current, multiply the secondary current by the turns-ratio.

IP = IS _ Turns-ratio IP = 0.417 _ 4.17 IP = 1.74 amps

To check this answer, compute the volt-amps of both windings.

Primary 120 _ 1.74 = 208.8

Secondary 500 _ 0.417 = 208.5

The slight difference in answers is caused by rounding off of values.

Since the primary voltage is less than the secondary voltage, the turns of wire in the primary will also be less. The primary turns will be found by dividing the turns of wire in the secondary by the turns-ratio.

LABORATORY EXERCISE

Name _______ Date __________

Materials Required:

480-240/1volt, 0.5-kVA control transformer Ohmmeter AC voltmeter, in-line or clamp-on. (If a clamp-on type is used, a 10:1 scale divider is recommended.) These experiments are intended to provide the electrician with hands-on experience dealing with transformers. The transformers used in these experiments are standard control transformers with two high-voltage windings rated at 240 volts each generally used to pro vide primary voltages of 480/240, and one low-voltage winding rated at 120 volts. The transformers have a rating of 0.5 kVA. The loads are standard 100 watt lamps that may be connected in parallel or series. It is assumed that the power supply is 208/120 volt three phase four wire. It is also possible used with a 240/120 volt three-phase high leg system, pro vided adjustments are made in the calculations.

As in industry, these transformers will be operated with full voltage applied to the windings.

The utmost caution must be exercised when dealing with these transformers. These transformers can provide enough voltage and current to seriously injure or kill anyone. The power should be disconnected before attempting to make or change any connections.

Caution: These transformers can provide enough voltage and current to seriously injure or kill anyone.

The transformer used in this experiment contains two high-voltage windings and one low voltage winding. The high-voltage windings are labeled H1 - H2 and H3 - H4. The low-voltage winding is labeled X1 - X2.

1. Set the ohmmeter to the Rx1 range and measure the resistance between the following terminals:

H1 - H2 ____________ ? H1 - H3 ____________ ? H1 - H4 ____________ ? H1 - X1 ____________ ? H1 - X2 ____________ ? H2 - H3 ____________ ? H2 - H4 ____________ ? H2 - X1 ____________ ? H2 - X2 ____________ ? H3 - H4 ____________ ? H3 - X1 ____________ ? H3 - X2 ____________ ? H4 - X1 ____________ ? H4 - X2 ____________ ? X1 - X2 ____________ ?

2. Using the information provided by the measurements from step 1, which sets or terminals form complete circuits within the transformer?

These circuits represent the connections to the three separate windings within the transformer.

3. Which of the windings exhibits the lowest resistance and why?

4. The H1 - H2 terminals are connected to one of the high-voltage windings and the H3 - H4 terminals are connected to the second high-voltage winding. Each of these windings is rated at 240 volts. When this transformer will be connected for 240 volt operation, the two high-voltage windings are connected in parallel to form one winding by connecting H1 to H3 and H2 to H4, as shown in Ill. I9. This will provide a 2:1 turns ratio with the low-voltage winding.

When this transformer is operated with 480 volts connected to the primary, the high voltage windings are connected in series by connecting H2 to H3 and connecting power to H1 and H4, as shown in Ill. I10. This effectively doubles the primary turns, pro viding a 4:1 turns-ratio with the low-voltage winding.

5. Connect the two high-voltage windings for parallel operation as shown in Ill. I9.

Assume a voltage of 208 volts is applied to the high-voltage windings. Compute the volt age that should be seen on the low-voltage winding between terminals X1 and X2. _ volts.

6. Make certain that the incoming power leads are connected to terminals H1 and H4 as shown in Ill. I9. Apply a voltage of 208 volts to the transformer and measure the voltage across terminals X1 and X2. ____ volts.

7. The measured voltage may be slightly higher than the computed voltage. The rated voltage of a transformer is based on full load. It is normal for the secondary voltage to be slightly higher when no load is connected to the transformer. Transformers are generally wound with a few extra turns of wire in the winding that's intended to be used as the load side. This helps overcome the voltage drop when load is added. The slight change in turns-ratio does not affect the operation of the transformer to a great extent.

Ill. I9 High-voltage windings connected in parallel.

Ill. I10 High-voltage windings connected in series.

8. Turn off the power to the transformer.

9. Disconnect the wires connected to the transformer and reconnect the transformer as shown in Ill. I10. The two high-voltage windings are connected in series by connecting H2 and H3 together. This connection changes the turns-ratio of the transformer from 2:1 to 4:1. Make certain that the incoming power is connected to terminals H1 and H4.

10. Assume that a voltage of 208 volts is applied to the high-voltage windings. Compute the voltage across the low-voltage winding. ___ volts

11. Turn on the power and apply a voltage of 208 volts to the transformer. Measure the volt age across terminals X1 and X2. ___ volts

12. Turn off the power. Disconnect the power lines that are connected to terminals H1 and H4. Don't disconnect the wire between terminals H2 and H3.

13. In the next part of the exercise, the low-voltage winding will be used as the primary and the high-voltage windings will be used as the secondary. If the high-voltage windings are connected in series, the turns-ratio will be 1:4, which means that the secondary voltage will be four times greater than the primary voltage. The transformer has now become a step-up transformer instead of a step-down transformer. Assume that a voltage of 120 volts is connected to terminals X1 and X2. If the high-voltage windings are connected in series, compute the voltage across terminals H1 and H4. ___ volts

14. Connect the transformer as shown in Ill. I11. Make certain that the voltage applied to terminals X1 and X2 is 120 volts and not 208 volts. Also make certain that the AC voltmeter is set for a higher range than the computed value of voltage in step 13.

Caution: The secondary voltage in this step will be 480 volts or higher. Use extreme caution when making this measurement. Be sure to wear safety glasses at all times.

Ill. I11 The incoming power is connected to terminals X1 and X2.

Ill. I12 The transformer has a turns-ratio of 1:2.

15. Turn on the power and measure the voltage across terminals H1 and H4. ____ volts

16. Turn off the power supply.

17. Disconnect the lead between terminals H2 and H3. Reconnect the transformer so that the high-voltage windings are connected in parallel by connecting H1 and H3 together and H2 and H4 together as shown in Ill. I12. Don't disconnect the power leads to terminals X1 and X2. The transformer now has a turns-ratio of 1:2.

18. Assume that a voltage of 120 volts is connected to the low-voltage winding. Compute the voltage across the high-voltage winding.____ volts

19. Make certain the power leads are still connected to terminals X1 and X2. Turn on the power and apply 120 volts to terminals X1 and X2. Measure the voltage across terminals H1 and H4.___ volts

20. Turn off the power supply and disconnect all leads to the transformer. Return the components to their proper place.

QUIZ:

1. What is a transformer?

2. What are common efficiencies for transformers?

3. What is an isolation transformer?

4. All values of a transformer are proportional to its:

5. A transformer has a primary voltage of 480 volts and a secondary voltage of 20 volts. What is the turns-ratio of the transformer?

6. If the secondary of the transformer in question 5 supplies a current of 9.6 amperes to a load, what is the primary current (disregard excitation current)?

7. Explain the difference between a step-up and a step-down transformer.

8. A transformer has a primary voltage of 240 volts and a secondary voltage of 48 volts.

What is the turns-ratio of this transformer?

9. A transformer has an output of 750 volt-amps. The primary voltage is 120 volts. What is the primary current?

10. A transformer has a turns-ratio of 1:6. The primary current is 18 amperes. What is the secondary current?

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