Single-Phase Transformers (part 3)

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Autotransformers

Autotransformers are one-winding transformers. They use the same winding for both the primary and secondary. The primary winding is between points B and N and has a voltage of 120 volts applied to it. If the turns of wire are counted between points B and N, it can be seen that there are 120 turns of wire. Now assume that the selector switch is set to point D. The load is now connected between points D and N. The secondary of this transformer contains 40 turns of wire. If the amount of voltage applied to the load is to be calculated the following formula can be used:

120 turns ES = 4800 V-turns

ES = 40 V

+++++33 Reactors are used to help prevent inrush current from becoming excessive when power is first turned on. Switch Reactor Low-impedance load

+++++34 Magnetic domain in neutral position.

+++++38 Isolation transformer.

+++++35 Domain influenced by a north magnetic field.

+++++36 Domain influenced by a south magnetic field.

+++++37 The core of an inductor contains an air gap.

+++++41 An autotransformer has only one winding used for both the primary and secondary.

Assume that the load connected to the secondary has an impedance of 10 ohms. The amount of current flow in the secondary circuit can be calculated using the formula:

I = 4A

The primary current can be calculated by using the same formula that was used to calculate primary current for an isolation type of transformer:

The amount of power input and output for the autotransformer must be the same, just as they are in an isolation transformer:

Primary Secondary

120 V x 1.333 A = 160 VA

40 V x 4 A = 160 VA

Now assume that the rotary switch is connected to point A. The load is now connected to 160 turns of wire. The voltage applied to the load can be calculated by

Notice that the autotransformer, like the isolation transformer, can be either a step-up or step-down transformer.

If the rotary switch shown were to be removed and replaced with a sliding tap that made contact directly to the transformer winding, the turns ratio could be adjusted continuously. This type of transformer is commonly referred to as a Variac or Powerstat depending on the manufacturer. A cutaway view of a variable autotransformer.

The windings are wrapped around a tape-wound toroid core inside a plastic case. The tops of the windings have been milled l at to provide a commutator.

A carbon brush makes contact with the windings.

+++++42 Cutaway view of a Powerstat. Shaft Brush holder Carbon brush Powerkote coil Core Base Shaft bearings End forms Radiator Gold-plated commutator Terminal board

Autotransformers are often used by power companies to provide a small

increase or decrease to the line voltage. They help provide voltage regulation to large power-lines. A three-phase autotransformer is shown.

This transformer is contained in a housing filled with transformer oil, which acts as a coolant and prevents moisture from forming in the windings.

The autotransformer does have one disadvantage. Because the load is connected to one side of the powerline, there is no line isolation between the incoming power and the load. This can cause problems with certain types of equipment and must be a consideration when designing a power system.

Transformer Polarities

To understand what is meant by transformer polarity, the voltage produced across a winding must be considered during some point in time. In a 60-herz AC circuit, the voltage changes polarity 60 times per second. When discussing transformer polarity, it’s necessary to consider the relationship between the different windings at the same point in time. It’s therefore assumed that this point in time is when the peak positive voltage is being produced across the winding.

+++++43 Three-phase autotransformer.

Polarity Markings on Schematics:

When a transformer on a schematic diagram, it’s common practice to indicate the polarity of the transformer windings by placing a dot beside one end of each winding. These dots signify that the polarity is the same at that point in time for each winding. For example, assume the voltage applied to the primary winding is at its peak positive value at the terminal indicated by the dot. The voltage at the dotted lead of the secondary will be at its peak positive value at the same time.

This same type of polarity notation is used for transformers that have more than one primary or secondary winding. An example of a transformer with a multisecondary.

+++++44 Transformer polarity dots.

+++++45 Polarity marks for multiple secondaries.

The polarity of transformer windings can be determined by connecting them as an autotransformer and testing for additive or subtractive polarity, often referred to as a boost or buck connection. This is done by connecting one lead of the secondary to one lead of the primary and measuring the voltage across both windings. The transformer shown in the example has a primary voltage rating of 120 volts and a secondary voltage rating of 24 volts. This same circuit has been redrawn to show the connection more clearly. Notice that the secondary winding has been connected in series with the primary winding. The transformer now contains only one winding and is therefore an autotransformer. When 120 volts are applied to the primary winding, the voltmeter connected across the secondary indicates either the sum of the two voltages or the difference between the two voltages. If this voltmeter indicates 144 volts (120 V + 24 V=144 V), the windings are connected additive (boost) and polarity dots can be placed. Notice in this connection that the secondary voltage is added to the primary voltage.

If the voltmeter connected to the secondary winding indicates a voltage of 96 volts (120 x 2 24 V=96 V), the windings are connected subtractive (buck) and polarity dots are placed.

+++++46 Connecting the secondary and primary windings forms an autotransformer.

+++++47 Redrawing the connection.

+++++48 Placing polarity dots to indicate additive polarity.

+++++49 Polarity dots indicate subtractive polarity.

+++++50 Arrows help indicate the placement of the polarity dots.

Using Arrows to Place Dots:

To help in the understanding of additive and subtractive polarity, arrows can be used to indicate a direction of greater-than or less-than values. … arrows have been added to indicate the direction in which the dot is to be placed. In this example, the transformer is connected additive, or boost, and both arrows point in the same direction. Notice that the arrow points to the dot. … it’s seen that values of the two arrows add to produce 144 volts.

…arrows have been added to a subtractive, or buck, connection. In this instance, the arrows point in opposite directions and the voltage of one tries to cancel the voltage of the other. The result is that the smaller value is eliminated and the larger value is reduced.

+++++51 The values of the arrows add to indicate additive polarity (boost connection).

+++++52 The arrows help indicate subtractive polarity.

+++++53 The values of the arrows subtract (buck connection).

+++++54 At no load, the primary current lags the voltage by 90 deg. Applied voltage -- Primary current

+++++55 The secondary voltage lags the primary current by 90 deg. Secondary voltage; Primary current

Voltage and Current Relationships in a Transformer

When the primary of a transformer is connected to power but there is no load connected to the secondary, current is limited by the inductive reactance of the primary. At this time, the transformer is essentially an inductor and the excitation current is lagging the applied voltage by 90 degrees. The primary current induces a voltage in the secondary. This induced volt age is proportional to the rate of change of current. The secondary voltage is maximum during the periods that the primary current is changing the most (0 degrees, 180 degrees, and 360 degrees), and it will be zero when the primary current is not changing (90 degrees and 270 degrees). A plot of the primary current and secondary voltage shows that the secondary voltage lags the primary current by 90 degrees. Because the secondary voltage lags the primary current by 90 degrees and the applied voltage leads the primary current by 90 degrees, the secondary voltage is 180 degrees out of phase with the applied voltage and in phase with the induced voltage in the primary.

+++++56 Voltage and current relationships of the primary and secondary windings.

Applied voltage Secondary current Primary current Secondary voltage

+++++57 Testing a transformer with an ohmmeter.