Single-Phase Transformers (part 2: Isolation Transformers)

Isolation Transformers

The transformers are isolation transformers. This means that the secondary winding is physically and electrically isolated from the primary winding. There is no electric connection between the primary and secondary winding. This transformer is magnetically coupled, not electrically coupled. This line isolation is often a very desirable characteristic. The isolation transformer greatly reduces 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 has the same input and output voltages and is used for the purpose of isolation only.

The reason that the isolation transformer can greatly reduce any voltage spikes before they reach the secondary is because of the rise time of current through an inductor. Recall fthat DC in an inductor rises at an exponential rate. As the current increases in value, the expanding magnetic field cuts through the conductors of the coil and induces a volt age that is 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 is. Spike voltages and currents are generally of very short duration, which means that they increase in value very rapidly. 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.

The basic construction of an isolation transformer.

A metal core is used to provide good magnetic coupling between the two windings. The core is generally made of laminations stacked together. Laminating the core helps reduce power losses caused by eddy current induction.

+++++3 DC through an inductor rises at an exponential rate.

Exponential curve Time Current

+++++4 Voltage spikes are generally of very short duration.

Spike voltage Sine wave voltage Duration of voltage spike

+++++5 The isolation transformer greatly reduces the voltage spike. Primary Secondary Load

+++++6 Basic construction of an isolation transformer. Winding; Iron core; Winding

+++++7 Magnetic field produced by AC. Magnetic field

+++++8 The magnetic field of the primary induces a voltage into the secondary.

Basic Operating Principles:

… one winding of an isolation transformer has been connected to an AC supply, and the other winding has been connected to a load. As current increases from zero to its peak positive point, a magnetic field expands outward around the coil. When the current decreases from its peak positive point to ward zero, the magnetic field collapses. When the current increases toward its negative peak, the magnetic field again expands but with an opposite polarity of that previously. The field again collapses when the current decreases from its negative peak toward zero. This continually expanding and collapsing magnetic field cuts the windings of the primary and induces a voltage into it. This induced voltage opposes the applied voltage and limits the current flow of the primary.

When a coil induces a voltage into itself, it’s known as self-induction.

Excitation Current:

There will always be some amount of current flow in the primary of any volt age transformer regardless of type or size even if there is no load connected to the secondary. This current flow 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 that it’s often omitted when making calculations.

Mutual Induction:

Because the secondary windings of an isolation transformer are wound on the same core as the primary, the magnetic field produced by the primary winding also cuts the windings of the secondary. This continually changing magnetic field induces a voltage into the secondary winding. The ability of one coil to induce a voltage into another coil is called mutual induction. The amount of voltage induced in the secondary is determined by the ratio of the number of turns of wire in the secondary to those in the primary. For example, assume the primary has 240 turns of wire and is connected to 120 VAC. This gives the transformer a volts-per-turn ratio of 0.5 (120 V/240 turns=0.5 V per turn). Now assume the secondary winding contains 100 turns of wire. Because the transformer has a volts-per-turn ratio of 0.5 volt per turn, the secondary voltage is 50 volts (100 turns x 0.5 V/turn=50 V per turn).

Transformer Calculations:

In the following examples, values of voltage, current, and turns for different transformers are calculated.

Assume that the isolation transformer has 240 turns of wire on the primary and 60 turns of wire on the secondary. This is a ratio of 4:1 (240 turns/60 turns=4). Now assume that 120 volts are 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=ohms. The next problem is to calculate the current flow in the secondary and primary windings. The current flow of the secondary can be calculated using Ohm's law because the voltage and impedance are known:

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

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

Primary Secondary

120 x 1.5=180 VA

30 x 6=180 VA

In this example, assume that the primary winding contains 240 turns of wire and the secondary contains 1200 turns of wire. This is a turns ratio of 1:5 (1200 turns/240 turns=5). Now assume that 120 volts are connected to the primary winding. Calculate 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 2400 ohms. Find the amount of current flow in the primary and secondary windings. The current flow in the secondary winding can be calculated using Ohm's law:

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

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

Primary Secondary

120 V x 1.25 A=150 VA

600 V x 0.25 A=150 VA

+++++9 Calculating transformer values using the turns ratio.

Calculating Isolation Transformer Values Using the Turns Ratio

As illustrated in the previous examples, transformer values of voltage, current, and turns can be calculated using formulas. It’s also possible to calculate these same values using the turns ratio. To make calculations using the turns ratio, a ratio is established that compares some number to 1, or 1 to some number. For example, assume a transformer has a primary rated at 240 volts and a secondary rated at 96 volts. The turns ratio can be calculated by dividing the higher voltage by the lower voltage:

This ratio indicates that there are 2.5 turns of wire in the primary winding for every 1 turn of wire in the secondary. The side of the transformer with the lowest voltage will always have the lowest number (1) of the ratio.

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

The primary current can be found using the turns ratio. Recall that the volt- amperes of the primary must equal the volt-amperes of the secondary. Because the primary voltage is greater, the primary current will have to be less than the secondary current:

Primary Secondary

240 V x 1.6 A=384 VA 96 V x 4 A=384 VA

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

NP=NS x turns ratio; NP=150 turns x 2.5; NP=375 turns

In the next example, assume an isolation transformer has a primary volt age of 120 volts and a secondary voltage of 500 volts. The secondary has a load impedance of 1200 ohms. The secondary contains 800 turns of wire.

The turns ratio can be found by dividing the higher voltage by the lower voltage:

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:

IP=IS x turns ratio

IP=0.417Ax4.17

IP=1.739 A

+++++10 Calculating transformer values.

+++++11 Transformer with calculated values.

To check this answer, calculate the volt-amperes of both windings:

Primary -- Secondary

120 Vx1.739A=208.68VA

500 Vx0.417A=208.5VA

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

Because the primary voltage is less than the secondary voltage, the turns of wire in the primary is less also:

+++++11 shows the transformer with all calculated values.

Multiple-Tapped Windings:

It’s not uncommon for isolation transformers to be designed with windings that have more than one set of lead wires connected to the primary or secondary. These are called multiple-tapped windings. The transformer contains a secondary winding rated at 24 volts. The primary winding contains several taps, however. One of the primary lead wires is labeled C and is the common for the other leads. The other leads are labeled 120 volts, 208 volts, and 240 volts. This transformer is designed in such a manner that it can be connected to different primary voltages without changing the value of the secondary voltage. In this example, it’s assumed that the secondary winding has a total of 120 turns of wire. To maintain the proper turns ratio, the primary would have 600 turns of wire between C and 120 volts, 1040 turns between C and 208 volts, and 1200 turns between C and 240 volts.

The isolation transformer contains a single primary winding. The secondary winding, however, has been tapped at several points.

One of the secondary lead wires is labeled C and is common to the other lead wires. When rated voltage is applied to the primary, voltages of 12 volts, 24 volts, and 48 volts can be obtained at the secondary. It should also be noted that this arrangement of taps permits the transformer to be used as a center-tapped transformer for two of the voltages. If a load is placed across the lead wires labeled C and 24, the lead wire labeled 12 volts becomes a center tap. If a load is placed across the C and 48 lead wires, the 24 volts lead wire becomes a center tap.

In this example, it’s assumed that the primary winding has 300 turns of wire. To produce the proper turns ratio would require 30 turns of wire between C and 12 volts, 60 turns of wire between C and 24 volts, and 120 turns of wire between C and 48 volts.

+++++12 Transformer with multiple-tapped primary winding. Primary winding; Secondary winding

+++++13 Transformer secondary with multiple taps.

+++++14 Transformer with multiple secondary windings. Primary winding; Secondary windings

The isolation transformer shown is similar to the transformer. The transformer however, has multiple secondary windings instead of a single secondary winding with multiple taps. The advantage of the transformer is that the secondary windings are electrically isolated from each other. These secondary windings can be either step-up or step-down depending on the application of the transformer.

Calculating Values for Isolation Transformers with Multiple Secondaries

When calculating the values of an isolation transformer with multiple secondary windings, each secondary must be treated as a different transformer. For example, the transformer contains one primary winding and three secondary windings. The primary is connected to 120 VAC and contains 300 turns of wire. One secondary has an output voltage of 560 volts and a load impedance of 1000 ohms; the second secondary has an output voltage of 208 volts and a load impedance of 400 ohms; and the third secondary has an output voltage of 24 volts and a load impedance of 6 ohms. The current, turns of wire, and ratio for each secondary and the current of the primary will be found.

The first step is to calculate the turns ratio of the first secondary. The turns ratio can be found by dividing the smaller voltage into the larger:

+++++15 Calculating values for a transformer with multiple secondary windings.

The current flow in the first secondary can be calculated using Ohm's law:

The number of turns of wire in the first secondary winding is found using the turns ratio. Because this secondary has a higher voltage than the primary, it must have more turns of wire:

NS1=NP x turns ratio

NS1=300 turnsx4.67

N=1401 turns

The amount of primary current needed to supply this secondary winding can be found using the turns ratio also. Because the primary has less voltage, it requires more current:

IP(FIRST SECONDARY)=IS1 x turns ratio

IP(FIRST SECONDARY)=0.56Ax4.67

IP(FIRST SECONDARY)=2.61A

The turns ratio of the second secondary winding is found by dividing the higher voltage by the lower:

The amount of current flow in this secondary can be determined using Ohm's law:

Because the voltage of this secondary is greater than the primary, it has more turns of wire than the primary. The number of turns of this secondary is found using the turns ratio:

NS2=NP x turns ratio

NS2=300 turns x 1.73

NS2=519 turns

The voltage of the primary is less than this secondary. The primary there fore requires a greater amount of current. The amount of current required to operate this secondary is calculated using the turns ratio:

IP(SECOND SECONDARY)=IS2 x turns ratio

IP(SECOND SECONDARY)=0.52A x 1.732

IP(SECOND SECONDARY)=0.9A

The turns ratio of the third secondary winding is calculated in the same way as the other two. The larger voltage is divided by the smaller:

The primary current is found using Ohm's law:

The output voltage of the third secondary is less than the primary. The number of turns of wire is therefore less than the primary turns:

The primary has a higher voltage than this secondary. The primary current is therefore less by the amount of the turns ratio:

The primary must supply current to each of the three secondary windings. Therefore, the total amount of primary current is the sum of the currents required to supply each secondary:

IP(TOTAL)=IP1 1 IP2 1 IP3 IP(TOTAL)=2.61A 1 0.9A 1 0.8A

IP(TOTAL)=4.31 A The transformer with all calculated values.

+++++16 The transformer with all calculated values.

+++++17 Distribution transformer.

+++++18 The voltage from either line to neutral is 120 volts. The voltage across the entire secondary winding is 240 volts.

+++++19 The voltages across the secondary are out of phase with each other.

+++++20 Loads of 240 volts connect directly across the secondary winding.

Water heater Electric heat Central air conditioner

+++++21 The neutral carries the sum of the unbalanced load.

Distribution Transformers:

A common type of isolation transformer is the distribution transformer. This type of transformer changes the high voltage of power company distribution lines to the common 240/120 volts used to supply power to most homes and many businesses. In this example, it’s assumed that the primary is connected to a 7200-volt line. The secondary is 240 volts with a center tap. The center tap is grounded and becomes the neutral conductor or common conductor. If voltage is measured across the entire secondary, a voltage of 240 volts is seen. If voltage is measured from either line to the center tap, half of the secondary voltage, or 120 volts, is seen. The reason this occurs is that the grounded neutral conductor becomes the center point of two out-of-phase voltages. If a vector diagram is drawn to illustrate this condition, you will see that the grounded neutral conductor is connected to the center point of the two out-of-phase voltages. Loads that are intended to operate on 240 volts, such as water heaters, electric-resistance heating units, and central air conditioners are connected directly across the lines of the secondary.

Loads that are intended to operate on 120 volts connect from the center tap, or neutral, to one of the secondary lines. The function of the neutral is to carry the difference in current between the two secondary lines and maintain a balanced voltage. …, one of the secondary lines has a current flow of 30 amperes and the other has a current flow of 24 amperes. The neutral conducts the sum of the unbalanced load. In this example, the neutral current is 6 amperes (30A-24A=6A).

+++++22 Control transformer with fuse protection added to the secondary winding.

+++++23 Control transformer connected for 240-volt operation.

Control Transformers:

Another common type of isolation transformer found throughout industry is the control transformer. The control transformer is used to reduce the line voltage to the value needed to operate control circuits. The most common type of control transformer contains two primary windings and one secondary. The primary windings are generally rated at 240 volts each, and the secondary is rated at 120 volts. This arrangement provides a 2:1 turns ratio between each of the primary windings and the secondary. For example, assume that each of the primary windings contains 200 turns of wire. The secondary will contain 100 turns of wire.

One of the primary windings is labeled H1 and H2. The other is labeled H3 and H4. The secondary winding is labeled X1 and X2. If the primary of the transformer is to be connected to 240 volts, the two primary windings are connected in parallel by connecting H1 and H3 together and H2 and H4 together. When the primary windings are connected in parallel, the same voltage is applied across both windings. This has the same effect as using one primary winding with a total of 200 turns of wire. A turns ratio of 2:1 is maintained, and the secondary voltage is 120 volts.

If the transformer is to be connected to 480 volts, the two primary windings are connected in series by connecting H2 and H3 together. The incoming power is connected to H1 and H4. Series-connecting the primary windings has the effect of increasing the number of turns in the primary to 400. This produces a turns ratio of 4:1. When 480 volts are connected to the primary, the secondary voltage will remain at 120.

+++++24 Control transformer connected for 480-volt operation.

+++++25 The primary windings of a control transformer are crossed.

The primary leads of a control transformer are generally cross-connected. This is done so that metal links can be used to connect the primary for 240- or 480-volt operation. If the primary is to be connected for 240-volt operation, the metal links will be connected under screws. Notice that leads H1 and H3 are connected together and leads H2 and H4 are connected together. Compare this connection with the connection shown.

If the transformer is to be connected for 480-volt operation, terminals H2 and H3 are connected. Compare this connection with the connection.

+++++26 Metal links connect transformer for 240-volt operation.

+++++27 Control transformer connected for 480-volt operation.

Transformer Core Types:

Several types of cores are used in the construction of transformers. Most cores are made from thin steel punchings laminated together to form a solid metal core. The core for a 600-mega-volt-ampere (MVA) three-phase transformer. Laminated cores are preferred because a thin layer of oxide forms on the surface of each lamination and acts as an insulator to reduce the formation of eddy currents inside the core material. The amount of core material needed for a particular transformer is determined by the power rating of the transformer. The amount of core material must be sufficient to prevent saturation at full load. The type and shape of the core generally determine the amount of magnetic coupling between the windings and to some extent the efficiency of the transformer.

The transformer is known as a core-type transformer. The windings are placed around each end of the core material. As a general rule, the low-voltage winding is placed closest to the core and the high-voltage winding is placed over the low-voltage winding.

+++++28 Core of a 600-MVA three-phase transformer.

+++++29 A core-type transformer.

+++++30 A shell-type transformer.

+++++31 A transformer with an H-type core.

+++++32 A toroid transformer.

The shell-type transformer is constructed in a similar manner to the core type, except that the shell type has a metal core piece through the middle of the window. The primary and secondary windings are wound around the center core piece with the low-voltage winding being closest to the metal core. This arrangement permits the transformer to be surrounded by the core and provides excellent magnetic coupling. When the transformer is in operation, all the magnetic flux must pass through the center core piece. It then divides through the two outer core pieces.

The H-type core shown is similar to the shell-type core in that it has an iron core through its center around which the primary and secondary windings are wound. The H core, however, surrounds the windings on four sides instead of two. This extra metal helps reduce stray leakage flux and improves the efficiency of the transformer. The H-type core is often found on high-voltage distribution transformers.

The tape-wound core or toroid core is constructed by tightly winding one long continuous silicon steel tape into a spiral. The tape may or may not be housed in a plastic container, depending on the application. This type of core does not require steel punching laminated together.

Because the core is one continuous length of metal, flux leakage is kept to a minimum. Flux leakage is the amount of magnetic flux lines that don’t follow the metal core and are lost to the surrounding air. The tape-wound core is one of the most efficient core designs available.

Transformer Inrush Current:

A reactor is an inductor used to add inductance to the circuit. Although transformers and reactors are both inductive devices, there is a great difference in their operating characteristics. Reactors are often connected in series with a low-impedance load to prevent inrush current (the amount of current that flows when power is initially applied to the circuit) from becoming excessive. Transformers, however, can produce extremely high inrush currents when power is first applied to the primary winding. The type of core used when constructing inductors and transformers is primarily responsible for this difference in characteristics.

Magnetic Domains:

Magnetic materials contain tiny magnetic structures in their molecular material known as magnetic domains. These domains can be affected by outside sources of magnetism. +++++ a magnetic domain that has not been polarized by an outside magnetic source.

Now assume that the north pole of a magnet is placed toward the top of the material that contains the magnetic domains. Notice that the structure of the domain has changed to realign the molecules in the direction of the outside magnetic field. If the polarity of the magnetic pole is changed, the molecular structure of the domain changes to realign itself with the new magnetic lines of flux. This external influence can be produced by an electromagnet as well as a permanent magnet.

In certain types of cores, the molecular structure of the domain snaps back to its neutral position when the magnetizing force is removed. This type of core is used in the construction of reactors or chokes. A core of this type is constructed by separating sections of the steel laminations with an air gap. This air gap breaks the magnetic path through the core material and is responsible for the domains returning to their neutral position once the magnetizing force is removed.

The core construction of a transformer, however, does not contain an air gap. The steel laminations are connected together in such a manner as to pro duce a very low reluctance path for the magnetic lines of flux. In this type of core, the domains remain in their set position once the magnetizing force has been removed. This type of core "remembers" where it was last set. This was the principle of operation of the core memory of early computers. It’s also the reason that transformers can have extremely high inrush currents when they are first connected to the powerline.

+++++39 Magnetic domains are left in the neutral position. Turn-off point; Magnetizing current

+++++40 Domains are set at one end of magnetic polarity.

The amount of inrush current in the primary of a transformer is limited by three factors:

1. the amount of applied voltage,

2. the resistance of the wire in the primary winding, and …

3. the flux change of the magnetic field in the core. The amount of flux change determines the amount of inductive reactance produced in the primary winding when power is applied.

+++++ a simple isolation-type transformer. The AC applied to the primary winding produces a magnetic field around the winding. As the current changes in magnitude and direction, the magnetic lines of flux change also. Because the lines of flux in the core are continually changing polarity, the magnetic domains in the core material are changing also. As stated previously, the magnetic domains in the core of a transformer remember their last set position. For this reason, the point on the waveform at which current is disconnected from the primary winding can have a great bearing on the amount of inrush current when the transformer is reconnected to power. For example, assume the power supplying the primary winding is disconnected at the zero crossing point. In this instance, the magnetic domains would be set at the neutral point. When power is restored to the primary winding, the core material can be magnetized by either magnetic polarity. This permits a change of flux, which is the dominant current-limiting factor. In this instance, the amount of inrush current would be relatively low.

If the power supplying current to the primary winding is interrupted at the peak point of the positive or negative half cycle, however, the domains in the core material will be set at that position. +++++ this condition. It’s assumed that the current was stopped as it reached its peak positive point. If the power is reconnected to the primary winding during the positive half cycle, only a very small amount of flux change can take place. Because the core material is saturated in the positive direction, the primary winding of the transformer is essentially an air-core inductor, which greatly decreases the inductive characteristics of the winding. The inrush current in this situation would be limited by the resistance of the winding and a very small amount of inductive reactance.

This characteristic of transformers can be demonstrated with a clamp-on ammeter that has a "peak-hold" capability. If the ammeter is connected to one of the primary leads and power is switched on and off several times, the amount of inrush current varies over a wide range.