Rotating Magnetic Field

The operating principle for all three-phase motors is the rotating magnetic field. There are 3 elements that trigger the magnetic field to turn. These are:

1. the fact that the voltages in a three-phase system are 120° out of phase with each other.
2. the fact that the three voltages alter polarity at routine intervals.
3. the arrangement of the stator windings around the inside of the motor.

3 phase operating principles and rotating magnetic field

rotating magnetic field Rotating Magnetic Field

3 phase stator and voltages

The image above shows 3 AC sine waves 120° out of phase with each other, and the stator winding of a three-phase motor. The stator illustrates a two-pole three phase motor. 2 pole implies that there are 2 poles per phase. AC motors do not normally have actual pole pieces like this image, but they will be utilized below to aid in comprehending how the rotating magnetic field is developed in a three-phase motor.

Notice that pole pieces A1 and A2 are found opposite each other. The exact same is true for poles B1 and B2 and C1 and C2. Pole pieces A1 and A2 are wound in such a manner that when current flows with the winding they will develop opposite magnetic polarities. This is likewise true for poles B1 and B2 and C1 and C2. The windings of poles B1 and C1 are wound in the exact same direction in relation to each other, but in opposite instructions from the winding of pole A1. The beginning end of the winding for poles A1 and A2 is connected to Line 1, the beginning end of the winding for poles B1 and B2 is linked to Line 2, and the beginning end of the winding for poles C1 and C2 is connected to Line 3. The finish ends of all three windings are joined to form a wye connection for the stator.

rotating magnetic field 2 Rotating Magnetic Field

magnetic field between poles A1 and A2

To understand exactly how the magnetic field turns around the inside of the stator (image above). A dashed line labeled A has been drawn with the 3 sine waves of the three-phase system. This line is utilized to show the condition of the 3 voltages at this point in time. The arrows drawn inside the motor indicate the greatest concentration of magnetic lines of flux; the arrows are pointing in the direction that shows magnetic lines of flux from north to south. Line 1 has actually reached its maximum peak voltage in the favorable direction and Lines 2 and 3 are less than maximum and in the adverse direction. The magnetic field is focused in between poles A1 and A2. Weaker lines of magnetic flux likewise exist in between poles B1 and B2 and C1 and C2. Also note that poles A1, B1, and C1 are all a south magnetic polarity. Poles A2, B2, and C2 form a north magnetic polarity.

rotating magnetic field 3 Rotating Magnetic Field

Magnetic field is between phases A and B

In the image above, line B is drawn at a time when the voltage of Line 3 is zero and the voltages of Lines 1 and 2 are less than optimum but opposite in polarity. The magnetic field is now concentrated in between the pole pieces of phases A and B. Phase C has no current flow at this time and therefore no electromagnetic field.

rotating magnetic field 4 Rotating Magnetic Field

Magnetic field between phases B and C

Line D indicates when Line 1 is zero and Lines 2 and 3 are less than max and opposite in polarity (image above). The electromagnetic field is now concentrated between the poles of stages B and C. At the end of one total cycle the magnetic field finishes a complete 360° of rotation. The speed of the turning magnetic field is 3600 rpm in a two-pole motor connected to a 60-Hz line.

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Transformer Core Types

There are many transformer core types used in the construction of transformers. A lot of cores are made from thin steel knock outs laminated together to form a solid metal core. Laminated cores are preferred because a thin layer of oxide forms on the area of each lamination and acts as an insulator to minimize the buildup of eddy currents inside the core material. The quantity of core material required for a specific transformer is determined by the power value of the transformer. The quantity of core product needed, must be met to prevent saturation at complete load. The type and form of the core normally determines the amount of magnetic coupling in between the windings and to some extent the performance of the transformer.

 

Transformer Core Types Transformer Core Types

Core-Type Transformer

Transformer core types

The transformer illustrated above is referred to as a core-type transformer. The windings are placed around each end of the center material. As a general guideline, the low-voltage winding is placed closest to the core and the high-voltage winding is put over the low-voltage winding.

 

Shell type transformer Transformer Core Types

Shell-type transformer

Shell-type transformer

The shell-type transformer is built in a similar manner to the core kind, except that the shell type has a metal core piece through the middle of the window (above). The primary and secondary windings are wound around the center core piece with the low-voltage winding being closest to the metal center. This plan permits the transformer to be bordered by the core and offers excellent magnetic coupling. When the transformer functions, all the magnetic flux needs to go through the center core piece. It then divides through the two external center pieces.

 

H type core transformer Transformer Core Types

H-type core transformer

H-type Core

The H-type core received (above) is similar to the shell-type core because it has an iron core through its center around which the primary and secondary windings are wound. The H core, nonetheless, surrounds the windings on 4 sides instead of two. This extra metal helps minimize stray leakage flux and enhances the efficiency of the transformer. The H-type center is commonly discovered on high-voltage distribution transformers.

 

toroid transformer Transformer Core Types

toroid transformer

Tape-Wound Core

The tape-wound core or toroid core (above) is constructed by tightly winding one long constant silicon steel tape into a spiral. The tape might or could not be housed in a plastic container, relying on the application. This sort of center does not require steel knock outs laminated together. Because the center is one constant length of metal, flux leakage is kept to a minimum. Flux leakage is the amount of magnetic flux lines that do not follow the metal center and are lost to the bordering air. The tape-wound core is one of the most reliable core designs readily available.

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Control Transformer

A typical kind of isolation transformer found throughout the electrical 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 typical type of control transformer contains two primary windings and one secondary. The primary windings are generally valued at 240 volts each, and the secondary is valued at 120 volts. This arrangement provides a 2:1 turns ratio in between each of the primary windings and the secondary. For instance, presume that each of the primary windings contains 200 turns of wire. The secondary will consist of 100 turns of wire.

control transformer  Control Transformer

control transformer 240V AC

One the primary windings in the image above is identified H1 and H2. The other is identified H3 and H4. The secondary winding is designated X1 and X2. If the primary of the transformer is to be connected to 240 volts, the two primary windings are linked in parallel by connecting H1 and H3 together and H2 and H4 together. When the primary windings are linked in parallel, the same voltage is used around both windings. This has the exact same impact as making use of 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 linked in series by connecting H2 and H3 together (shown below #2). The inbound power is linked to H1 and H4. When connecting the primary windings in series it has the effect of raising 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 continue to be at 120.

control transformer links Control Transformer

240V and 480V hookups

 

control transformer 480v Control Transformer
control transformer 480v

 

 

 

 

 

 

 

The primary leads of a control transformer are usually cross-connected. This is done so that metal links can be made use of to link the primary for 240 volts or 480 volt operation. If the primary is to be linked for 240-volt operation, the metal links will be linked under screws. Notice that leads H1 and H3 are linked together and leads H2 and H4 are linked together. Contrast this hookup with the previous connection.
If the transformer is to be linked for 480-volt operation, terminals H2 and H3 are to be linked as shown above. Contrast this link with the hookup previously imaged.

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Distribution Transformer

A typical type of isolation transformer is the distribution transformer. This kind 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 buildings. In this example, it is presumed that the primary is linked 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 around the whole 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 (image below).

transformer values

The reason this takes place is that the grounded neutral conductor becomes the center point of two out of phase voltages. If a vector diagram is drawn to show this condition, you will see that the grounded neutral conductor is linked to the center point of the two out of phase voltages (image below). Loads that are meant to operate on 240 volts, such as water heating systems, electric resistance heating rooms, and central air conditioning conditioners are connected directly throughout the lines of the secondary.
Loads that are meant to operate on 120 volts link from the center tap, or neutral, to among the secondary lines. The function of the neutral is to hold the distinction in current between the two secondary lines and keep a well balanced voltage.

Voltages are out of phase

distribution transformer neutral Distribution Transformer

The neutral carries the unbalanced load

 

 

 

 

 

 

In the image above, 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 instance, the neutral current is 6 amperes (30A - 24A = 6A).

Distribution Transformer Construction

A Distribution transformer is made the same way smaller sized transformers are made. Most utilize a “C” or “E” shaped core created from laminated sheet steel stacked and either glued together with a liquid bond or secured with steel straps. The low current, high voltage primaries are wound from enamel coated copper wire and the high current, low voltage secondaries are twisted using a thick bow of aluminum or copper insulated with resin twined paper. The whole assembly is done to heal the resin then dunked in a big powder coated steel tank. Next it is filled with high purity mineral oil, which is inert and non-conductive. The mineral oil helps eliminate heat and protects the transformer from dampness, which will stay on the surface of the oil. The storage tank is quickly depressurized to rid the transformer of any wetness that would cause arcing. A gasket is then placed on top to protect it from weather elements.

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Auto Transformer

Auto transformers are one-winding transformers. They use the same winding for both the primary and secondary. The primary winding in the image below is between points B and N and has a voltage of 120 volts put on it. If the turns of wire are counted in 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 linked in between points D and N. The secondary of this transformer consists of 40 turns of wire. If the quantity of voltage put on the load is to be calculated, the following formula can be made use of:

Auto transformer Auto Transformer

Rotary Switch
Auto-transformers have only one winding for both the primary ans secondary

 

auto transformer formula Auto Transformer

 

 

 

 

 Auto transformer calculations

Presume that the load linked to the secondary has an impedance of 10 ohms. The amount of current flow in the secondary circuit can be computed making use of the formula:

auto transformer formulas Auto Transformer

The primary current can be computed by using the same formula that was utilized to calculate primary current for an isolation sort of transformer:

primary current formula Auto Transformer

The quantity of power input and output for the auto transformer must coincide, just as they are in an isolation transformer:

primary current Auto Transformer

Now assume that the rotary switch is connected to point A. The load is now linked to 160 turns of wire. The voltage put on the load can be computed by:

auto transformer voltage Auto Transformer

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

If the rotary switch revealed above were to be gotten rid of and replaced with a sliding tap that made contact straight to the transformer winding, the turns ratio could be adjusted continuously. This kind of transformer is commonly described as a Variac or Power-stat depending on the supplier. A cutaway view of a changeable auto transformer is revealed below. The windings are coiled around a tape-wound toroid center inside a plastic case. The tops of the windings have been milled flat to offer a commutator. A carbon brush makes contact with the windings.

inside auto transformer Auto Transformer

Auto transformers are often made use of by power companies to provide a little increase or decrease to the line voltage. They help offer voltage law to big power lines. The auto transformer does have one drawback. Because the load is connected to one side of the power line, there is no line isolation between the incoming power and the load. This can trigger problems with particular sorts of equipment and have to be a factor to consider when making a power system.

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Isolation Transformers

Isolation transformers indicates that the secondary winding is physically and electrically isolated from the primary winding. There is no electric hookup between the primary and secondary winding. The transformer is magnetically combined, not electrically paired. The line isolation is frequently a very desirable attribute. Isolation transformers significantly minimizes any voltage increases that stem on the supply side, prior to they are moved 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 function of isolation only.

isolation transformer Isolation Transformers

An isolation transformer has its primary and secondary windings electrically separated from each other.

The reason that the isolation transformers can considerably reduce any voltage spikes prior to they reach the secondary is because of the grow time of current through an inductor. DC in an inductor increases at a rapid rate. As the current rises in value, the broadening magnetic field cuts through the conductors of the coil and causes a voltage that is opposed to the used voltage. The amount of induced voltage is proportional to the rate of change of current.

transformer voltage spike Isolation Transformers

DC through an inductor. Short duration voltage spikes

This simply implies that the much faster current tries to increase, the greater the opposition to that increase is. Spike voltages and currents are generally of very brief period, which means that they raise in value very quickly. This rapid modification of value triggers the opposition to the change to enhance just as quickly. By the time the spike has been transferred to the secondary winding of the transformer, it has been removed or greatly lowered.

Basic Operation of Isolation Transformers

isolation transformer magnetic Isolation Transformers

Isolation Transformer Magnetic Field

One winding of  an isolation transformer has been linked to an AC supply, and the various other winding has actually been connected to a load. As current boosts from absolutely nothing to its peak positive point, a magnetic field expands outward around the coil. When the current decreases from its peak positive point towards zero, the magnetic field collapses. When the current boosts towards its negative peak, the magnetic field once again broadens however with an opposite polarity of that previously. The area again breaks down when the current lowers from its negative peak toward zero. This continuously broadening and breaking down magnetic field cuts the windings of the primary and induces a voltage into it. This induced voltage opposes the used voltage and limits the current flow of the primary. When a coil induces a voltage into itself, it is called self-induction.

isolation transformer construction Isolation Transformers

Construction of Isolation Transformer

The standard building of isolation transformers is revealed above. A metal center is made use of to provide great magnetic coupling between the two windings. The center is usually made of lamination’s stacked together. Laminating the center helps minimize power losses triggered by eddy current induction.

Excitation Current

There will always be some amount of current flow in the primary of any voltage transformer despite type or size even if there is no load linked to the secondary. This current flow is called the excitation current of the transformer. The excitation current is the quantity of current needed to allure the core of the transformer. The excitation current remains constant from no load to complete load. As a general rule, the excitation current is such a small part of the full load current that it is commonly left out when making computations.

Mutual Induction

Because the secondary windings of an isolation transformer are wound on the same core as the primary, the electromagnetic field produced by the primary winding also cuts the windings of the secondary. This continuously altering electromagnetic field causes a voltage into the secondary winding. The ability of one coil to cause a voltage into another coil is called shared induction. The amount of voltage caused in the secondary is identified by the ratio of the number of turns of wire in the secondary to those in the primary.

isolation transformer mutual induction Isolation Transformers

isolation transformer mutual induction

Multiple Tapped Windings

It is not uncommon for isolation transformers to be created 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 revealed above includes a secondary winding rated at 24 volts. The primary winding consists of several taps, nevertheless. One of the primary lead wires is labeled C and is the typical for the various 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 linked to different primary voltages without altering the value of the secondary voltage. In this example, it is presumed that the secondary winding has a total amount of 120 turns of wire. To preserve the correct turns ratio, the primary would have 600 turns of wire in between C and 120 volts, 1040 turns between C and 208 volts, and 1200 turns between C and 240 volts.

Multiple Tapped Windings Isolation Transformers

Multiple Tapped Transformer Primary Winding

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Single Phase Transformers

Transformers are among the most common devices discovered in the electric field. They vary in size from less than one cubic inch to the dimension of rail vehicles. Their ratings can easily vary from milli-volt-amperes (mVA) to giga-volt-amperes (GVA). It is crucial that anyone operating in the electric industry have an understanding of transformer types and connections. This article offers transformers intended for use in single phase installations (hence the term single phase transformers). There are 2 major sorts of single phase transformers, isolation transformers and auto-transformers.

A transformer is a magnetically run equipment that could alter values of voltage, current, and impedance without a change of regularity. Transformers are the most efficient machines known to man. Their effectiveness commonly range from 90 % to 99 % at complete load. Transformers can be broken down in to 3 categories:.

1. Isolation transformers
2. Auto-transformers
3. Current transformers

Values of a transformer are proportional to its turn ratio

All values of a transformer are equal to its turns proportion. This does not mean that the specific number of turns of wire on each winding need to be known to determine various worth’s of voltage and current for a transformer. Exactly what ought to be known is the proportion of turns. As an example, assume a transformer has 2 windings. One winding, the primary, has 1000 turns of wire; and the other, the secondary, has 250 turns of wire. The turns ratio of this transformer is 4 to 1, or 4:1 (1000 turns / 250 turns = 4). This indicates there are four turns of wire on the primary for every one turn of cable on the secondary.

Various formulas can be utilized to discover the values of voltage and current for a transformer. The following is a checklist of basic solutions.

Transformer Formulas

The primary winding of a transformer is the power input winding. It is the winding that is connected to the inbound power quantity. The secondary winding is the load winding, or output winding. It is the edge of the transformer that is connected to the forced load.

 

Isolation transformers

isolation transformer Single Phase Transformers

An isolation transformer has its primary and secondary windings electrically separated from each other.

Isolation transformers implies that the secondary winding is physically and electrically separated from the primary winding. There is no electric link in between the primary and secondary winding. This transformer is magnetically paired, not electrically coupled. This line isolation is frequently a very preferable feature. The isolation transformer greatly lowers any voltage spikes that originate on the quantity side before they are moved to the load edge. Some isolation transformers are built with a turns ratio of 1:1. A transformer of this type has the very same input and outcome voltages and is used for the purpose of isolation only.

transformer voltage spike Single Phase Transformers

DC through an inductor. Short duration voltage spikes

The reason that isolation transformers can considerably reduce any sort of voltage increases prior to they hit the secondary is due to the rise time of current through an inductor. DC in an inductor increases at an exponential rate. As the current boosts in value, the broadening electromagnetic field cuts through the conductors of the coil and generates a voltage that is opposed to the used voltage. The quantity of induced voltage is symmetrical to the value of modification of current. This merely indicates that the faster current tries to increase, the higher the opposition to that rise is. Spike voltages and current flows are typically of really short duration, meaning that they enhance in value incredibly rapidly. This quick change of value induces the opposition to the change to increase just as swiftly. By the time the spike has actually been moved to the secondary winding of the transformer, it has actually been gotten rid of or substantially lessened.

Auto-transformers

Auto-transformers are one-winding transformers. They utilize the exact same winding for both the primary and secondary. The primary winding is in between stages B and N and has a voltage of 120 volts applied to it. If the turns of cable are counted between points B and N, it can be seen that there are 120 turns of wire. Now presume that the selector change is set to direct D. The load is now connected in between stages D and N. The secondary of this transformer contains 40 turns of cable.

Auto transformer Single Phase Transformers

Auto-transformers have only one winding for both the primary ans secondary

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