The DIAC

A DIAC is a device containing five semiconductors layers (PNPNP) that behaves like two PN PN diodes connected back to back. It can conduct in either direction once the breakover voltage is exceeded. The symbol for DIAC is shown below.

It turns on when the applied voltage in either direction exceeds Vbo.Once it is turned on , a DIAC remains on until its current falls below IH.

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The Three Wire Thyrsitor or SCR

The most important member of the thyristor family is the three wire thyristor , also known as the silicon controlled rectifier or SCR. This device was developed and given by the name SCR by the General Electric Company in 1958. The name thyrsistor was adopted later by the international Electrotechnical comission. The symbol for a three wire thyristor or SCR is shown below.

As the name suggests the SCR is a controlled rectifier or a diode. Its voltage current characterisctics with the gate lead open is the same as that of a PN PN diode.
               Three wire thyrsitor or SCRs are by far the most common devices used in power control circuits. They are widely used for switching or rectification applications and are currently available in ratings ranging from a few amperes upto a maximum of about 3000A.
In summary , an SCR is

• Turns on when a voltage Vd applied to it exceeds Vbo.
• Has a breakover voltage Vbo whose level is controlled by the amount of gate current Ig present in the SCR.
• Turns off when the current Id flowing it drops below Ih.
• Blocks all current flow in the reverse direction until the maximum reverse voltage is exceeded.

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PN-PN Diode

Thyristor is the generic name given to a family of semiconductors devices which are made up of four semiconductor layers. One member of this family is the two wire thyristor also known as PN-PN diode or trigger diode. This device's name in the IEEE standard for the graphic symbols is reverse blocking diode type thyristor.Its symbol is shown in figure below.

             The PN-PN diode is a rectifier or a diode with an unusual voltage current characteristics in the forward biased region. The characteristics curve consists of three regions:

• The reverse blocking region
• The forward blocking region
• The conducting region

          In the reverse blocking region the PN PN diode behaves as an ordinary diode and all current flow until the reverse breakdown voltage is reached.In the conducting region the PN PN diode again behaves as an ordinary diode , allowing large amounts of current to flow with very little voltage drop.

In a summary PN-PN diode is :

• Turns on when the applied voltage Vd exceeds Vbo
• Turns off when the current Id drops below Ih
• Blocks all currents flow in the reverse direction untill the maximum reverse voltage exceeds

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Diodes

The Question is what is a diode ? A diode is a semiconductor device designed to conduct current in one direction only.The symbol for this device is

A diode is designed to conduct current from its anode to its cathode , but not in the opposite direction.
                   When a voltage is applied to the diode in the forward direction , a large current flow results.When a voltage is applied to the diode in the reverse direction , the current flow is limited to very very small value. If a large enough reverse voltage is applied to the diode , eventually the diode will break down and allow current to flow in the reverse direction.
                   Diodes are rated by the amount of power they can safely dissipate and by the maximum reverse voltage that they can take before breaking down. The power dissipated by the diode during forward operation is equal to the forward voltage drop across the diode times the current flowing through it.The maximum reverse voltage of a diode is known as its peak inverse voltage (PIV).
                  Diodes are also rated by thier switching time.that is by the time it takes to go from the off state to the on state and vice versa. Because power diodes are large , high power devices with a lot of stored charge in their junctions they switch states much more slowly then the diodes in electronic circuits.For the very fast switching applications , special diodes called fast recovery high speed diodes are employed.

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Introduction to Power Electronics

Here now , we are going to discuss a brief introduction to the high power electronics components and to the circuits in which they are employed. Several major types of semiconductor diodes and devices are used in motor control circuits.

                    Among them the most important are:

Diode

PNPN diode (Two wire thyristor)

SCR diode (Three wire thyristor)

GTO thyrsistor

DIAC

TRIAC

Power Transistors

Insulated gate bipolar transistor

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Single Phase Transformers

The ideal transformers described can ofcourse never actually be made.What can be produced are real transformers- two or more coils of wire physically wrapped around a ferromagnetic core.The characteristics of real transformer approximate the characteristics of an ideal transformer , but only to a degree.This section deals with the behaviour of real transformers.
To understand the operation of real transformer , see figure ,

it shows a transformer consisting of two coils of wire wrapped around a transformer core.The primary of the transformer is connected to an ac power source and the secondary winding is open circuited.The hysteresis curve of the transformer is given.

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The Ideal Transformer

An ideal transformer is a loseless device with an input winding and an output winding.The relationships between the iput voltage and the output voltage and between the input current and the output current are given by two simple equations.

             The transformer shown above has Np turns of wire on its primary side and Ns turns wire on its secondary side.The relationship between the voltage Vp(t) applied to the primary side of the transfomer and the voltage Vs(t) produced on the secondary side is
                                                   
                                                       Vp(t)  = Np
                                                       Vs(t)      Ns
where a is defined to be the turns ratio of the transformer.
                                                          a= Np
                                                               Ns
The relationship between the current i p(t) flowing into the primary side of the transformer and the current i s(t) flowing out of the secondary side of the transformer is
                                                   Np ip(t) = Ns is(t)
or
                                                ip(t) = 1
                                                is(t)     a

Notice that the phase angle of Vp is the same as angle of Vs and the phase angle of Ip is the same as the phase angle of Is.
Dot And Cross Convention :
                                                                      The realtionship in dot and cross product will become.

• If the primary voltage is positive at the dotted end of the winding with respect to the undotted end , then the secondary voltage will be positive at the dotted end also. Voltage polarities are the same with respect to the dots on each side of the core.
• If the primary current of the transformer flows into the dotted end of the primary winding , the secondary current will flow out of the dotted end of the secondary winding.

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Types Of Transfomer

                            Power  transformers are given a variety of different names , depending on their use in power systems.

Unit Transformer

A transformer connected to the output of generator and used to step up  its voltage is to transmission levels ( 110 kV) is sometimes called a unit transformer.

Substation Transformer

The transformer at the other end of the transmission line , which steps the voltage down from transmission levels to distribution level (from 2.3 to 34.5 kV) is called a substation transformer.

Distribution Transformer

The transformer that takes the distribution voltage and steps it down to the final voltage at which the power actually is used (110,208,220 kV) is called distribution transformer.

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Construction Of Transformer

The principle purpose of a transformer is to convert AC power at one voltage level to AC power of the same frequency at another level.Transformers are also used for a variety of purposes e.g voltage sampling , current sampling and impedance transformer , but i will describe first power transformer.
Power transformers are constructed on one of the two types of cores.One type of construction consists of a simple rectangular laminated piece of steeel with the transformer windinngs wrapped around two sides of the rectangular core. This type of construction is known as core form as shown.

                The primary and secondary windings in a physical transformer are wrapped one on top of the other with low voltage winding innermost.Such an arrangement serves two purposes.

• It simplifies the problem of insulating the high voltage winding from the core.
• It results in much less leakage flux.

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Transformers

A transformer is a device that changes AC electric power at one voltage to AC electric power at another voltage level through the action of a magnetic field.It consists of two or more coils of wire wrapped around a common ferromagnetic core.These coils are usually not directly connected.The only connections between the coils is the magnetic flux present within the core.

One of the transformer windings is connected to a source of AC electric power and the second transformer winding supplies electric power to loads.The transformer winding connected to the power source is called the primary windings or input windings , and the winding connected to the loads is called the secondary winding or output winding.If there is a third winding on the transformer , it is called tertiary winding

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Introduction to Electrical Machinery

An electrical machinery is a device that can convert either mechanical energy to electrical energy or electrical energy to mechanical energy.When such a device is used to convert mechanical energy to electrical energy , it is called generator and when it converts electrical energy to mechanical energy , it is called a motor. Since any given electrical machine can convert power in either direction , any machine can be used as either generator or a motor.
          The transformer is an electrical device that is closely related to electrical machines.It converts ac electrical energy at one level to ac electrical energy at another voltage level.Since transformers operate on the same principles as generators and motors, depending on the action of a magnetic field to accomplish the change in voltage level , they are usually studied together with motors and generators.
          These three types of electric devices are ubtiquitous in modern daily life. Electric motors in the home run refrigerators , freezers , vaccume cleaners , blenders, air conditioners, fans and many similar appliance.In the workplace , motors provide the motive power for almost all tools. Of course, generators are necessary to supply the power used by all these motors.

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WELCOME

Hi, I am Stephen J. Chapman .I am here with my new and revised book for newbie’s of electrical devices and there issues in detail.

I hope you will love to find it useful in all manners and the material I wrote for the beginners to get acquainted with all AC and DC Electric Machinery Fundamentals.
Hoping to get a positive reply from all people getting interest in it.
Thanks

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