AC System and DC System

DC system :

Advantages of DC Transmission :

There are two conductors used in DC transmission while three conductors required in AC transmission. There are no Inductance and Surges (High Voltage waves for very short time) in DC transmission. Due to absence of inductance, there are very low voltage drop in DC transmission lines comparing with AC (if both Load and sending end voltage is same). A DC System has a less potential stress over AC system for same Voltage level. Therefore, a DC line requires less insulation. In DC System, There is no interference with communication system. In DC Line, Corona losses are very low. In High Voltage DC Transmission lines, there are no Dielectric losses. In DC Transmission system, there are no difficulties in synchronizing and stability problems. DC system is more efficient than AC, therefore, the rate of price of Towers, Poles, Insulators, and conductor are low so the system is economical. In DC System, the speed control range is greater than AC System. There is low insulation required in DC system (about 70%). The price of DC cables is low (Due to Low insulation) In DC Supply System, the Sheath losses in underground cables are low. DC system is suitable for High Power Transmission based on High Current transmission.

The length DC Transmission lines are greater than AC lines.

Disadvantages of DC Transmission:

Due to commutation problem, Electric power can’t be produce at High (DC) Voltage. For High Voltage transmission, we cannot step the level of DC Voltage (As Transformer cannot work on DC). There is a limit of DC Switches and Circuit breakers (and costly too) Motor generator set is used for step down the level of DC voltage and the efficiency of Motor-generator set is low than transformer. So the system makes complex and costly. The level of DC Voltage cannot be change easily. So we cannot get desire voltage for Electrical and electronics appliances (such as 5 Volts, 9 Volts 15 Volts, 20 and 22 Volts etc.) directly from Transmission system. AC system : Advantages of AC Transmission System : AC Circuit breakers is cheap than DC Circuit breakers. The repairing and maintenance of AC substation is easy and inexpensive than DC Substation. The Level of AC voltage may be increased or decreased step up and Step down transformers. Disadvantages of AC Transmission  System: In AC line, the size of conductor is greater than DC Line. The Cost of AC Transmission lines are greater than DC Transmission lines. Due to Skin effect, the losses in AC system are more. In AC Lines, there is Capacitance, so continuously power loss when no load on lines or Line is open. Other line losses are due to inductance. More insulation required in AC System Also corona Losses occur In AC System,   There is telecommunication interference in AC System. There are stability and synchronizing problems in AC System. DC System is more efficient than AC System. There are also re-active power controlling problems in AC System follow us : https://www.youtube.com/channel/UC8uzr-PXQxLpAiVKA7S1rRQ?disable_polymer=true https://www.facebook.com/Engi.Prog/?ref=bookmarks

Classification and Types Of Power Diodes

Major Classification of Diodes:

·        Standard Diodes or General Purpose Diodes:

Standard or general purpose diodes have a comparatively high reverse recovery time, when compared to other diodes. Due to this reason they are used in applications which are not time sensitive and generally run on low speeds.

Usually the reverse recovery time for general purpose diodes varies between 20 micro seconds to 30 micro seconds which is quite a lot. Typical low speed applications for general purpose diodes include the power diode being used as a rectifier or in a converter, where the frequency input is quite low.

·        Fast Recovery Diodes

As their name suggests, these are the type of power diodes which have a relatively faster reverse recovery time, which usually varies from 2 micro seconds to 5 micro seconds. With such a fast recovery time, they can be easily used in high speed switching applications where the time is of great importance.

Due to their property of fast reverse recovery, they are also comparatively expensive as compared to the general purpose diodes.

·        Scotty Diodes:

Sometimes we face problem in charge storage in a pn-junction. This thing can be minimized to a great extent in a Scotty diode. A Scotty diode sets a barrier potential, i.e. a metal layer is deposited on n-type silicon.

As the rectification depends upon the majority charge carriers, so this layer prevents the recombination of these majority charge carriers, and a fast recovery can also be achieved in this way.

Types of Power Diodes:

·        Silicon Diodes

The diode detector verifies the radiation response and electrical characteristics of the PIN diode, which is a diode made out of silicon. Silicone is capable of handling high electrical fields without losing any of the electricity.

·        PIN Diodes

PIN photodiodes are used as radiation detectors that are used for X-ray measurements and for charged particles. The PIN diode spectrometer uses the radiation detector, signal processing electronics and a computer that analyzes and stores the information. Bias voltage is delivered to the detector. The charged particles or X-rays are sent through a preamplifier, an amplifier and a multichannel analyzer system (MCA). The charged particle creates a charged motion that influences the electric field, which produces a current pulse for the external circuit. An integrated current pulse is created that is proportional to the energy lost, whether that energy be the charged particles or the X-ray photons. The MCA is either a dedicated instrument or an analog-to-digital converter that is connected and interfaced to the computer.

·        Reverse-Biased Diodes

Diode detectors can be reverse-biased, which causes very little current to pass through the diode until the reverse breakdown voltage is surpassed, according to Newport.

·        Small Signal Diodes

Diode detectors operate at low voltages in the square law region. These diodes are known as small signal types, according to Newport. The higher voltage diodes operate more in the linear region and are known as the large signal types.

·        Log Detector Amplifiers

Log detector amplifiers use a series of amplifiers and diode detectors to detect output voltage. These detectors have a very good range for detection, but they are very large, which can make them inconvenient in some circumstances.

·        Gunn Diodes

Gunn diodes are diodes that are used as local oscillators that have negative resistance. This power diode produces very little noise and can operate at a very high frequency. These diodes are commonly used with microwaves because of the diode's efficiency and the frequency of the operation.

·        Epicap

The epicap a voltage-variable diode used for electronic tuning. When circuits are tuned electronically and electrically, they can function very quickly and reliably in a small form, according to Educypedia. These diodes are also more thermally stable and have less current leakage.

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Brushless Dc motor

In order to make the operation more reliable, more efficient, and less noisy the recent trend has been to use brushless D.C (BLDC) motors. They are also lighter compared to brushed motors with the 

same power output.

Why BLDC motors?

The brushes in conventional D.C motors wear out over the time and may cause sparking. This is illustrated in the Fig.1. As a result the conventional D.C motors require occasional maintenance. Controlling the brush sparking in them is also a difficult affair.

Figure.1))

Thus the brushed D.C motor should never be used for operations that demand long life and reliability. Fort this reason and the other reasons listed in the introduction, BLDC motors are used in most of the modern devices. Efficiency of a BLDC motor is typically around 85-90%, whereas the conventional brushed motors are only 75-80% efficient. BLDC motors are also suitable for high speed applications (10000 rpm or above). The BLDC motors are also well known for their better speed control.

The Basic working

The rotor and stator of a BLDC motor are shown in the Fig.2. It is clear that, the rotor of a BLDC motor is a permanent magnet.

(figure.2)

The stator has a coil arrangement, as illustrated; The internal winding of the rotor is illustrated in the Fig.3 (core of the rotor is hidden here). The rotor has 3 coils, named A, B and C.

(figure.3)        

Out of these 3 coils, only one coil is illustrated in the Fig.4 for simplicity. By applying DC power to the coil, the coil will energize and become an electromagnet.

(figure.4)

The operation of a BLDC is based on the simple force interaction between the permanent magnet and the electromagnet. In this condition, when the coil A is energized, the opposite poles of the rotor and stator are attracted to each other (The attractive force is shown in green arrow). As a result the rotor poles move near to the energized stator.

(figure.5)

As the rotor nears coil A, coil B is energized. As the rotor nears coil B, coil C is energized. After that, 

coil A is energized with the opposite polarity (compare the last part of Fig.6 with Fig.5).

(figure.6)

This process is repeated, and the rotor continues to rotate. The DC current required in the each coil is shown in the following graph