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Messages - Md. Khalid Hasan

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Electrical Machine / Starting Of Slip-Ring Motors
« on: April 10, 2019, 04:08:10 PM »
Slip-ring motors are started with full line voltage, as external resistance can be easily added in the rotor circuit with the help of slip-rings. A star connected rheostat is connected in series with the rotor via slip-rings as shown in the fig. Introducing resistance in rotor current will decrease the starting current in rotor (and, hence, in stator). Also, it improves power factor and the torque is increased. The connected rheostat may be hand-operated or automatic.
As, introduction of additional resistance in rotor improves the starting torque, slip-ring motors can be started on load.
The external resistance introduced is only for starting purposes, and is gradually cut out as the motor gathers the speed.

Electrical Machine / Starting Of Squirrel Cage Motors
« on: April 10, 2019, 04:07:40 PM »
Starting in-rush current in squirrel cage motors is controlled by applying reduced voltage to the stator. These methods are sometimes called as reduced voltage methods for starting of squirrel cage induction motors. For this purpose, following methods are used:
By using primary resistors
Star-delta switches
1. Using Primary Resistors:
primary resistors starting of induction motor
Obviously, the purpose of primary resistors is to drop some voltage and apply a reduced voltage to the stator. Consider, the starting voltage is reduced by 50%. Then according to the Ohm's law (V=I/Z), the starting current will also be reduced by the same percentage. From the torque equation of a three phase induction motor, the starting torque is approximately proportional to the square of the applied voltage. That means, if the applied voltage is 50% of the rated value, the starting torque will be only 25% of its normal voltage value. This method is generally used for a smooth starting of small induction motors. It is not recommended to use primary resistors type of starting method for motors with high starting torque requirements.
Resistors are generally selected so that 70% of the rated voltage can be applied to the motor. At the time of starting, full resistance is connected in the series with the stator winding and it is gradually decreased as the motor speeds up. When the motor reaches an appropriate speed, the resistances are disconnected from the circuit and the stator phases are directly connected to the supply lines.
2. Auto-Transformers:
auto transformer starting of induction motors
Auto-transformers are also known as auto-starters. They can be used for both star connected or delta connected squirrel cage motors. It is basically a three phase step down transformer with different taps provided that permit the user to start the motor at, say, 50%, 65% or 80% of line voltage. With auto-transformer starting, the current drawn from supply line is always less than the motor current by an amount equal to the transformation ratio. For example, when a motor is started on a 65% tap, the applied voltage to the motor will be 65% of the line voltage and the applied current will be 65% of the line voltage starting value, while the line current will be 65% of 65% (i.e. 42%) of the line voltage starting value. This difference between the line current and the motor current is due to transformer action. The internal connections of an auto-starter are as shown in the figure. At starting, switch is at "start" position, and a reduced voltage (which is selected using a tap) is applied across the stator. When the motor gathers an appropriate speed, say upto 80% of its rated speed, the auto-transformer automatically gets disconnected from the circuit as the switch goes to "run" position.
The switch changing the connection from start to run position may be air-break (small motors) or oil-immersed (large motors) type. There are also provisions for no-voltage and overload, with time delay circuits on an autostarter.

Small three phase induction motors can be started direct-on-line, which means that the rated supply is directly applied to the motor. But, as mentioned above, here, the starting current would be very large, usually 5 to 7 times the rated current. The starting torque is likely to be 1.5 to 2.5 times the full load torque. Induction motors can be started directly on-line using a DOL starter which generally consists of a contactor and a motor protection equipment such as a circuit breaker. A DOL starter consists of a coil operated contactor which can be controlled by start and stop push buttons. When the start push button is pressed, the contactor gets energized and it closes all the three phases of the motor to the supply phases at a time. The stop push button de-energizes the contactor and disconnects all the three phases to stop the motor.
In order to avoid excessive voltage drop in the supply line due to large starting current, a DOL starter is generally used for motors that are rated below 5kW.

Electrical Machine / Working Principle And Types Of An Induction Motor
« on: April 10, 2019, 04:01:45 PM »
Induction Motors are the most commonly used motors in many applications. These are also called as Asynchronous Motors, because an induction motor always runs at a speed lower than synchronous speed. Synchronous speed means the speed of the rotating magnetic field in the stator.

There basically 2 types of induction motor depending upon the type of input supply - (i) Single phase induction motor and (ii) Three phase induction motor.

Or they can be divided according to type of rotor - (i) Squirrel cage motor and (ii) Slip ring motor or wound type

Difference In Working
Synchronous motor: Stator poles rotate at the synchronous speed (Ns) when fed with a three phase supply. The rotor is fed with a DC supply. The rotor needs to be rotated at a speed near to the synchronous speed during starting. If done so, the rotor poles get magnetically coupled with the rotating stator poles, and thus the rotor starts rotating at the synchronous speed.
Synchronous motor always runs at a speed equal to its synchronous speed.
i.e. Actual speed = Synchronous speed
or  N = Ns = 120f/P
Learn more about working of a synchronous motor here.
Induction motor: When the stator is fed with two or three phase AC supply, a Rotating Magnetic Field (RMF) is produced. The relative speed between stator's rotating magnetic field and the rotor will cause an induced current in the rotor conductors. The rotor current gives rise to the rotor flux. According to Lenz's law, the direction of this induced current is such that it will tend to oppose the cause of its production, i.e. relative speed between stator's RMF and the rotor. Thus, the rotor will try to catch up with the RMF and reduce the relative speed.
Induction motor always runs at a speed which is less than the synchronous speed.
i.e. N < Ns
Learn more about working of induction motor here.

ঝড়-বৃষ্টির কারণে রাজধানী ঢাকায় বাতাসের মান উন্নত হয়েছে। এয়ার কোয়ালিটি ইনডেক্সে (একিউআই) মঙ্গলবার সন্ধ্যা ৭টা ২ মিনিটে ২২তম অবস্থানে উঠে এসেছে ঢাকা। আগে এর স্কোর ছিল ৮২, যা নির্দেশ করে বাতাসের গুণগত মান মাঝারি মানের।

মঙ্গলবার সন্ধ্যায় রাজধানীতে কালবৈশাখী ঝড় আঘাত হানে। এ সময় বাতাসের তীব্রতা ছিল ঘণ্টায় ৮৫ কিলোমিটার। সন্ধ্যা ৬টা থেকে পরবর্তী এক ঘণ্টায় ১১ মিলিমিটার বৃষ্টিপাত রেকর্ড করেছে আবহাওয়া অধিদপ্তর।

এদিকে আজ সকাল ৮টায় এয়ার কোয়ালিটি ইনডেক্সে (একিউআই) ঢাকার স্কোর ৮০ তে অবস্থান করলেও ৯টায় তা ৮৩তে দাঁড়ায়।

অন্যদিকে ১৫৩ স্কোর পেয়ে দূষিত বাতাসের শহরগুলোর তালিকায় প্রথমে রয়েছে দিল্লি। এর পরে আছে ব্রাসেলস ও চিয়াঙ মাই।

উল্লেখ্য, প্রতিদিনের বাতাসের মান নিয়ে তৈরি করা একিউআই সূচক একটি নির্দিষ্ট শহরের বাতাস কতটুকু নির্মল বা দূষিত সে সম্পর্কে মানুষকে তথ্য দেয়। এছাড়া বাতাস কোন ধরনের স্বাস্থ্য ঝুঁকি তৈরি হতে পারে কিনা তা জানায়।

ঢাকা দীর্ঘদিন ধরেই দূষিত বাতাস নিয়ে হিমশিম খাচ্ছে। গ্রীষ্মকালে শহরটিতে বায়ু দূষণ চরমে উঠলেও বর্ষায় অবস্থার উন্নতি দেখা যায়।

Signal Processing & Control System / Re: Adaptive cruise control
« on: April 10, 2019, 02:58:56 PM »
Thank you for sharing.

Signal Processing & Control System / Re: lane keeping system
« on: April 10, 2019, 02:58:43 PM »
Thank you for sharing.

আবহাওয়া সম্পর্কে নির্ভুল পূর্বাভাস পেতে ২০০ উপজেলায় স্বয়ংক্রিয় আবহাওয়া স্টেশন স্থাপনে স্থানীয় সরকার বিভাগের সঙ্গে চুক্তি করেছে প্রতিরক্ষা মন্ত্রণালয়।

গতকাল মঙ্গলবার সচিবালয়ে স্থানীয় সরকার বিভাগের সম্মেলনকক্ষে এই চুক্তি স্বাক্ষর হয়।

উপজেলা পরিষদের ভূমি ব্যবহার করে কৃষিভিত্তিক স্বয়ংক্রিয় আবহাওয়া পর্যবেক্ষণাগার স্থাপনের জন্য প্রয়োজন হবে মাত্র ২২০ বর্গফুট জায়গা।

স্থানীয় সরকার বিভাগের সিনিয়র সচিব এস এম গোলাম ফারুক এবং প্রতিরক্ষা মন্ত্রণালয়ের সচিব আখতার হোসেন ভূঁইয়া চুক্তিতে স্বাক্ষর করেন।

অনুষ্ঠানের প্রধান অতিথি স্থানীয় সরকার, পল্লী উন্নয়ন ও সমবায়মন্ত্রী মো. তাজুল ইসলাম বলেন, অঞ্চলভিত্তিক আবহাওয়ার পূর্বাভাস প্রাপ্তি অর্থনৈতিক কর্মকাণ্ডকে বেগবান করবে। বিশেষত কৃষিক্ষেত্রে আবহাওয়ার পূর্বাভাস কাজে লাগিয়ে লাভবান হওয়া যাবে।

অনুষ্ঠানে আরও উপস্থিত ছিলেন স্থানীয় সরকার প্রতিমন্ত্রী স্বপন ভট্টাচার্যসহ সংশ্লিষ্ট মন্ত্রণালয় ও অধিদপ্তরের ঊর্ধ্বতন কর্মকর্তারা।

In a report released today entitled Rebuild The Power System That Supports Japan, Keidanren underlined the importance of energy policy to the resource-poor country.

Energy is "the economic backbone of the nation" and so discussions on the country's energy future should be held in public, based on long-term perspectives, scientific information and modelling, it said.

Japan is in a "crisis situation" of low investment and in the electricity business it has stagnated, except in certain areas, such as nuclear safety measures, it said.

"Japan stands at a crossroads as to whether it can maintain the foundations of technology and industrial competitiveness built up as a small resource country and a technological nation," it said, adding that investment in R&D, generating reserve and new capacity construction had come to a halt.
In the current situation, it is challenging to maintain the pre-requisites of energy policy -  secure supply, affordability and environmental acceptability, it said. Meanwhile Japan is expecting to evolve into what Keidanren called Society 5.0, whereby social problems can be solved using advanced technology, Japan's population will age and decline, while in common with other places it expects decentralised energy sources supported by digitalisation to become very significant in regional and national supply. A highly decentralised future could see great variety in Japan, given its geography, distribution of renewable sources and dense clusters of population and industry. If it is to be successful, such a system needs planning.
For these reasons the report calls for the government to provide clarity for energy policy after 2030, so that business can set "a timely and proportionate" investment strategy.

Recently, growing electrical energy market and increasing integration of renewable energy sources (RES) in power systems have lead to new challenges on network planning step and operation, thus it is required to investigate and analyze properly the impacts of integrated RES on the power system. In this paper, the electricity transmission network with wind farms (WF) is modeled. For the grid model, a part of Izmir region is chosen due to the amount of installed generation plants based RES especially wind energy in this region. The comparison between unexpected variations to voltage profile of the power system before and after RES integration to the power system is demonstrated. In the modeling of the electricity transmission network with RES, Digsilent/Power Factory is used as software. The different case studies in integration of different amounts of RES are implemented on the developed grid model. As a result of the conducted case studies, effects of RES on existing power system are evaluated and graphics obtained from the simulation are presented. Especially, the voltage profile of power system is examined.

Power System and Renewable Energy / Renewable Energy in Power Systems
« on: April 10, 2019, 02:48:40 PM »
Renewable Energy (RE) sources differ from conventional sources in that, generally they cannot be scheduled, they are much smaller than conventional power stations and are often connected to the electricity distribution system rather than the transmission system.  The integration of such time variable ‘distributed’ or ‘embedded’ sources into electricity networks requires special consideration.
This new book addresses these special issues and covers the following:

The characteristics of conventional and RE generators with particular reference to the variable nature of RE from wind, solar, small hydro and marine sources over time scales ranging from seconds to months
The power balance and frequency stability in a network with increasing inputs from variable sources and the technical and economic implications of increased penetration from such sources  with special reference to demand side management

The increasing penetration of photovoltaic (PV) systems in distribution networks often causes overvoltage problems. One solution to address this issue is the provision of reactive power (RP) by the PV converters. This can cause increased power losses on the PV converters leading to additional operational costs. However, the manufacturers of commercially available PV converters provide data regarding the converter losses only under unity power factor (PF). The data are also limited regarding the technical details of the PV converters. This paper presents a methodology to estimate analytically the power losses in two-stage PV converters under an RP provision based on the efficiency curves at PF = 1 for different PV voltages and the limited information given in the PV converter datasheet. The losses are separately estimated for the dc–dc converter and the dc–ac inverter, because the losses on the former are not affected by the RP, while the losses on the latter are. The method is validated with field measurements of PV converter losses under RP provision and with detailed simulations.

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