This document introduces basic components of DC circuits. It defines a complete circuit as one where electron flow can travel from negative to positive terminals, and an incomplete circuit as one where flow is disrupted. Circuits must have four basic components: conductors to carry current, a load that uses power, a power supply to provide energy, and optional controllers to manage the circuit. Schematic symbols represent components in schematic diagrams, which are blueprints showing a circuit's construction. Common symbols are introduced for wires, loads like LEDs, power sources like batteries and cells, and switches as controllers. Assignments include an electronics reference chart to document websites on topics.
Applications of power electronics in HVDCKabilesh K
Role of Power electronics in HVDC and Transmission system. What are the components of Power electronics used in HVDC. Types of HVDC Links. Advantages of HVDC over HVAC.
This document provides an overview of power system planning and load forecasting. It discusses that load forecasting is the first crucial step for any power system planning study, as it involves predicting future load behavior. It describes different load forecasting techniques including extrapolation methods that use historical load data and trend curves, and correlation methods that relate loads to economic and demographic factors. The document also discusses factors that affect load forecasting like time of day, weather, customer class, and economics. Overall it provides a high-level introduction to the concepts and process of load forecasting for power system planning.
This document discusses power system security. It defines power system security as the probability of the system operating within acceptable ranges given potential changes or contingencies. It outlines the key steps in power system security including: (1) monitoring the current system state, (2) contingency analysis to evaluate potential risks, and (3) corrective action analysis to maintain security through preventative or automatic corrective actions.
Harmonics create pollution in our power system just like carbon dioxide and other gases create air pollution. It has adverse effects directly or indirectly on equipment like motors, transformers, induction heaters, etc. It leads to energy loss due to poor power factor.
Following content has been covered:
- The definition of harmonics is briefly interpreted.
- Factors which are responsible for harmonics current generation is discussed.
- Often the failure of equipment like motors, transformer, etc. has been put on harmonics current. But this is not always the case. This ambiguity is being tried to clear by putting content "What harmonics are not"? so that readers who are associated with operation and maintenance can efficiently do analysis and find the root cause of failure of equipment.
- IEEE Std. 519-1992, 2014 has been interpreted.
Introduction to Wireless Power Transfer and WitricityMln Phaneendra
Wireless Power Transfer has the ability to deliver major advancements in industries and applications that are dependent on physical, contacting connectors, which can be unreliable and prone to failure.
Vector control is a more advanced and precise method of controlling AC induction motors compared to scalar control. It involves transforming the motor currents and voltages into a rotating reference frame to obtain decoupled control similar to a DC motor. This allows for independent control of flux and torque for faster dynamic response and better performance than scalar control. The basic implementation of vector control uses Clarke and Park transformations to convert between stationary and rotating reference frames in the controller. It provides DC motor-like precision in speed and torque control of induction motors.
These slides are all about Phasor Measurement Units (PMUs). An introduction to PMU is presented as a preliminary knowledge for the course 'Distribution Generation and Smart Grid'. Your valuable suggestions are welcome.
This document introduces basic components of DC circuits. It defines a complete circuit as one where electron flow can travel from negative to positive terminals, and an incomplete circuit as one where flow is disrupted. Circuits must have four basic components: conductors to carry current, a load that uses power, a power supply to provide energy, and optional controllers to manage the circuit. Schematic symbols represent components in schematic diagrams, which are blueprints showing a circuit's construction. Common symbols are introduced for wires, loads like LEDs, power sources like batteries and cells, and switches as controllers. Assignments include an electronics reference chart to document websites on topics.
Applications of power electronics in HVDCKabilesh K
Role of Power electronics in HVDC and Transmission system. What are the components of Power electronics used in HVDC. Types of HVDC Links. Advantages of HVDC over HVAC.
This document provides an overview of power system planning and load forecasting. It discusses that load forecasting is the first crucial step for any power system planning study, as it involves predicting future load behavior. It describes different load forecasting techniques including extrapolation methods that use historical load data and trend curves, and correlation methods that relate loads to economic and demographic factors. The document also discusses factors that affect load forecasting like time of day, weather, customer class, and economics. Overall it provides a high-level introduction to the concepts and process of load forecasting for power system planning.
This document discusses power system security. It defines power system security as the probability of the system operating within acceptable ranges given potential changes or contingencies. It outlines the key steps in power system security including: (1) monitoring the current system state, (2) contingency analysis to evaluate potential risks, and (3) corrective action analysis to maintain security through preventative or automatic corrective actions.
Harmonics create pollution in our power system just like carbon dioxide and other gases create air pollution. It has adverse effects directly or indirectly on equipment like motors, transformers, induction heaters, etc. It leads to energy loss due to poor power factor.
Following content has been covered:
- The definition of harmonics is briefly interpreted.
- Factors which are responsible for harmonics current generation is discussed.
- Often the failure of equipment like motors, transformer, etc. has been put on harmonics current. But this is not always the case. This ambiguity is being tried to clear by putting content "What harmonics are not"? so that readers who are associated with operation and maintenance can efficiently do analysis and find the root cause of failure of equipment.
- IEEE Std. 519-1992, 2014 has been interpreted.
Introduction to Wireless Power Transfer and WitricityMln Phaneendra
Wireless Power Transfer has the ability to deliver major advancements in industries and applications that are dependent on physical, contacting connectors, which can be unreliable and prone to failure.
Vector control is a more advanced and precise method of controlling AC induction motors compared to scalar control. It involves transforming the motor currents and voltages into a rotating reference frame to obtain decoupled control similar to a DC motor. This allows for independent control of flux and torque for faster dynamic response and better performance than scalar control. The basic implementation of vector control uses Clarke and Park transformations to convert between stationary and rotating reference frames in the controller. It provides DC motor-like precision in speed and torque control of induction motors.
These slides are all about Phasor Measurement Units (PMUs). An introduction to PMU is presented as a preliminary knowledge for the course 'Distribution Generation and Smart Grid'. Your valuable suggestions are welcome.
This presentation on Power Quality Improvement Techniques: A Review presented by Sahid Raja Khan student of B. Tech. Electrical Engineering of Compucom Institute of Technology and Management Jaipur. It describes the improvement technique of Power Quality at GSS and other Substations including Generating Stations.
EHV (extra high voltage) AC transmission refers to equipment designed for voltages greater than 345 kV. Higher transmission voltages increase efficiency by reducing transmission losses and current, decrease infrastructure costs, and increase transmission capacity. However, they also present safety and interference risks. New technologies like FACTS (flexible AC transmission systems) help maximize the benefits of EHV transmission by enabling voltage control and power flow management. There is growing support for expanding national EHV transmission grids to facilitate large-scale renewable energy integration and inter-regional power sharing.
concept of resilience and self healing in smart gridKundan Kumar
The document discusses concepts related to resilience and self-healing in smart grids. It defines a smart grid as an electrical grid using communications technologies to improve efficiency. Key functions include enabling customer participation and accommodating different generation options. Self-healing is the ability of a system to automatically restore itself without human intervention. For the electrical grid, this means timely detection of issues and minimizing loss of service through reconfiguring resources. The transmission and distribution components can be modeled using graph theory to analyze resilience. Automatic meter reading is one approach for distribution grids.
DISTRIBUTED GENERATION ENVIRONMENT WITH SMART GRIDNIT MEGHALAYA
This document discusses distributed generation and the smart grid environment. It provides an introduction to the need for changes in energy generation, delivery, and use to establish sustainability and restore environmental balance. The document then discusses different forms of renewable energy sources and distributed generation. It describes some of the challenges of distributed generation and how a smart grid can help solve these issues. Finally, it discusses components of the smart grid like advanced metering infrastructure and phasor measurement units, and the benefits of integrating distributed generation with the smart grid.
Wide area monitoring systems (WAMS) are essentially based on the new data acquisition technology of phasor measurement and allow monitoring transmission system conditions over large areas in view of detecting and further counteracting grid instabilities.
Distribution Automation: Control functions– Communication system –Consumer Information Service– Geographical Information Systems. SCADA –block diagram –functions. Energy Management: Supply Side Management–Demand Side Management–Technologies Implementation, Dispersed Generation
The document discusses the components and structure of an electric power system. It describes how power is generated at power stations and stepped up in voltage for transmission over long distances before being stepped down for distribution to consumers. The key components are generators, transformers, transmission lines, control equipment, and distribution systems. Power flows from generation through transmission and distribution before reaching ultimate consumers.
This document discusses advanced metering infrastructure (AMI). It defines AMI as a system that allows for two-way communication between utilities and smart meters, enabling near real-time collection and transfer of energy usage data. The key components of an AMI system include smart meters, communications infrastructure, home area networks, a meter data management system, and operational gateways. While costly to implement, AMI provides benefits like improved reliability, lower energy costs, and reduced electricity theft. The document also examines AMI in the context of India's power grid and estimates costs associated with deployment.
- Frequency control is important to maintain required receiving end voltage and stable operation when systems are interconnected. Automatic generation control (AGC) is used to maintain power balance and constant system frequency as load changes.
- AGC has three components - primary control provides immediate response to load changes, secondary control corrects tie-line flows, and economic dispatch schedules units economically. It acts based on changes in generator speed and frequency.
- For multi-area systems, AGC must restore frequency and scheduled tie-line flows in each area while ensuring areas absorb their own load changes. Area control error (ACE) is used to adjust control settings to drive ACE to zero and balance the system.
The document discusses supervisory control and data acquisition (SCADA) systems. It defines SCADA and provides a brief history. It describes common SCADA components like remote terminal units (RTU), programmable logic controllers (PLC), human-machine interfaces, and data acquisition servers. It discusses the system components, future trends moving to networked systems, and applications in power system automation including intelligent electronic devices and automation processes. It concludes that India is moving towards greater power grid automation for increased efficiency and standardization.
Power Quality is a combination of Voltage profile, Frequency profile, Harmonics contain and reliability of power supply.
The Power Quality is defined as the degree to which the power supply approaches the ideal case of stable, uninterrupted, zero distortion and disturbance free supply.
presentation on POWER THEFT IDENTIFICATION SYSTEMGaurav Shukla
This document summarizes a seminar presentation on a microcontroller-based power theft identification system. It introduces power theft as the illegal use of electrical power without paying the supplier. It then describes two common ways that power theft occurs: slowing down electricity meters with magnets, and inverting meters to make them count backwards. The proposed system architecture integrates a wireless network with the electrical grid to monitor multiple points using data aggregation algorithms. A microcontroller like a PLC would be programmed to detect theft and control the electrical distribution in response.
Protection against overvoltage
overvoltage
causes of overvoltage
lightning
types of lightning strokes
harmful effect of lightning
protection against lightning
Power system planning & operation [eceg 4410]Sifan Welisa
The document discusses power load forecasting and substation planning. It explains that accurate load forecasting is important for power system planning and operation. Several load forecasting methods are described, including those based on historical load data, economic factors, and standardized load curves. Load forecasts can be short, medium, or long-term. The document also discusses factors to consider in substation planning and design, such as location, equipment requirements, and configuration. Feasibility studies are important for assessing potential hydroelectric and substation projects.
Reactive power compensation is used to improve the performance of AC power systems. There are various methods of reactive power compensation including shunt compensation, series compensation, static VAR compensators, and static synchronous compensators. Shunt compensation devices such as capacitors and reactors are connected in parallel to transmission lines to regulate voltage. Series compensation uses capacitors connected in series to transmission lines to increase power transfer capability. Static VAR compensators and static synchronous compensators use thyristor-based voltage sourced converters to dynamically inject or absorb reactive power and control voltage. Reactive power compensation provides benefits such as improved power factor, voltage regulation, reduced losses, and increased power transfer capacity.
The document discusses planning for HVDC transmission and modern trends in HVDC technology. When planning HVDC transmission, the key factors to consider are cost, technical performance, and reliability. Modern trends aim to reduce converter station costs while improving reliability and performance. This includes advances in power semiconductors, converter control technology, development of DC breakers, conversion of existing AC lines, and operation with weak AC systems. Emerging technologies discussed are active DC filters, capacitor commutated converters, and ultra-high voltage DC transmission.
Power system security refers to the probability that a power system will remain stable and within acceptable operating limits given potential disturbances or contingencies. There are three main operating states: preventive, emergency, and restorative. In the preventive state, the system operates normally and can withstand credible contingencies. The emergency state occurs when limits are violated, and the goal is to relieve stress. In the restorative state, parts of the system have lost power and the goal is to restore the system to normal. Security assessment involves system monitoring, contingency analysis to evaluate risks, and preventive and corrective actions. On-line security assessment continuously monitors the system using real-time measurements and updates assessments as conditions change.
This presentation on Power Quality Improvement Techniques: A Review presented by Sahid Raja Khan student of B. Tech. Electrical Engineering of Compucom Institute of Technology and Management Jaipur. It describes the improvement technique of Power Quality at GSS and other Substations including Generating Stations.
EHV (extra high voltage) AC transmission refers to equipment designed for voltages greater than 345 kV. Higher transmission voltages increase efficiency by reducing transmission losses and current, decrease infrastructure costs, and increase transmission capacity. However, they also present safety and interference risks. New technologies like FACTS (flexible AC transmission systems) help maximize the benefits of EHV transmission by enabling voltage control and power flow management. There is growing support for expanding national EHV transmission grids to facilitate large-scale renewable energy integration and inter-regional power sharing.
concept of resilience and self healing in smart gridKundan Kumar
The document discusses concepts related to resilience and self-healing in smart grids. It defines a smart grid as an electrical grid using communications technologies to improve efficiency. Key functions include enabling customer participation and accommodating different generation options. Self-healing is the ability of a system to automatically restore itself without human intervention. For the electrical grid, this means timely detection of issues and minimizing loss of service through reconfiguring resources. The transmission and distribution components can be modeled using graph theory to analyze resilience. Automatic meter reading is one approach for distribution grids.
DISTRIBUTED GENERATION ENVIRONMENT WITH SMART GRIDNIT MEGHALAYA
This document discusses distributed generation and the smart grid environment. It provides an introduction to the need for changes in energy generation, delivery, and use to establish sustainability and restore environmental balance. The document then discusses different forms of renewable energy sources and distributed generation. It describes some of the challenges of distributed generation and how a smart grid can help solve these issues. Finally, it discusses components of the smart grid like advanced metering infrastructure and phasor measurement units, and the benefits of integrating distributed generation with the smart grid.
Wide area monitoring systems (WAMS) are essentially based on the new data acquisition technology of phasor measurement and allow monitoring transmission system conditions over large areas in view of detecting and further counteracting grid instabilities.
Distribution Automation: Control functions– Communication system –Consumer Information Service– Geographical Information Systems. SCADA –block diagram –functions. Energy Management: Supply Side Management–Demand Side Management–Technologies Implementation, Dispersed Generation
The document discusses the components and structure of an electric power system. It describes how power is generated at power stations and stepped up in voltage for transmission over long distances before being stepped down for distribution to consumers. The key components are generators, transformers, transmission lines, control equipment, and distribution systems. Power flows from generation through transmission and distribution before reaching ultimate consumers.
This document discusses advanced metering infrastructure (AMI). It defines AMI as a system that allows for two-way communication between utilities and smart meters, enabling near real-time collection and transfer of energy usage data. The key components of an AMI system include smart meters, communications infrastructure, home area networks, a meter data management system, and operational gateways. While costly to implement, AMI provides benefits like improved reliability, lower energy costs, and reduced electricity theft. The document also examines AMI in the context of India's power grid and estimates costs associated with deployment.
- Frequency control is important to maintain required receiving end voltage and stable operation when systems are interconnected. Automatic generation control (AGC) is used to maintain power balance and constant system frequency as load changes.
- AGC has three components - primary control provides immediate response to load changes, secondary control corrects tie-line flows, and economic dispatch schedules units economically. It acts based on changes in generator speed and frequency.
- For multi-area systems, AGC must restore frequency and scheduled tie-line flows in each area while ensuring areas absorb their own load changes. Area control error (ACE) is used to adjust control settings to drive ACE to zero and balance the system.
The document discusses supervisory control and data acquisition (SCADA) systems. It defines SCADA and provides a brief history. It describes common SCADA components like remote terminal units (RTU), programmable logic controllers (PLC), human-machine interfaces, and data acquisition servers. It discusses the system components, future trends moving to networked systems, and applications in power system automation including intelligent electronic devices and automation processes. It concludes that India is moving towards greater power grid automation for increased efficiency and standardization.
Power Quality is a combination of Voltage profile, Frequency profile, Harmonics contain and reliability of power supply.
The Power Quality is defined as the degree to which the power supply approaches the ideal case of stable, uninterrupted, zero distortion and disturbance free supply.
presentation on POWER THEFT IDENTIFICATION SYSTEMGaurav Shukla
This document summarizes a seminar presentation on a microcontroller-based power theft identification system. It introduces power theft as the illegal use of electrical power without paying the supplier. It then describes two common ways that power theft occurs: slowing down electricity meters with magnets, and inverting meters to make them count backwards. The proposed system architecture integrates a wireless network with the electrical grid to monitor multiple points using data aggregation algorithms. A microcontroller like a PLC would be programmed to detect theft and control the electrical distribution in response.
Protection against overvoltage
overvoltage
causes of overvoltage
lightning
types of lightning strokes
harmful effect of lightning
protection against lightning
Power system planning & operation [eceg 4410]Sifan Welisa
The document discusses power load forecasting and substation planning. It explains that accurate load forecasting is important for power system planning and operation. Several load forecasting methods are described, including those based on historical load data, economic factors, and standardized load curves. Load forecasts can be short, medium, or long-term. The document also discusses factors to consider in substation planning and design, such as location, equipment requirements, and configuration. Feasibility studies are important for assessing potential hydroelectric and substation projects.
Reactive power compensation is used to improve the performance of AC power systems. There are various methods of reactive power compensation including shunt compensation, series compensation, static VAR compensators, and static synchronous compensators. Shunt compensation devices such as capacitors and reactors are connected in parallel to transmission lines to regulate voltage. Series compensation uses capacitors connected in series to transmission lines to increase power transfer capability. Static VAR compensators and static synchronous compensators use thyristor-based voltage sourced converters to dynamically inject or absorb reactive power and control voltage. Reactive power compensation provides benefits such as improved power factor, voltage regulation, reduced losses, and increased power transfer capacity.
The document discusses planning for HVDC transmission and modern trends in HVDC technology. When planning HVDC transmission, the key factors to consider are cost, technical performance, and reliability. Modern trends aim to reduce converter station costs while improving reliability and performance. This includes advances in power semiconductors, converter control technology, development of DC breakers, conversion of existing AC lines, and operation with weak AC systems. Emerging technologies discussed are active DC filters, capacitor commutated converters, and ultra-high voltage DC transmission.
Power system security refers to the probability that a power system will remain stable and within acceptable operating limits given potential disturbances or contingencies. There are three main operating states: preventive, emergency, and restorative. In the preventive state, the system operates normally and can withstand credible contingencies. The emergency state occurs when limits are violated, and the goal is to relieve stress. In the restorative state, parts of the system have lost power and the goal is to restore the system to normal. Security assessment involves system monitoring, contingency analysis to evaluate risks, and preventive and corrective actions. On-line security assessment continuously monitors the system using real-time measurements and updates assessments as conditions change.
O mașină utilizează energie pentru a aplica forțe și a controla mișcarea pentru a efectua o acțiune intenționată. Mașinile pot fi conduse de animale și de persoane, prin forțe naturale, cum ar fi vântul și apa și prin energie chimică, termică sau electrică, și includ un sistem de mecanisme care modelează intrarea actuatorului (un actuator, sau servomotor, este o componentă a unei mașini responsabilă de mișcarea și controlul unui mecanism sau sistem, de exemplu prin deschiderea unei supape) pentru a realiza o aplicare specifică a forțelor de ieșire și a mișcării. Acestea pot include, de asemenea, computere și senzori care monitorizează performanța și planificarea mișcărilor, adesea numite sisteme mecanice.
1. MASINA DE CURENT CONTINUU(MCC)
Generalitati
1.Elemente constructive
Masina de curent continuu,MCC, se construieste pentru o gama larga de puteri(zeci de
wati,pana la mii de kilowati),turatii si tensiuni nominale (pana la 2000V)
Masina de curent continuu se utilizeaza in regim de generator(in istalatiile de producere
a energiei electrice),motor(tractiune electrica, masini de ridicat si transportat,in actionari
care necesita reglaj larg si continuu al vitezei) si frana.
Masina de curent continuu se compune din doua parti constructive de baza:statorul care
reprezinta inductorul si rotorul care reprezinta indusul.
Masinile de curent continuu pot fi:
Masini heteropolare-sistemul inductor este format dintr-o susccesiune alternanta
de poli nord si sud
Masini homopolare-functionarea lor se bazeaza pe discul lui Faraday
Statorul este partea imobila a masinii,care are ca elemente constructive principale:
carcasa(jugul statoric), polii de excitatie si infasurarea concentrata respectiva de
curent continuu, polii de comutatie(auxiliari) si infasurarea concentrata
corespunzatoare, scuturile(capacele) frontale cu lagare cu rulmenti sau de alunecare,
sistemul perii si portperii, cutia de borne.
Rotorul este partea mobila a masinii, constituit din cateva elemente constructive
principale:miezul(pachetul)rotoric,care prezinta la periferie dinti,repartizati uniform,
iar spre interior jugul rotoric fixat pe arbore,infasurarea rotorica distribuita uniform in
crestaturi ale miezului rotoric, colectorul, ventilatorul.
Vom da in cele ce urmeaza o scurta descriere a elementelor constructive principale
ale masinii de curent continuu.
Carcasa (jugul statoric) reprezinta partea imobila in care se fixeaza polii de excitatie
si prin care masina este fixata in fundatie prin intermediul unei talpi de prindere si
buloane(fig 3.1).La masinile de putere mai mare de cateva sute de wati, carcasa si
jugul statoric(care serveste drept drum de inchidere al fluxului magnetic produs de
polii de excitatie) reprezinta una si aceeasi piesa constructiva.Pentru a se oferi
fluxului magnetic o reluctanta cat mai mica, carcasa se construieste din fonta si otel
turnat, uneori din tabla groasa si otel sudata.
1
3. Polii de excitatie(principali) se construiesc din tole de otel electrotehnic de 1-2mm
grosime(fig.3.3),stranse pachet cu ajutorul unor buloane nituite. Polii se prind in carcasa
prin buloane.Ei poarta bobinele de excitatie strabatute de curentul de excitatie. Bobinele
de excitatie se realizeaza dintr-un conductor rotund sau profilat de cupru. Conductorul
este izolat pentru a nu se produce scurtcircuite intre spirele bobinei. Bobinele polilor de
excitatie se leaga intre ele in serie sau paralel si se alimenteaza prin bornele din cutia de
borne.
Polii de comutatie(auxiliari)(fig 3.4), sunt constituiti dintr-un miez de fier masiv sau
din tole si au de regula o forma paralelipipedica. Acestia sunt situati in axa neutra a
masinii-mijlocul distantei dintre polii principali.
Miezul rotoric(fig. 3.1) se contruieste din tole de otel electrotehnic(fig.3.5),de forma
circulara cu dinti si crestaturi,de profil foarte variat(fig. 3.5,b).De obicei grosimea acestor
tole este de 0,5-1mm.Tolele separate se izoleaza una de alta printr-un strat subtire de lac
3
4. sau printr-un strat de oxid.Grosimea izolatiei este de 0,03-0,05mm.O astfel de constructie
a miezului are ca scop reducerea curentilor turbionari care se dezvolta in miez la rotirea
sa in campul magnetic.Curentii turbionari duc la pierderi de energie care se transforma in
caldura.La miez masiv, aceste pierderi desi ar fi foarte mari ar duce la reducerea
randamentului masinii si la o incalzire foarte ridicata.
Infasurarea rotorica(fig. 3.1) consta din sectii,care se pregatesc pe sabloane speciale
si se aseaza in crestaturile miezului(fig. 3.6, a).Infasurarea se izoleaza de miez cu grija si
se consolideaza in crestaturi, cele mai deseori cu ajutorul unor pene speciale de lemn sau
alt material izolant(fig. 3.6,b)
Sectiunile infasurarii rotorice se leaga la colector ,care este un subansamblu
caracteristic masinii de curent continuu
. Colectorul(fig. 3.7) are forma cilindrica,fiind construit din placute de
cupru,denumite lamele,izolate una fata de cealalta printr-un strat de micanita si de
asemenea izolate de piesele de strangere.La masinile de putere mica,colectorul se
realizeaza din lamele solidarizate si totodata izolate intre ele cu ajutorul unui material
rasinos sintetic.Colectorul se invarteste solidar cu rotorul masinii
.
4
5. Infasurarea de excitatie se executa din conductor de cupru izolat,sub forma unor
bobine concentrice montate pe miezul polilor principali.Bobinele de excitatie se leaga
in serie au in paralel,astefl incat sa se obtina un inductor heteropolar.Infasurarea de
excitatie se alimenteaza in curent continuu.
Infasurarea de compensare este dispusa in crestaturi inchise practicate in talpa
polilor principali, in zona situate spre intrefier.Aceasta infasurare se conecteaza in
serie cu infasurarea indusului si are rolul de a anihila sau diminua efectele
fenomenului de reactie a indusului.
2)Regimurile energetice de functionare ale masinii de curent
continuu
Masina de curent continuu poate functiona in trei regimuri din punctul de vedere al
transformarii energetice efectuate:de generator,de motor si de frana.In regimul de
generator,masina transforma puterea mecanica primita pe la arbore de la un motor
(care antreneaza masina) in putere electrica debitata intr-o retea de curent continuu. In
regimul de motor,masina transforma puterea electrica primita de la o retea de curent
continuu in putere mecanica cedata pe la arbore unui mecanism sau unei instalatii
mecanice.Precum se remarca,in regim de motor masina realizeaza transformarea
inversa de putere in comparativ cu regimul generator. In fine,in regim de frana
electrica,masina primeste putere mecanica pe la arbore si putere electrica de la o retea
de curent continuu si le transforma ireversibil in timp in caldura,dezvoltand totodata
un cuplu necesar franarii unui mecanism sau unei instalatii mecanice.
In cele ce urmeaza vom studia mai in detaliu principiul de functionare in regim de
frana electrica,vom stabili ecuatiile generale de functionare stationara si bilantul de
puteri.
3)FRANAREA MASINILOR DE CURENT CONTINUU
Fenomenele de baza la utilizarea masinii de curent continuu pentru franare in
sistemele de actionari.Caracteristici mecanice.Franarea dinamica si recuperativa.
Franarea in actionarile electrice se realizeaza in mai multe scopuri:
-pentru mentinerea constanta a vitezei unui sistem supus actiunii unor cupluri datorite
fortelor gravita-tionale sau inertiei;
-pentru mentinerea in nemiscare a unui sistem supus actiunii unor cupluri exterioare;
-pentru micsorarea vitezei unui sistem (in vederea modificarii regimului tehnologic de
functionare sau opririi rapide).
In cele ce urmeaza vom analiza diferite moduri de utilizare a masinii electrice de
curent continuu pentru a frana un sistem mecanic.De la bun inceput trebuie aratat ca
franarea pe aceasta cale prezinta avantaje nete in comparatie cu franarea mecanica(se
elimina uzura sabotilor si tobei de franare, se reduc dimensiunile instalatiei de
franare,se asigura un control sigur al valorii cuplului de franare, se asigura un control
5
6. sigur al valorii cuplului de franare,uneori se poate recupera o parte din energia
cinetica care intervine in procesul de franare).
3.1.Franarea in regim de frana propriu-zisa
Regimul de frana propriu zisa se utilizeaza deseori in actionarile electrice in cele doua
variante,pornind de la regimul de baza de motor:
-prin variatia unei rezistente inseriate cu infasurarea rotorului,trecerea in regimul de
frana facandu-se prin inversarea sensului vitezei rotatiei la aceeasi polaritatea a
tensiunii la borne;
-prin inversarea polaritatii tensiunii la borne si intercalarea unei rezistente in serie cu
infasurarea rotorului la acelasi sens de rotatie;
Franarea propriu zisa prin inversarea sensului de rotatie se intalneste curent in
instalatiile de ridicat.Sa presupunem deci o asemenea instalatie(de exemplu un pod
rulant)actionata de o masina de curent continuu,fie aceasta cu excitatie independenta
constanta.Sa consideram ca masina functioneaza in regim de motor si ridica o
anumita greutate cu o viteza relativ importanta.Pe caracteristica mecanica naturala a
in regim de motor din figura 3.7.1, fie A punctual de functionare respectiv.Odata
ajunsa greutatea la o anumita inaltime,se pune problema micsorarii vitezei de ridicatie
, lucru care se realizeaza la un acelasi cuplu si tensiune aplicata a retelei,prin
intercalarea in serie cu infasurarea rotorica a unei rezistentei RF.
Masina functioneaza in continuare ca motor,prezentand o noua caracteristica
mecanica de panta mai mare.Intr-adevar,caracteristica mecanica naturala are
urmatoare expresie analitica.
Ra
Ω= Ωo- 2 2
M,
k E ΨE
UA
unde: Ωo= reprezinta viteza unghiulara de mers in gol ideal(cand masina nu
k E ΨE
2 2
are nici un fel de cuplu rezistent la arbore),iar caracteristica mecanica corepunzatoare
unei rezistente in serie RF va fi :
R A + RF
Ω= Ωo- M, (3.36)
k E ΨE
2 2
evidentiand o panta proportionala cu RA+RF
Pe noua caracteristica mecanica,notata cu b in figura 3.7.1 punctul de functionare va fi B.
La un moment dat apare necesitatea opririi greutatii la o anumita inaltime si deplasarea ei
pe orizontala.In acest caz,viteza masinii devine zero,iar masina trebuie sa dezvolte in
continuare acelasi cuplu electromagnetic.
Asa cum arata carcateristica c din figura 3.71, se pot realiza cerintele de mai sus cu
ajutorul unei rezistente R’F crescute,intercalata in circuitul rotoric.In punctul de
functionare C de pe caracteristica c,masina nu este nici motor,nici frana propriu
zisa,aflandu-se la granita de separatie dintre cele doua regimuri.Masina primeste putere
6
7. electrica care se transforma,in timp,in caldura in rezistenta totala RA+R’F si nu dezvolta
nici nu primeste putere mecanica,deoarece Ω=0.
Pentru a cobori greutatea,deci a inverse sensul de rotatie fata de situatia anterioara,trebuie
conectata o rezistenta serie R”F de valoare si mai ridicata,punctul de functionare trecand
in D in zona vitezelor negative(in cadranul IV al planului Ω,M).Variindu-se rezistenta RF
in continuare,se poate varia viteza de coborare a greutatii.In aceasta ultima situatie,
masina electrica lucreaza in regim de frana propriu-zisa,asa cum se poate verifica imediat
pe baza celor aratate anterior.Ea primeste putere mecanica de la arbore pe baza scaderii in
timp a energiei potentiale a greutatii in campul gravific al Pamantului.In acelasi timp,ea
absoarbe putere electrica de la retea.Puterea totala absorbita este transformata prin efect
Joule in rezistenta RA+RF’.Masina dezvolta un cuplu electromagnetic de acelasi sens ca si
in regimul de motor,numai ca de data aceasta cuplul are sens invers fata de viteza de
rotatie,devenind cuplu de franare care se opune cuplului dezvoltat de greutate.
3.1.1)Franarea propriu-zisa prin inversarea polaritatii tensiunii
Franarea propriu-zisa prin inversarea polaritatii tensiunii se intalneste la numeroase
actionari electrice in care apare problema opririi rapide a instalatiei mecanice antrenate de
7
8. o masina electrica.Pentru a fixa ideile,sa ne referim la cazul actionarii unui laminar
reversibil.In asemenea instalatii se pune problema ca dupa ce masina electrica a
functionat in regim de motor, rotind valturile laminorului intr-un anumit sens,sa se
franeze rapid intreaga instalatie si apoi sa se accelereze valturile in sens contrar.In acest
scop, dupa ce masina a functionat ca motor(presupus cu excitatie independenta constanta
ca sens si marime) intr-un anumit sens de rotatie, se inverseaza sensul tensiunii UA
aplicate la bornele masinii, care trece in regim de frana propriu-zisa, pana cand viteza
devine nula, iar apoi in continuare in regim de motor cu sens invers de rotatie.
In regimul de motor cu sens dreapta ,se aplica la borne o tensiune de o anumita polaritate,
punctul de functionare fiind A pe caracteristica naturala a din figura 3.72.La un moment
dat este nevoie a se frana intreaga instalatie. Pentru aceasta se intrerupe brusc alimentarea
masinii(circuitul de excitatie nu se intrerupe), se conecteaza in serie cu infasurarea
rotorica o rezistenta RF convenabila si se alimenteaza masina cu tensiune de polaritate
inversata. Noua caracteristica mecanica de functionare a masinii este notata cu b in figura
3.72. Expresia sa analitica rezulta din relatia (3.36) cu conditia schimbarii semnului
vitezei Ωo de mers in gol ideal(deloarece tensiune U schimba de semn)
R A + RF
Ω= -Ωo- M
k E ΨE
2 2
Punctul de functionare sare brusc din pozitia A in pozitia B pe noua caracteristica
mecanica(in cadranul II al planului Ω,M)la aceeasi viteza de rotatie la care se invartea
masina in momentul intreruperii regimului de functionare ca motor.
In noul punct B de functionare,masina lucreaza in regim de frana propriu-zisa pentru
sensul dreapta de rotatie.Intr-adevar,ea absoarbe in continuare puterea electrica de la
reteaua de alimentare(tensiunea schimba de semn,curentul de asemenea ,deoarece si
t.e.m. indusa si tensiunea la borne actioneaza acum in concordanta pentru a schimba
sensul curentului IA) si,in acelasi timp,absoarbe putere mecanica pe la arbore pe seama
micsorarii in timp a energiei cinetice acumulate in masele in rotatie ale laminorului.
Toata aceasta putere absorbita se transforma in caldura in rezistenta RA+RF.Cuplul
dezvoltat de masina in punctul B este de semn schimbat fata de cel corespunzator
punctului A(s-a inversat curentul IA la acelasi curent de excitatie) si actioneaza in sens
contrar cuplului de inertie al maselor in miscare.Acest bilant sumar de puteri corespunde
intru totul cu cel prezentat la principiul de functionare al masinilor de curent continuu in
regim de frana propriu zisa.
Motorul de franare descris mai sus este cunoscut in literatura si sub numele de franare
contra curent sau prin legaturi inverse.Subliniem faptul ca un asemenea regim nu este cu
nimic distinct de regimul de frana propriu-zisa descris mai inainte,ceea ce difera fiind
doar maniera de trecere a masinii din regimul de motor in cel de frana propriu-zisa.
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9. Daca in situatia analizata mai inainte se lasa masina sa functioneze conform caracteristicii
b din figura 3.72 atunci instalatia se franeaza si viteaza se micsoreaza treptat(punctul B se
deplaseaza catre punctul C),la un moment dat instalatia se opreste(punctul C),dupa care
ea se accelereaza in sensul stanga,masina electrica trecand din nou in regim de motor
pentru sensul stanga(cadranul III din planul Ω,M).
Franarea automatizata se realizeaza cu ajutorul releelor si al contactoarelor electrice.In
cele ce urmeaza se va descrie un sistem de comanda automatizata a pornirii reversibile a
unui motor electric prevazut cu franare automatizata la trecerea de la sensul dreapta la cel
stanga.
Pentru franare se utilizeaza un releu de tensiune intr-un montaj special aratat in figura
3.73,in care R1,R2 reprezinta rezistentele de pornire,iar R3 rezistenta suplimentara de
franare.Releul de franare are bobina d care actioneaza contactul d si care comanda
alimentarea boninei c3 a unui contactor de scurtcircuitare a rezistentei R3 [ prin
intermediul contactului principal(de forta c3)].
Releul d trebuie sa comande conectarea rezistentei Ra de franare ori de cate ori se
inverseaza sensul de rotatie al motorului si este nevoie de regimul de franare propriu-zisa
pentru a se scurta timpii morti.Dupa ce a realizat franarea in apropierea vitezei Ω = 0 a
masinii,releul trebuie sa comande scurtcircuitarea rezistentei R3’ pentru ca pornirea in
sens invers sa se faca la cuplu ridicat.Rezistenta Ra trebuie sa ramana scurtcircuitata atat
in timpul pornirii(indiferent de sens), cat si al functionarii normale a motorului.
9
10. Sa notam cu Ud tensiunea la bornele bobinei releului de franare si cu U tensiunea
retelei.Sa notam cu R3’ fractiunea din rezistenta R3 care ramane in afara prizei releului de
tensiune d.Atunci cand s-a comandat inversarea sensului de rotatie al motorului prin
inversarea polaritatii tensiunii aplicate (cu acelasi curent de excitatie) , se poate scrie:
U d = U − R3 ' I ,
I fiind curentul absorbit de motor.Dar din ecuatia de functionare a motorului rezulta
U + k E ΩΦ E
I= ,
R
in care R = R1+ R2+ R3+ RA.Prin urmare ,
R ' R '
U d = U 1 − 3 − 3 k E ΩΦE , (3.37)
R R
Se poate remarca faptul ca pentru Ω = −Ω0 , in care Ω reprezinta viteza
0
motorului la mers in gol ideal , U= k E Ω0 ΦE si Ud=U , aceasta indiferent de valoarea
rezistentei R3’.Asadar , ecuatia (3.37) reprezinta o familie
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11. de drepte functie de parametrul R3’ , dar toate dreptele Ω = f (U d ) trec prin punctul
H(- Ω ,U) , indiferent de R3’.Pentru R3’=R/2 se obtine dreapta reprezentat prin linia
0
intrerupta AH in fig.3.74.Pentru R3’=0 se obtine dreapta paralela cu axa ordonatelor.
De obicei, in practica, R3’=R/2, iar releul de franare d este astfel reglat incat
actioneaza numai pentru Ud>0,4U.Intro pozitie oarecare, in situatia cand masina
actioneaza in regim de motor (fig.3.74), se comanda inversarea sensului de
rotatie.Tensiunea la bornele releului de franare scade la valoarea corespunzatoare
punctului A, pentru care Ud<0,4U si deci releul d nu mai actioneaza, contactul d se
deschide, contactul c3 la fel si, prin urmare, rezistenta R3 este intercalata in serie cu
motorul.Pe masura ce viteza Ω scade,tensiunea Ud creste (fig.3.74,dreapta AH), iar
cand Ud=0,4U si Ω este foarte apropiat de zero (punctual B in figura 3.74), releul d
actioneaza si rezistenta R3 este scurtcircuitata.Cuplul dezvoltat de masina inregistreaza un
soc , tensiunea Ud devine egala cu U si releul de franare mentine contactul c3 inchis.Pe
urma intervin releele de accelerare, care scurtcircuiteaza succesiv rezistentele R1 si R2.
In figura 3.75 este redata schema completa de comanda a pornirii reversibile cu
franare contra curent.Releele de accelerare sunt relee de timp, conform schemei
cunoscute, d1 reprezinta bobina releului care realizeaza franarea la sensul de rotatie
“dreapta”, iar d2 este bobina releului care realizeaza franarea la sensul de rotatie
“stanga”.Cele doua relee nu functioneaza simultan, datorita existentei diodelor inseriate
cu bobine respective.
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12. Modul de functionare a schemei se poate stabili usor pe baza celor cunoscute din
paragrafele anterioare.Pentru pornirea in sens dreapta se actioneaza butonul b2.Pentru
reversare cu franare se actioneaza succesiv butoanele b1,b3.
3.2)Franarea in regim de generator cu recuperarea energiei
Sa presupunem ca o locomotiva electrica actionata de masini de curent continuu are de
urcat intr-o regiune deluroasa, trebuind sa invinga un anumit cuplu rezistent produs de
fortele gravitatiei si de frecari.Atat timp cat trenul urca, masinile electrice functioneaza in
regim de motor (fig. 3.76).Din ecuatiile de functionare corespunzatoare acestui regim
rezulta
U − k E ΩΦ E
IA = A ,
RA
cu ΦE =const.
Din aceasta expresie se observa ca , pe masura ce trenul se apropie de sfarsitul rampei
(urcusului) si cuplul rezistent scade, curentul IA (proportional cu cuplul) scade, iar viteza
Ω tinde sa creasca.Punctul de functionare fuge pe caracteristica din figura 3.76 din zona
corespunzatoare cuplurilor mari spre punctual B.
Odata terminat urcusul, trenul ajunge pe palier (teren plat) si apoi se inscrie in
panta (la vale).Pe panta de inceput, fortele gravitatiei, care actioneaza acum ca forte
active pot invinge la un moment dat fortele de frecari, iar cuplul rezistent la arbore al
masinii devine zero si odata cu el si curentul IA.In aceasta situatie , Ω = Ω0 (punctual
B).Masina electrica inceteaza de a mai fi motor, dar continua sa se roteasca in acelasi
sens ca mai inainte.Avem de a face cu regimul de mers in gol ideal.
Daca panta traseului se accentueaza, cuplul activ produs asupra arborelui
masinii electrice cuplata mecanic cu rotile trenului se mareste si trenul se accelereaza.
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13. Viteza de rotatie Ω depaseste valoarea Ω .Tensiunea electromotoare E0, pastrand si ea
0
acelasi sens, depaseste valoarea tensiunii UA la borne, iar curentul IA schimba de
sens.Cuplul electromagnetic dezvoltat de masina electrica schimba si el de sens odata cu
curentul (fluxul de excitatie se mentine acelasi in acest rastimp) si devine cuplu de
franare, care se opune accelerarii trenului si limiteaza valoarea vitezei de coborare.
Deoarece curentul IA a schimbat de sens, dar sensul tensiunii UA a ramas
acelasi, masina inceteaza de a mai primi energie electrica de la reteaua de alimentare si,
dimpotriva, debiteaza energie electrica in aceasta retea.Masina a devenit generator de
energie electrica pe seama micsorarii energiei potentiale a trenului in campul fortelor de
gravitatie.Masina electrica functioneaza intocmai ca o frana, dar cu recuperarea energiei,
ceea ce constituie un deosebit de apreciabil avantaj.Trenul electric care coboara dintr-o
regiune muntoasa spre campie poate produce energie electrica pentru trenul care urca
dinspre campie spre munte, in felul acesta puterea electrica necesara alimentarii de
ansamblu a liniei ferate electrificate reducandu-se sensibil.Acest fapt reprezinta unul
dintre avantajele esentiale ale tractiunii electrice indeosebi in zonele de profil variat fata
de tractiunea cu locomotive cu aburi sau diesel-electrice.
Semnalam faptul ca in figura 3.76 regimul de frana ca generator cu
recuperarea energiei are loc in cadranul al doilea, pe prelungirea caracteristicii mecanice
corespunzatoare regimului de motor dincolo de punctul B de mers in gol ideal.
Franarea prin mers ca generator a unui motor cu excitatie serie este posibila
numai in cazul in care se trece la excitatie in derivatie sau independenta.
3.3)Franarea in regim de generator fara recuperare(franarea dinamica)
Atat in tractiune electrica, cat si in alte actionari, pentru franari bruste se utilizeaza
deseori asa numita franare dinamica.
Sa consideram o masina electrica de curent continuu cu excitatie independenta
functionand in regim de motor (fig. 3.77) si actionand ca o locomotive electrica.Atunci
cand dorim sa franam brusc trenul, se deconecteaza masina de la retea si se inchide
circuitul rotoric pe o rezistenta R,curentul de excitatie ramanand acelasi.
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14. Masina,care continua sa se roteasca in acelasi sens,se transforma in generator care
debiteaza energeie electrica in rezistenta R.Aceasta energie provine din energia cinetica
acumulata de tren.Se poate calcula in asa mod rezistenta R,incat energia cinetica a
trenului sa se cheltuiasca repede sub forma de caldura in rezistenta R,in infasurarea
rotorului masinii si prin frecari mecanice,iar trenul sa se opreasca in scurt timp.
In aceasta perioada,masina electrica are t.e.m de acelasi sens ca mai inainte.Sensul
curentului in infasurarea rotorica este impus acum de aceasta t.e.m si este deci diferit de
sensul curentului IA din regimul de motor.Cuplul dezvoltat de masina este si el de sens
contrar, adica un cuplu de franare(fig.3.77)
Ecuatiile de functionare ale masinii in aceasta situatie sunt:
Eo=kEΩΦE =(R+RA)IA+ΔUp
k E ΩΦ E − ∆U P
M = −k E I A Φ E = −k E Φ E
R + RA
Neglijand caderea de tensiune la perii,se poate scrie urmatoarea dependenta intre cuplul
de franare M si viteza de rotatie Ω
kEΦ2
2
M =− E
Ω
R + R2
Aceasta ecuatie reprezinta o dreapta,trecand prin origine(caracteristica b din figura 3.77),
de panta variabila functie de fluxul de excitatie si de rezistenta R.La un flux de excitatie
dat,cuplul de franare este cu atat mai mare si deci franarea este cu atat mai rapida cu cat
rezistenta R este mai mica.
Franarea dinamica se poate utiliza si in cazul motorului cu excitatie derivatie.Este usor de
aratat ca,in acest caz,la deconectarea masinii de la retea si la inchiderea circuitului rotoric
pe o rezistenta sunt indeplinite conditiile de autoexcitare pentru functionarea in regim de
generator.
In cazul motorului serie,pentru a putea fi indeplinite conditiile de autoexcitare,trebuie ca
odata cu conectarea infasurarii rotorului pe rezistenta R sa se inverseze si legaturile
infasurarii de excitatie;altminteri curentul produs de t.e.m. remanenta distruge campul
remanent.In plus,rezistenta R nu trebuie sa depaseasca o anumita valoare critica.
.
14