UNIVERSIDAD PONTIFICIA COMILLAS
ESCUELA TÉCNICA SUPERIOR DE INGENIERÍA (ICAI)
Design of a test bench
Autor: Alfredo Díaz Vaquero
Directora: Diana Florez
AUTORIZACIÓN PARA LA DIGITALIZACIÓN, DEPÓSITO Y DIVULGACIÓN EN ACCESO
ABIERTO (RESTRINGIDO) DE DOCUMENTACIÓN
1º. Declaración de la autoría y acreditación de la misma.
El autor D. ALFREDO DÍAZ VAQUERO, como ALUMNO de la UNIVERSIDAD PONTIFICIA COMILLAS (COMILLAS), DECLARA que es el titular de los derechos de propiedad intelectual, objeto de la presente cesión, en relación con la obra PROYECTO FIN DE GRADO: DESIGN OF A TEST BENCH1, que ésta es una obra original, y que ostenta la condición de autor en el sentido
que otorga la Ley de Propiedad Intelectual como titular único o cotitular de la obra.
En caso de ser cotitular, el autor (firmante) declara asimismo que cuenta con el consentimiento de los restantes titulares para hacer la presente cesión. En caso de previa cesión a terceros de derechos de explotación de la obra, el autor declara que tiene la oportuna autorización de dichos titulares de derechos a los fines de esta cesión o bien que retiene la facultad de ceder estos derechos en la forma prevista en la presente cesión y así lo acredita.
2º. Objeto y fines de la cesión.
Con el fin de dar la máxima difusión a la obra citada a través del Repositorio institucional de la Universidad y hacer posible su utilización de forma libre y gratuita ( con las limitaciones que más adelante se detallan) por todos los usuarios del repositorio y del portal e-ciencia, el autor CEDE a la Universidad Pontificia Comillas de forma gratuita y no exclusiva, por el máximo plazo legal y con ámbito universal, los derechos de digitalización, de archivo, de reproducción, de distribución, de comunicación pública, incluido el derecho de puesta a disposición electrónica, tal y como se describen en la Ley de Propiedad Intelectual. El derecho de transformación se cede a los únicos efectos de lo dispuesto en la letra (a) del apartado siguiente.
3º. Condiciones de la cesión.
Sin perjuicio de la titularidad de la obra, que sigue correspondiendo a su autor, la cesión de derechos contemplada en esta licencia, el repositorio institucional podrá:
(a) Transformarla para adaptarla a cualquier tecnología susceptible de incorporarla a internet; realizar adaptaciones para hacer posible la utilización de la obra en formatos electrónicos, así
como incorporar metadatos para realizar el registro de la obra e incorporar “marcas de agua” o cualquier otro sistema de seguridad o de protección.
(b) Reproducirla en un soporte digital para su incorporación a una base de datos electrónica, incluyendo el derecho de reproducir y almacenar la obra en servidores, a los efectos de garantizar su seguridad, conservación y preservar el formato.
(c) Comunicarla y ponerla a disposición del público a través de un archivo abierto institucional, accesible de modo libre y gratuito a través de internet.2
(d) Distribuir copias electrónicas de la obra a los usuarios en un soporte digital. 3
4º. Derechos del autor.
El autor, en tanto que titular de una obra que cede con carácter no exclusivo a la Universidad por medio de su registro en el Repositorio Institucional tiene derecho a:
a) A que la Universidad identifique claramente su nombre como el autor o propietario de los derechos del documento.
b) Comunicar y dar publicidad a la obra en la versión que ceda y en otras posteriores a través de cualquier medio.
c) Solicitar la retirada de la obra del repositorio por causa justificada. A tal fin deberá ponerse en contacto con el vicerrector/a de investigación (curiarte@rec.upcomillas.es).
d) Autorizar expresamente a COMILLAS para, en su caso, realizar los trámites necesarios para la obtención del ISBN.
d) Recibir notificación fehaciente de cualquier reclamación que puedan formular terceras personas en relación con la obra y, en particular, de reclamaciones relativas a los derechos de propiedad intelectual sobre ella.
2 En el supuesto de que el autor opte por el acceso restringido, este apartado quedaría redactado en los siguientes términos:
(c) Comunicarla y ponerla a disposición del público a través de un archivo institucional, accesible de modo restringido, en los términos previstos en el Reglamento del Repositorio Institucional
5º. Deberes del autor.
El autor se compromete a:
a) Garantizar que el compromiso que adquiere mediante el presente escrito no infringe ningún derecho de terceros, ya sean de propiedad industrial, intelectual o cualquier otro.
b) Garantizar que el contenido de las obras no atenta contra los derechos al honor, a la intimidad y a la imagen de terceros.
c) Asumir toda reclamación o responsabilidad, incluyendo las indemnizaciones por daños, que pudieran ejercitarse contra la Universidad por terceros que vieran infringidos sus derechos e intereses a causa de la cesión.
d) Asumir la responsabilidad en el caso de que las instituciones fueran condenadas por infracción de derechos derivada de las obras objeto de la cesión.
6º. Fines y funcionamiento del Repositorio Institucional.
La obra se pondrá a disposición de los usuarios para que hagan de ella un uso justo y respetuoso con los derechos del autor, según lo permitido por la legislación aplicable, y con fines de estudio, investigación, o cualquier otro fin lícito. Con dicha finalidad, la Universidad asume los siguientes deberes y se reserva las siguientes facultades:
a) Deberes del repositorio Institucional:
- La Universidad informará a los usuarios del archivo sobre los usos permitidos, y no garantiza ni asume responsabilidad alguna por otras formas en que los usuarios hagan un uso posterior de las obras no conforme con la legislación vigente. El uso posterior, más allá de la copia privada, requerirá que se cite la fuente y se reconozca la autoría, que no se obtenga beneficio comercial, y que no se realicen obras derivadas.
- La Universidad no revisará el contenido de las obras, que en todo caso permanecerá bajo la responsabilidad exclusiva del autor y no estará obligada a ejercitar acciones legales en nombre del autor en el supuesto de infracciones a derechos de propiedad intelectual derivados del depósito y archivo de las obras. El autor renuncia a cualquier reclamación frente a la Universidad por las formas no ajustadas a la legislación vigente en que los usuarios hagan uso de las obras.
- La Universidad adoptará las medidas necesarias para la preservación de la obra en un futuro.
b) Derechos que se reserva el Repositorio institucional respecto de las obras en él registradas:
ü6ffiix§
Mad rid,
^ ..|..!.ou
Declaro, bajo mi rcsponsabilidad, que elProyecto presentado con eltltulo
Design of a test bench
en la ETS de lngenierla - lCAl de la Universidad Pontificia Comillas en el
curso acadrámico 2CI1512016 es de miautorla, original e inédito y
no ha sido presentado con anterioridad a otros efectos. El Prcyecto no es
plagio de otro, nitotal ni parcialmente y la irrformación que ha sido tomada
de otros documentos está debidamente referenciada.
r"rn ,/.(.t
0É.,
rul6
Autorizada la entrega delproyecto
EL DIRECTOR DEL PROYECTO
Fecha: 261 051 2016
W Bo del Coordinador de ProYectos
JÉ
Design of a test bench
RESUMEN
Las prácticas de laboratorio son esenciales en la enseñanza de la ingeniería.
Son el complemento perfecto para comprender la complicada teoría. Su
importancia no es sólo técnica ya que contribuyen a fomentar el liderazgo,
el trabajo en equipo y la capacidad de cumplir con estrictas fechas de entrega.
En el laboratorio de Máquinas Térmicas de la escuela de Ingeniería HEI se
está utilizando una tecnología anticuada para llevar a cabo sus prácticas de
laboratorio. De acuerdo a los tiempos de avances tecnológicos en los que
vivimos y a la incesante búsqueda de métodos educativos innovadores, el
departamento de Energía, Electricidad y Automática quiere reemplazar el
antiguo equipo por uno nuevo. Tienen a disposición un motor diésel de un
Toyota Yaris. Este motor va a ser utilizado para enseñar los principios de
funcionamiento de un motor de combustión interna a los alumnos de la
universidad.
El objetivo principal de este proyecto es diseñar una solución de freno
eléctrico usando la tecnología más apropiada en términos técnicos y
económicos. Para elegir el método de frenado más adecuado, se llevará a
cabo un estudio de las diferentes tecnologías existentes. Previamente, se
desarrollará un estudio del banco de ensayo que se encuentra actualmente en
el laboratorio.
ABSTRACT
Laboratory practices are essential in engineering education. They are the
perfect complement to understand the complicated theory. Their importance
is not only technical. They contribute to foster leadership, team-work and the
capability to meet strict deadlines.
In the laboratory of Thermal Machines of HEI School of Engineering are
using an antiquated technology in a test bench for the laboratory practices.
According to the high-tech times that we live in and the incessant search of
innovative educational methods, the Energies, Electricity and Automation
department of HEI wants to replace the old equipment with a new one. They
have at disposal a Toyota Yaris diesel engine. This engine is going to be used
to teach the principles of operation of internal combustion engines to
university students.
The main objective of this project is to design of a new electrical brake
solution for the diesel motor test bench using the most accurate technology
in technical and economical terms. To choose the most appropriate braking
method, a study of the different braking technologies is held. Previously, a
study of the current test bench components and operation is developed.
Acknowledgments
First, thanks to my project leaders Pascale Bouvier and Diana Florez who followed my progress and helped me during the project. I would also like to thank Patrice Seigner who has advised me in certain technical aspects.
Thanks to the International Relationships department, specially to Anne Lacour, for their understanding and support during my stay in France.
Finally, I am truly grateful to my family. Without their emotional and economical support this rewarding experience would not have been possible. Special thanks to my parents and my sister because they have always been there when I have needed it.
Contenido
1. Introduction ... 6
1.1 Context ... 6
1.2 Problem ... 6
1.3 Objectives ... 7
1.4 Constraints ... 7
2. Study of the current test bench in the laboratory of Thermal Machines ... 8
2.1 Composition and operation of the current test bench ... 8
2.2 Diesel engine ... 9
2.3 Electrical braking system... 9
2.3.1 DC machine ... 9
2.3.2 External braking resistor bank ... 10
2.3.3 Electrical control cabinet ... 11
2.4. Engine data panel ... 12
2.5 Test ... 14
3. Study of electrical braking systems... 15
3.1 DC motor braking system ... 15
3.2 AC motor braking systems ... 15
3.2.1 DC injection braking ... 15
3.2.2 Motor flux braking ... 16
3.2.3 Dynamical Braking ... 17
3.2.4 Regenerative braking or IGBT bridge configuration ... 18
3.3 Eddy-current or Foucault current dynamometer ... 19
3.4 Powder dynamometer ... 21
3.5 Conclusion of the study ... 21
4. Design of the new test bench ... 23
4.1 Diesel engine ... 24
4.2 Drive operation ... 26
4.3 Selection of asynchronous machine ... 28
4.3.2 Dimensioning the asynchronous machine ... 31
4.4 Dimensioning the braking resistor ... 39
4.7.1 Protections of the frequency converter ... 47
4.7.2 Protections upstream of the frequency converter ... 47
4.7.3 Protections downstream of the frequency converter ... 50
4.8 Wires ... 51
5.Cost estimation ... 54
5.1 Dynamic brake system cost estimation ... 54
5.2 Eddy current and Regenerative dynamometer cost estimation ... 55
Conclusions ... 56
List of figures
Figure 1: Block diagram of the current test bench ... 8
Figure 2: Compound Motor scheme ... 10
Figure 3: External braking resistor bank ... 11
Figure 4: Electric cabinet ... 12
Figure 5: Engine data panel ... 13
Figure 6: Scheme of DC injection braking. ... 16
Figure 7:Dynamical braking configuration. ... 17
Figure 8: Regenerative braking or IGBT bridge configuration. ... 18
Figure 9: Scheme of the analogy between a power transmission system and an IGBT bridge configuration... 19
Figure 10: Lateral view of eddy current dynamometer and frontal view of the rotor. [24] ... 20
Figure 11: Scheme of the new electrical braking system ... 23
Figure 12: EOBD plug and adaptor. ... 25
Figure 13: Data transmission architecture. ... 25
Figure 14: Torque-Speed map for drive applications... 26
Figure 15: Explanation about the effect of the speed reducer in torque, rotational speed and power transmitted ... 32
Figure 16: Scheme of the test bench configuration with the gearbox included ... 33
Figure 17: Simplified equivalent circuit in L of the asynchronous machine ... 35
Figure 18: : Parameter values of the asynchronous machine ... 38
Figure 19: Sensorless vector control scheme. ... 41
Figure 20: Flux vector control scheme. ... 42
Figure 21: Voltage-frequency curve ... 44
Figure 22: λ-frequency curve ... 45
Figure 23:Torque-frequency curve ... 46
Figure 24: Circuit Breaker Curve MA. ... 49
List of tables
Table I: Diesel engine technical data ... 9
Table II: DC Machine technical data ... 9
Table III: Current, voltage, power and resistance values during the practical work ... 11
Table IV: List of measured parameter and measure instrument ... 13
Table V: Measurements values acquired during the laboratory practical work ... 14
Table VI: Summary of conclusions ... 22
Table VII: Diesel engine technical data ... 24
Table VIII: List of measured parameters and the new measure instrument ... 24
Table IX: Rotational speed depending on their pair of poles ... 30
Table X: Theoretical characteristics of the asynchronous machine ... 34
Table XI: Asynchronous Machine Technical Data ... 34
Table XII: Approximate errors of the asynchronous machine parameters ... 36
Table XIII: Braking resistor ... 39
Table XIV: Speed Accuracy and Torque response of the different controls. ... 42
Table XV: Frequency Converter Technical Data ... 42
Table XVI: Circuit Breaker Technical Data ... 50
Table XVII: Operating current for each type of insulating material ... 52
Table XVIII: Dynamical brake system cost estimation ... 54
Table XIX: Eddy current and AC dynamometer cost estimation ... 55
Appendix I: Current test bench documents Appendix II: Asynchronous machine Appendix III: Braking resistor Appendix IV: Frequency converter Appendix V: Electrical protections
1.
Introduction
1.1
Context
“Engineering is a practical discipline. It is a hands-on profession where doing is key.”
“The function of the engineering profession is to manipulate materials, energy, and information, thereby creating benefit for humankind. To do this successfully, engineers must have a knowledge of nature that goes beyond mere theory.”
Lyle D. Feisel and Albert J. Rosa. The Role of the Laboratory in Undergraduate Engineering Education.
Laboratory practices are essential in engineering education. They are the perfect complement to understand the complicated theory. Their importance is not only technical. They contribute to foster leadership, team-work and the capability to meet strict deadlines.
In the laboratory of Thermal Machines of HEI School of Engineering are using an antiquated technology in a test bench for the laboratory practices. According to the high-tech times that we live in and the incessant search of innovative educational methods, the Energies, Electricity and Automation department of HEI wants to replace the old equipment with a new one. They have at disposal a Toyota Yaris diesel engine. This engine is going to be used to teach the principles of operation of internal combustion engines to university students.
1.2
Problem
The university wants to carry out the same laboratory practice in the new motor that they held in the current test bench. It’s important to understand the operation and to analyse each element of the current test bench.
To develop the test, it is necessary to design a variable charge to control the motor’s torque maintaining constant the rotation speed. The students have to design a brake system to simulate this variable charge.
1.3
Objectives
The main objective of this project is to design of a new brake solution for the diesel motor test bench using the most accurate technology in technical and economical terms. To choose the most appropriate electrical braking method, a study of the different braking technologies is held. This main objective has implicit the following tasks:
-Analyse and determinate how the old configuration of the test bench works in order to use some components, if possible, for the new test bench.
-Develop a comparative study of different electrical braking systems and choose the most adequate according to technical and economic guidelines.
-Determinate and dimension the necessary equipment for the test bench’s assembly with the chosen technology.
-Estimate the price that the proposed solution will cost to HEI.
1.4
Constraints
The final model designed is theoretical. If the construction or the assembly of the solution proposed are impossible due to external causes to the student (for example the university can’t afford the equipment required), they will not be responsible for the unsuccessful result.
2. Study of the current test bench in the laboratory of
Thermal Machines
The theoretical concepts, that students acquire and the report that they have to write during the laboratory practice, are not going to change with the implementation of a new test bench or at least they must be very similar. Thus, to design the new test bench it is important to understand first how the old test bench works and take into account all the necessary data to do the calculations for the report.
2.1 Composition and operation of the current test bench
As shown in figure 1, the current test bench is composed of five basic blocks: a diesel engine, a DC machine, an electrical cabinet, a variable braking resistor bank and a diesel engine panel. Each element is going to be explained in detail. A shaft directly connects the diesel engine to the DC machine. Therefore, the velocity ratio between the rotational speed of the diesel engine shaft and the DC machine rotor is 1:1.
The diesel engine converts thermal energy into mechanical energy generating a torque in the shaft. The rotational speed in the shaft is constant due to the opposite torque given by the DC generator. The DC machine converts the mechanical energy in electrical energy which is dissipated in the resistor.
To start the diesel engine, the DC machine starts acting as a motor. Once the diesel engine is running, the operation of the DC machine is changed, with a selector, into generator mode by a technician.
2.2 Diesel engine
The diesel engine has a single cylinder, four strokes and a water cooling system. It is also equipped with an exhaust pipe and a silencer. The most important technical data is collected in table I.
Diesel Engine Technical Data
Power 5 hp
Bore 87,3 mm
Stroke 110 mm
Compression ratio 16,5:1
Maximum speed 1800 rpm
Type Petter PHIW
Table I: Diesel engine technical data
2.3 Electrical braking system
It is composed of a DC machine and an external resistor bank. The operation of the DC machine is controlled from an electrical cabinet.
Dynamic or rheostatic braking is the method used by the test bench. The DC generator transforms the kinetic energy, furnished by the diesel motor, into electrical energy that is dissipated in the braking resistor bank as heat.
2.3.1 DC machine
The DC machine was made by BKB Electric Motors, English company located in Birmingham that was dissolved around 1992. The information available of this machine is the nameplate and the data sheet. This DC machine can act as a motor or as a generator. This means that it is possible to operate in the four quadrants. It is used as a motor to start the diesel engine and as a generator to transform the kinetic energy into electrical energy to dissipate it in the braking resistor. The most important technical data is collected in table II.
DC Machine Technical Data
Power 7 kW
Voltage 220 V
Current 31.8 A
Speed 1800 rpm
Maximum speed 2500 rpm
Insulation Class E
Type Compound
The type of machine, as specified in the nameplate, is compound. This means that it is a combination of a shunt and a series machine. This type of machines performs more approximately to a shunt motor than a series machine. Figure 2 shows the electric scheme of its internal configuration.
Figure 2: Compound Motor scheme [11]
Speed and torque of the DC machine are controlled with voltage and current respectively:
- The speed of the armature is directly proportional to the armature voltage. The rotational direction can be changed by switching the polarity.
- The torque is directly proportional to the armature current.
2.3.2 External braking resistor bank
The DC generator is used to simulate a variable load for the diesel engine. The electrical power generated by the DC generator is dissipated as heat. The variable torque and the constant speed are obtained by varying the resistance value of the resistor bank with the selector installed in the electrical cabinet.
Using the data of the voltage and the current that were measured, the power generated by the DC machine and the approximate resistance value are calculated with the following formulas respectively.
= ∙ [W]
= [ ]
Where:
P: electrical power generated by the DC machine measured in Watts [W]. : DC voltage applied to the DC machine in Volts [V].
The results are collected in table III.
Test Number 1 2 3 4 5 6
I (A) 0 5 10 15 20 26
U (V) 130 130 130 130 130 130 Electrical Power
(W) 0 650 1300 1950 2600 3380
Resistance (Ω) - 26 13 8.67 6.5 5
Table III: Current, voltage, power and resistance values during the practical work
As observed in figure 3, there is not an additional refrigeration system in the external braking resistor bank. Thus, the heat is dissipated in the air by natural convection.
Figure 3: External braking resistor bank
2.3.3 Electrical control cabinet
From this cabinet voltage and current are measured with an internal voltmeter1 and
ammeter2. As shown in figure 4, it also includes all the necessary components to control
the DC machine and the resistor bank:
- Field rheostat3: to regulate the voltage.
- Motor/generator selector4: it allows to operate in the four quadrants.
- Load control5: the external resistance value is changed with the selector
- Emergency stop button6: this button cuts the power supply in an emergency case.
- Starter7: to start the DC machine.
Figure 4: Electric cabinet
2.4. Engine data panel
In this panel all the parameters that concern to the diesel engine are measured. The only parameter that does not appear in the panel is the torque that it is measured by spring balance. In figure 5 is shown the engine data panel with all the parameters indicated.
1 Voltmeter 2 Ammeter
4 Motor/Generator Selector
6 Emergency stop button
7 Starter
8 Armature speed control
5 Load control 3 Field rheostat
Figure 5: Engine data panel
In table IV, all the required diesel engine parameters are gathered and also the devices needed to acquire them. These are the parameters that students have to collect to elaborate the practical work report.
Parameters Measured by
Rotational speed Tachometer Exhaust gas temperature Temperature sensor
Air flow supply Flowmeter Cooling water flow Rotameter Inlet and outlet temperature of cooling
water Temperature sensor Injected fuel consumption Fuel gauge and chronometer
Torque Spring balance
Table IV: List of measured parameter and measure instrument
Rotational Speed
Exhaust gas temperature
Inlet temperature of cooling water
Outlet temperature of cooling water
Cooling water flow Injected fuel flow
2.5 Test
With regard to understand the operation of the test bench, some tests have been carried out. These tests are the same that students usually do in their practical work sessions. In each test the power torque provided by the diesel engine is increased and also the resistance value is changed to dissipate the necessary energy. In table V can be appreciated the data collected during a typical practical work.
Test number 1 2 3 4 5 6
I(A) 0 5 10 15 20 26
U(V) 130 130 130 130 130 130
Electrical Power (W) 0 650 1300 1950 2600 3380
F (N) 4 26 44 64 79 108
Speed (rev/min) 1238 1259 1217 1246 1169 1248
Cooling water flow (L/h) 15 30 40 40 40 80
Inlet Temp. water (˚C) 12,2 12,2 10,6 10,7 10,6 10,5
Outlet Temp. water (˚C) 50,6 52,7 47,4 54,4 61,4 52,2
Exhaust gas temperature
(˚C) 136 180 216 278 334 421
Time (s) 236 178 144 110 90 67
Table V: Measurements values acquired during the laboratory practical work
In Appendix I are attached all the documents that were used to acquire the technical data of each element of the test bench in order to understand the operation.
3. Study of electrical braking systems
With regard to select the most appropriate braking method for the diesel engine, they are going to be analysed to understand their operation and to check if they can meet the braking requirements.
3.1 DC motor braking system
The DC motor braking system is the same configuration as it is installed in the laboratory. Its components and operation were previously described in the chapter 2 ‘Study of the existent test bench’.
3.2 AC motor braking systems
The scheme is very similar to a dc motor braking system but its components and operation mode are completely different. In this part electrical braking methods are going to be explained with the purpose of choosing the most adequate system for our application.
3.2.1 DC injection braking
Once the AC voltage supply is disconnected from the stator, a DC voltage is applied to the stator windings. Thus, a DC current is injected into two of the phases. This DC current circulates in the stator windings creating a stationary magnetic field that causes a negative slip producing a braking torque. When this magnetic field is cut by the rotor a current appears in the rotor windings. The energy generated during the braking is dissipated as heat in the rotor windings. If the current is too elevated or the time that is applied is excessive, this can lead to overheating problems. [14]
The DC current value depends on the time that is required to stop the motor. The shorter the time, the higher the current. It is not recommended to surpass the rated current of the motor to avoid overheating problems. [7]
This type of braking is not appropriated for our application because it requires the disconnection from the AC supply to brake. So, it is not possible to control the torque or the speed. In addition, the continuous injection of DC current into the windings can be detrimental for the motor. This configuration is recommended for emergency braking. For example, to stop a motor with a huge inertia when there is a failure in the electricity supply.
Figure 6: Scheme of DC injection braking. [12]
3.2.2 Motor flux braking
It is also known as compound braking. The main principle of this method is to decelerate the drive system using motor losses. Basically, the operation is to use the motor as a ‘braking resistor’ to dissipate the energy. The motor flux and the magnetizing current increases when braking is required. The control used in these applications is the Direct Torque Control (DTC) [6] which makes it possible to always have control over the motor.
If we increase the current to decelerate, consequently motor losses also increase. The braking energy that motor can dissipate is directly proportional to the internal resistance of this motor. This is a drawback because the resistance value supposes a limit for the braking power. The continuous use of this kind of braking can overheat the motor causing irreparable damages.
In conclusion, this braking system is only appropriate for intermittent service and low power motors (typically below 5kW) due to electric and economical waste that supposes to increase the current to decelerate.
3.2.3 Dynamical Braking
Maintaining the DC bus voltage constant is the key of this configuration. When a certain value of DC bus voltage, which depends on the nominal voltage of the inverter, is exceeded the braking energy is conducted to resistor connected to the DC link where energy is dissipated. This supposes an advantage because if AC supply is interrupted the chopper can continue working. The scheme of the dynamical braking configuration is shown in figure 7.
3.2.4 Regenerative braking or IGBT bridge configuration
This configuration allows to return the braking power to the grid, that is the reason why it is known as regenerative braking or IGBT based regeneration. In figure 8 are shown the power flows in the two possible operation modes, motoring and generating.
Figure 8: Regenerative braking or IGBT bridge configuration. [9]
To change the operation mode from motor to generator and to inject energy into the grid the rectifier must be active and the inverter must be a current source inverter.
Applying the same regulation method as in power transmission grid, the IGBT can be controlled. This is expressed in the following formula.
= ∙ ∙ sin [ ]
Where:
- : is the power supply voltage measured in V.
- : is the rectifier voltage measured in V.
- : is the reactance value measured in Ω.
This formula means that adjusting the power angle between the two AC systems, the power flow can be regulated. In this case, the IGBT bridge represents the AC voltage supply which is connected to the motor through a choke, as we can see in figure 9.
Figure 9: Scheme of the analogy between a power transmission system and an IGBT bridge configuration. [6]
The DC bus voltage is always constant independently of the value and the direction of the power flow. The incoming power flow has to be the same as the outgoing power flow of the DC bus.
The benefits of using IGBT bridge configuration are numerous: small amount of harmonics; the DC bus voltage does not grow dangerously while braking and energy efficiency improvement. But the main drawback it is the cost which is too elevated, prices are around 130.000€. This solution is optimal when the braking is repeated, the amount of energy generated is significantly important and harmonics have to be minimal.
There are two options in order to use the energy created with this method. The first one is to store the braking energy in batteries connected to the DC link to be used when needed. The second option is to inject directly this energy into the grid, this election requires that the grid must allow an income power flow.
3.3 Eddy-current or Foucault current dynamometer
This braking method is based on the Faraday’s Law of electromagnetic induction and in the Lenz’s Law. Eddy currents are induced in a conductor when this is under the influence of a varying magnetic field. Eddy currents oppose to the magnetic field that generates them. With regard to understand the operation of eddy current braking the physical principles that this method involve are going to be explained. [23]
Faraday’s Law states that the induced electromagnetic force is proportional to the rate of change of a magnetic flux over the time.
Lenz’s Law states that the direction of an electromagnetic force is always the opposite of the change that has produced it. As described in the following equation.
Where:
- : induced electromagnetic force.
- : magnetic flux variation
-N: number of turns in the coil.
The composition of an eddy current brake is quite similar to a rotating electrical machine. This brake consists in two parts, as shown in figure 10.
Figure 10: Lateral view of eddy current dynamometer and frontal view of the rotor. [24]
-Electromagnetic stators: they are composed of coils disposed in a circular way around
the rotor. These coils are connected to the electrical power supply in order to create a magnetic field.
-Rotor: a notched disc which is connected by a shaft to the engine that is going to be
tested. They must be made of a non-ferromagnetic metal to induce eddy currents on it.
The electromagnetic stators create a time-varying magnetic field using the power supply. When the shaft starts to turn, eddy currents are induced in the rotor caused by the time-varying magnetic field. These eddy currents produce an opposite magnetic field, as it was demonstrated in Lenz’s Law. An electromagnetic force appears in the opposite way to the rotational direction creating a “braking torque”. The braking torque is controlled by varying the strength of the magnetic field. This magnetic field depends on the current that circulates in the stator windings. The kinetic energy is dissipated as heat by the eddy currents.
Depending on the cooling system there are two types of eddy currents dynamometers:
-Wet gap: the rotor or part of it is in contact with the water. This improves the heat
dissipation.
3.4 Powder dynamometer
The composition is the same as the eddy current dynamometer, as it is formed by stator and rotor. The main difference is that there is a metallic powder between stator and rotor. When a magnetic field is applied by the stator, caused by a current circulating in the stator coils, this powder changes its characteristics and it sticks on the rotor causing friction and braking the rotor. To dissipate the heat generated by the friction this type of dynamometer has a water cooling system.
3.5 Conclusion of the study
After the study, among all the possible options the technology chosen is the dynamical braking. The design is simple, the technology is well-known and it allows to control speed and torque with accuracy. All the components that this configuration requires for its construction and assembly are available in the market and there is no need to fabricate or ask for a custom-made product. AC drives have a lot of applications in the industry, so there are a lot of manufacturers so there is a wide range of commercial options and suppliers.
The design and construction from zero of eddy currents and powder dynamometer are complicated and requires to fabricate all the pieces in the university or ask manufacturers for custom-made pieces Thus, the possibility to construct one of these kind of dynamometers is neglected. But a market research is going to be developed to look for manufacturers that offer an integrated solution. The advantages of buying an integrated solution are:
- The installation of the equipment is handled by the manufacturer.
- In case of a failure in the equipment, manufacturers offer a warranty period or a maintenance service
In order to summarize the conclusions of the study, in table VI are collected the main reasons for accepting or rejecting each braking method analyzed. The braking method selected is highlighted in bold.
DC Injection braking Not possible to control speed and torque. Motor flux braking Only appropriate for intermittent service
and low power motors. Motor overheating.
Dynamical Braking Well known technology.
Easy control of speed and torque. Simple design.
Regenerative braking or IGBT bridge configuration
Elevated cost.
The grid must allow power flow injection.
Eddy-current dynamometer Difficult design and construction. Powder dynamometer Difficult design and construction.
Table VI: Summary of conclusions
In conclusion, the braking method that is going to be designed for our application is a dynamical braking system. The design of the system will alternate a supply market research for alternative methods as eddy current dynamometer, powder dynamometer or regenerative dynamometer.
4.
Design of the new test bench
The new test bench consists in two parts:
-Diesel engine, including a computer to monitor the diesel engine operation and an EOBD connector.
-Dynamic braking system.
The electrical equipment needed to build the dynamical braking system is composed of: - Rectifier: transforms AC voltage into DC voltage.
- DC link or DC bus.
- Rheostat or power dissipation resistor: dissipates the braking energy as heat. - Inverter: converts DC voltage into AC voltage.
- Asynchronous machine.
An optimal solution to control the asynchronous machine is to apply a frequency converter. This unit includes a voltage input to connect to the power supply, a rectifier, a DC bus, an inverter and an electrical output to connect to the asynchronous machine. The DC bus has terminal blocks to connect the power dissipation resistor. The scheme of the new test bench is shown figure 11.
4.1 Diesel engine
The technical characteristics of the diesel engine are the starting point of all the calculations to dimension the electrical braking system is. The most important characteristics are summarized in table VII.
Toyota Yaris II 1.4D4D 90 8V Turbo
Displacement / Cylinders 1364 cm3 / 4 cylinders
Maximum Torque 190 N∙m between 1800 and 2800 rev/min Maximum Power 67kW (90hp) at 3800 rev/min
Weight 150 kg
VIN Number VNKKC96360A178373 Table VII: Diesel engine technical data[5]
To acquire all the relevant parameters that the students need to elaborate the report, the EOBD supposes an excellent tool. All the sensors and thermometers that where included in the old configuration are not needed if the OBD data acquisition system is used. Furthermore, the amount of different variables measured that can be extracted with the EOBD can open a wide range of possibilities to improve the laboratory practices and to increase the theoretical concepts that can be learned by the students. In table VIII are collected all the required parameters that must be measured and their measure instrument.
Parameters Measured by
Rotational speed OBD
Exhaust gas temperature OBD
Air flow supply OBD
Cooling water flow OBD
Inlet and outlet temperature of cooling water
OBD
Injected fuel consumption OBD
Torque OBD
Table VIII: List of measured parameters and the new measure instrument
Since 2004, all the diesel vehicles that are manufactured and commercialized in Europe have to include an EOBD (European On-Board Diagnostics) plug. This EOBD plug is connected to sensors which are installed in the engine. Basically, the EOBD is a computer based system designed to acquire all the relevant parameters of the diesel engine performance to detect possible failures or simply to monitor the operation.
The EOBD plug is composed of 16 pins. Each car manufacturer uses the 16 PIN in a different way. In figure 12 is shown the EOBD plug and the adaptor that transfer the data from the engine to a computer.
Figure 12: EOBD plug and adaptor. [18]
The basic architecture of the OBD data transmission system is explained in figure 13.
4.2 Drive operation
Drive applications are divided in three groups depending on speed and torque direction criteria [6]:
-One-quadrant application: speed and torque acquire the same direction. The power flow goes from the motor to the load. Typical in pumps and fans where the load’s quadratic behaviour requires a variable torque.
-Two-quadrants application: the speed direction is always the same but the torque direction can be modified. So, the power flow can go in both directions. This means that the asynchronous machine can work as a motor or as a generator.
- Four-quadrant application: the direction of speed and torque can indifferently change.
In figure 14 we can observe the four quadrants and their respective rotational speed and torque directions.
Figure 14: Torque-Speed map for drive applications. [6]
In a brake application, the asynchronous machine must work in the decelerating quadrants (II and IV). Thus, the case is a four-quadrant application.
The first step is to define the drive operation. In this case, the diesel engine is going to be started before than the asynchronous machine.
Once the asynchronous machine is started, the constant speed is maintained as a result of an opposite torque when the diesel engine tries to accelerate. The torque of the diesel engine is going to be increased progressively keeping constant the rotational speed.
The power generated by the diesel engine is defined by:
= ∙ [ ]
Where:
-τ: is the torque in N∙m.
-ω: is the rotational speed in rad/s.
Thus, the incremental variation of the power generated by the diesel engine is going to depend only on the torque variation because the rotation speed is going to be the same during the whole the test.
The differential equation of motion for a drive system relates the torque with the angular velocity variation and moment of inertia, as it is observed.
− = ∙ [ ∙ ]
Where:
- : is the asynchronous machine torque in N∙m.
- : is the diesel engine torque in N∙m.
-J: is the moment of inertia of the drive system in kg/m2.
- : is the derivative of the rotational speed.
In this drive application, the objective is to achieve a stable speed. This means that the angular velocity is constant and therefore the derivative of the rotational speed with respect to time is equal to zero. The moment of inertia is constant and multiplied by the derivative of the rotational velocity with respect to time is equal to zero. Thus, isolating the torque of the asynchronous machine.
=
4.3 Selection of asynchronous machine
4.3.1 Selection of asynchronous machine
Type of asynchronous machineThe first step is to choose the type of asynchronous machine that it is going to be used. There are two types of asynchronous machines. The only difference between them is the type of rotor but their working principle is the same.
The stator generates a rotating magnetic field that induces another magnetic field in the rotor. The interaction between this two magnetic fields creates a torque that turns the rotor.
The two types of asynchronous machines are:
-Single cage or squirrel cage: the rotor is composed of bars, usually made of copper, integrated between two metal rings.
-Wound rotor: the rotor windings are similar to the stator windings and the number of poles of stator and rotor are the same. In the shaft, there are integrated slip rings that are in contact with brushes connected to an external resistance.
The wound rotor machines are usually expensive and require a high level of maintenance because the brushes and the slip rings need to be changed due to wear and tear. They were very popular because they allowed to control the speed in an easy way using a rheostat. But with the apparition of frequency converters and their sophisticated control methods, the use of this type of machines decreased.
The squirrel cage machine is cheaper, requires a low maintenance and it is possible to control the speed and the torque with a frequency converter. Thus, this is the type of asynchronous machine that is going to be used.
Methods of starting asynchronous machines
- Starting by reduced stator voltage: a reduced voltage is applied to the asynchronous machine when starting, and once it is rotating, increase the voltage to its nominal value. This starting method can be achieved by a star-delta start, only valid if the machine is going to work on delta, or an auto-transformer.
- Soft start by V/f ramp: starting from zero frequency, the V/f ratio is increased to arrive at a controlled rate until the selected speed.
- Flying start: when an asynchronous machine is started once the rotor is already turning, it is called ‘a flying start’ or ‘to start on the fly’. The inverter starts the machine with a reduced voltage and synchronizes the frequency with the rotational speed consequently. The installation of an encoder is not required, to use this function, using a sensorless vector control the frequency converter can estimate the rotational speed. An acceleration ramp is used by varying the V/f characteristic till the operation point is set. This option is only included in some frequency converters. Thus, this parameter must be taken into account when the frequency converter choice is made.
The only possible method to start the asynchronous machine when the rotor is already turning is the flying start. Thus, the availability of flying start is a requirement for the frequency converter.
Voltage
The required nominal voltage is at least 400V because it is a three phase asynchronous machine.
Torque and rotational speed
The starting point is the maximum torque and the rotational speed that the diesel engine can provide.
- The maximum torque that the diesel engine can provide is 190 N∙m.
- The maximum torque can be generated in a speed interval between 1800 rpm and 2800 rpm.
The synchronism angular velocity of an asynchronous machine depends on their poles. As the number of poles increases, the synchronism angular velocity decreases and the torque that the machine can provide increases. From two pair of poles, if the number of poles continue increasing the nominal power factor is going to get worse and the weight is going to increase. The synchronism angular velocity is calculated by.
= 60 ∙ [ ]
Where:
- : is the normalized rotational speed in rpm.
- : is the frequency measured in Hz. In Europe is 50Hz.
Electrical machines manufacturers offer, in their asynchronous machine catalogues for industrial applications, four normalized synchronism velocities which are 3000 rpm, 1500 rpm, 1000 rpm and 750 rpm. These normalized synchronism velocities depend on the asynchronous machine number of poles. One pair of poles for 3000 rpm, two pair of poles for 1500rpm, three pair poles for 1000 rpm and four pair poles for 750 rpm. Manufacturers do not produce squirrel cage machines beyond four pair of poles. In table IX are summarized the different rotational speeds depending on their pair of poles.
Pair of poles Rotational Speed
1 3000 rpm
2 1500 rpm
3 1000 rpm
4 750 rpm
Table IX: Rotational speed depending on their pair of poles
An important parameter that has to be taken into account is the slip. It is the difference between the synchronous rotational speed and the rotor rotational speed, as showed.
= − ∙ 100 [%]
Where:
- : is the synchronism normalized rotational speed in rpm.
- : is the rotor rotational speed in rpm.
- If the slip is positive the asynchronous machine acts as a motor. - If the slip is negative the asynchronous machine acts as a generator.
The typical slip value is between a 2% and a 5%.
In the steady state of this application the asynchronous machine has to act as a generator. Thus, the rotor speed, which is the rotational speed of the diesel engine shaft, has to be bigger than the synchronous speed to achieve a negative slip.
4.3.2 Dimensioning the asynchronous machine
The normalized rotational speed of the asynchronous machine has to be included in the interval of the diesel engine operation speed, as observed.
< <
Where:
- : is the minimum rotational speed of the diesel engine.
- : is the normalized synchronous speed of the asynchronous machine.
- : is the maximum rotational speed of the diesel engine.
None of the normalized rotational speed are included in this range, so a reduction gearbox, also known as speed reducer, has to be included in the configuration. To calculate the speed reducer ratio, the next formula is used.
=
Where:
- N: is the speed reducer ratio. It has to be a whole number.
- : is the rotational speed that the diesel engine is going to achieve. It is
measured in rpm or rad/s.
- : is the normalized synchronous speed of the asynchronous machine. It is measured in rpm or rad/s. It has to be in the same units as .
Regarding these two requirements, the implementation in the system is going to be analyzed for each normalized synchronous speed:
- 3000 rpm: The maximum rotational speed value that the diesel engine can provide to reach the maximum torque is 2800 rpm. If the rotor of the asynchronous machine runs at this velocity, the slip can only be positive. Thus, it is not possible to generate electrical energy. These machines are not valid for the diesel engine speed range.
- 1500 rpm: the minimal speed value that the diesel engine can provide for the maximum torque is 1800 rpm. The slip value for this speed with a 1500 rpm machine is a 20% which is a too elevated value to maintain in steady state. Even if a reducer with a 2:1 ratio is used, a rotational speed bigger than 2800 rpm is impossible to achieve, so these machines are not valid for the configuration.
- 1000 rpm: this speed is not in the interval between 1800 rpm and 2800 rpm, so in theory it is not possible to use these asynchronous machines. But if a speed reducer with a 2:1 ratio is used the diesel engine can work around 2000 rpm speed. This rotational speed is included in the interval and there is a margin for achieve positive and negative slip values.
- 750 rpm: this speed is not included in the interval between 1800 rpm and 2800 rpm. If a speed reducer with a 3:1 ratio is used, the diesel engine would have to run around 2250 rpm. This rotational speed is included in the interval and there is a margin for achieve positive and negative slip values.
The only two possibilities are to use a 1000 rpm or a 750 rpm machine. The option with the less pair of poles (1000 rpm) is chosen in order to have a better power factor. Choosing the 1000 rpm option the product torque∙rotational speed (τ∙ω) is going to be smaller and thus the power dissipated.
Supposing that the gearbox used as speed reducer ideal and does not change the power transmitted between the diesel engine and the asynchronous machine. The speed is reduced by half but the torque is increased to the double. As shown in figure 15.
Figure 15: Explanation about the effect of the speed reducer in torque, rotational speed and power transmitted
The configuration with the speed reducer integrated is shown in figure 16.
Figure 16: Scheme of the test bench configuration with the gearbox included
The nominal power of an asynchronous machine that appears in the nameplate is the result of multiplying the nominal torque by the nominal angular velocity.
= ∙ [ ]
However, in the dimensioning of a drive application the parameters that have to be considered are the maximum torque and the normalized synchronism velocity. Multiplying these parameters, the minimum power of the asynchronous machine is obtained.
> ∙ [ ]
> 2 ∙ 190 ∙1000 ∙ 2
Nominal current
The nominal current of the asynchronous machine is calculated as.
=
√3 ∙ ∙ cos [ ]
Where:
-P: is the electrical power in W.
-U: is the three phase voltage in V.
-cos : is the power factor.
The power factor and the efficiency depend on the electric machine chosen. To do the calculations, the supply voltage is 400V, the estimated is 0,8 and the power is the previously calculated.
The theoretical characteristics of the asynchronous machine needed are the following:
ωs 1000rpm
Rated Power 39.793,4 W
Rated Voltage 400 V
Minimum Current 77,46 A
Minimum Torque 380 N∙m
Table X: Theoretical characteristics of the asynchronous machine
In commercial catalogues, there are not asynchronous machines rated at 39kW. The most immediately proximate normalized nominal power that is offered is 45 kW. So, the data of the asynchronous machine that has to be used is contained in the following table. To make the analysis of the asynchronous machine, the technical data has been extracted from ABB data sheet which is attached in Appendix II.
Asynchronous Machine Technical Data
Commercial denomination 3GBA 283 110-ADL
Rated Power Pn 45 kW
Rated Voltage Un 400 V
Rated Current In 83,3 A
Rated frequency fn 50Hz
Starting Current 6∙In
Power factor Cos φ 0,84
Nominal Torque 434 N∙m
Moment of inertia 1,85 kg∙m2
4.3.2 Model of the asynchronous motor
The simplified equivalent circuit in L is used to simulate the asynchronous machine, as illustrated in figure 17.
Figure 17: Simplified equivalent circuit in L of the asynchronous machine
Where:
- : represents the losses in the copper of the stator.
- : represents the short-circuit reactance
- : represents the iron-loss resistance.
- : represents the magnetizing reactance.
- ⁄ : represents the variable rotor resistance.
- : represents the input current.
- : represents the magnetizing current.
- : represents the current circulating in the rotor.
The procedure followed to calculate all the parameters is taught the subject Electrical Machines of ICAI School of Engineering. The parameters of the asynchronous machine are going to be calculated approximately with the catalogue data. The results have an approximated error but they are very close to the exact value. The estimated error oscillates between 1% and 10% depending on the parameter. In the table XII it is observed the approximate error for each parameter.
Asynchronous Machine Parameter Approximate error 10%
1% 5% 5% 10%
Table XII: Approximate errors of the asynchronous machine parameters
Once the asynchronous motor will be available, the parameters can be calculated accurately using the data obtained of the no-load and short-circuit tests.
First, we calculate the nominal power in per-unit and the nominal slip to estimate , as shown.
= = 45000
√3 ∙ 400 ∙ 83,30= 0,78
=1000 − 990
1000 = 0,01
Where:
- : is the nominal power in per-unit.
- : is the nominal power in W.
- : is the base apparent power in VA.
The air-gap power in per-unit ( ) is defined by the following equation:
=
(1 − )= ∙ (1 − )
Where:
- : is the mechanical power in the rotor in per-unit.
- : is the slip in per-unit.
When the slip is much lower than 1 and the asynchronous machine is working under rated operating conditions the air-gap power is approximately the nominal power and the nominal power can be calculated in the following way to obtain .
≈ ≈ ∙ → ≈0,78
0.01= 0,0128
and are calculated with the efficiency of 100% and 75% of the load. The total losses are calculated with the efficiency and the nominal power for both cases and they are also calculated with and the . Finally, we set the two equations equal to each other.
1
− 1 ∙ = + ∙ → 1− 1 ∙ = + ∙
1
0,928− 1 ∙ 0.78 = + 0,78 ∙
1
0,93− 1 ∙ 3
4∙ 0.78 = + ( 3
4∙ 0,78 ) ∙
Solving this equation system with 2 unknowns, the values of and are obtained.
= 0,0619 = 0,0228
Using these values, we can calculate and .
= − = 0,0476
= = 1
0,0234= 43,86
The must be in pu referred to torque base MB to use the formula to calculate .
However, in the catalogue the is referred to to the nominal torque. Thus, we have to change the base of which is very simple
The value of the short-circuit reactance ( ) is obtained neglecting the value of in the square root of the formula in per-unit referred to torque base.
= ± 1
2 ∙ ( ± + ) → ≈
1
2 ∙ + = 0,305 pu
The last parameter to be calculated is the magnetizing reactance ( ). First, we calculate the reactive power and with this data we are able to calculate solving the resultant equation.
= 1 − cos = 0,542
= + ∙ ≈ 1 + ∙
≈ 1
− ∙ = 2,8
All the parameter values of the asynchronous machine are collected in figure 18.
4.4 Dimensioning the braking resistor
The peak voltage on the DC bus is related to the supply voltage as the following formula shows.
= √2 ∙ = 565.68
The capacitors bank of the frequency converter stores the energy until the DC bus voltage reaches a maximal value. When the value is reached the power is dissipated in the braking resistor. The resistance value depends on the power that has to be dissipated in the braking resistor. To calculate the resistance value, the highest power value that can be dissipated and the lowest value of DC bus voltage must be taken. Using this data, the maximum resistance value that can be used is obtained as shown.
= [ ]
= 565,68
39.793,4= 8 Ω
The maximum resistance value is 8 Ω. Thus, the braking resistance value has to be 8 Ω or below and the resistance must be able to dissipate the power previously calculated.
A power braking resistor is necessary to dissipate the braking energy. The resistor duty cycle is going to be 100% because they have to dissipate the energy generated by the diesel engine during the test duration. There are two types of dynamical braking resistor that can be used are:
-Wire grid resistor: this type of resistor is made of stainless steel strips forming grids.
-Punch grid resistor: this type of resistor is formed by grids of punched sheet steels composed of nickel chromium alloy.
In table XIII is collected the model of the braking resistors suitable for our design.
Model Description 45KW
Type: EWGRE (Wire Grid)
Wire grid Resistor
45KW Type: PUNCHED GRID
Punched grid Resistor Table XIII: Braking resistor
4.5 Selection of the frequency converter
To choose correctly the frequency converter the three main parameters which have to be checked are:
-Input voltage.
- Rated current.
-Rated power.
-IP degree.
Manufacturers usually offer in their catalogues normalized nominal power that matches with the asynchronous machine nominal power. Thus, to select it we have to look for a frequency converter with the same nominal power as the asynchronous machine.
Another parameter that must be taken into account is the IP protection. The first number indicates the degree of protection against solids and the second number indicates the degree of protection against liquids. The greater the number, the higher the protection degree. As the frequency converter is going to be installed in an enclosed space, an accurate degree of protection is IP 20. The frequency inverter is protected from touch by fingers and objects greater than 12 millimeters but it is not protected from liquids. [26].
It must be checked that in the specifications list appears the ‘flying start’ or ‘catch on fly’ mode. If the frequency converter has not this function it is not possible to start the asynchronous machine when its rotor is already turning.
Control
In this part, the variable speed control systems (scalar control and vector control) are going to be analyzed to choose the most convenient method for our application.
- V/f (Voltage/frequency) drives: this method is also known as ‘scalar control’. The operation key is to maintain constant the flux by increasing proportionally the input voltage and the frequency. It is based in an open-loop system, there is not feedback in the control scheme. Applying this control is not possible to achieve the nominal torque at low speeds.
The vector control is based on Space Vector Theory, the simplified explanation is that current is divided into three spatial vectors direct, quadrature and zero by applying Park and Clark transformations. In an equilibrated system, as a three phase system, zero component is neglected. Basically with this two current components, direct and quadrature, the asynchronous machine controlled in the following way.
-Id: direct component performs flux control.
The two different types of vector control are:
- Sensorless vector control: introducing the most important nominal parameters of the asynchronous machine (voltage, current, frequency, speed, power factor) and the control automatically calculates the model of this machine to estimate its operation. Voltage and current are constantly measured to estimate the speed. In figure 19 we can observe the control scheme.
Figure 19: Sensorless vector control scheme. [15]
- Flux vector control: for this type of control it is required to install an encoder in order to measure the rotational speed of the motor. The position of flux vector is estimated using the measured current and speed. This control is especially suitable for high precision applications. The control scheme is represented in figure 20.
Figure 20: Flux vector control scheme. [15]
To avoid the encoder installation, which is not necessary if the control method is different than the flux vector control, and to obtain a good speed accuracy and torque response, regarding the values of table XV, the most appropriate option is the sensorless vector control.
V/f Sensorless Vector Control
Flux Vector Control with
encoder Speed Accuracy 1% 0.5% 0,001% Torque response 100 ms 1-10 ms 1-10 ms
Encoder No No Yes
Table XIV: Speed Accuracy and Torque response of the different controls. [14]
The WEG-700 is the frequency converter that meets with our requirements as we can observe in table XVI where all the important parameters of the frequency converter are collected. In appendix IV the data sheet and technical information of the frequency converter are attached.
Frequency Converter Technical Data
Power 45 kW
Voltage 380-415 V
Input Current 88 A
Control V/F, Sensorless Vector, Flux Vector.
Flying Start Available
4.6 Variable frequency drive (VFD) curves
It is necessary to set limits to draw the VFD curves. The maximum voltage is the most restrictive between the voltage supply and the maximum voltage of the frequency converter. In this case, the voltage supply is 400V and the maximum voltage of the frequency converter is 415V. Thus, the voltage limit is 400V or 1 per-unit.
The air-gap flux ( ) in per-unit is defined by the following equation.
= [pu]
Where:
- : is the asynchronous machine fed voltage in per-unit.
- : is the asynchronous machine fed frequency in per-unit.
In all these curves there are two well differentiated regions:
- Constant torque region: from 0 to 50Hz fed frequency. In this region the air-gap in per-unit is always equal to one.
- Constant power region or field wakening region: above 50Hz of fed frequency. In this region the capability to generate torque of the asynchronous machine is reduced by the increase of the frequency.
All these curves are symmetrical with respect to the vertical axis. The positive frequency axis represents the rotational forward direction and the negative axis represents the rotational reverse direction. It is only represented the frequency in the positive axis.
Voltage-Speed curve
Voltage and frequency must vary in the same proportion to keep the air-gap flux constant until 50Hz. From 50Hz onwards, the frequency can increase, but the voltage keeps constant because it has reached its maximal value (1 per-unit). By this reason, the air-gap field value decreases under its nominal value creating the field weakening region.
Air-gap flux-frequency
From zero to 50Hz the air-gap flux keeps constant and it maintains its nominal value due to the proportional increase of the voltage and the frequency that we can observe in figure 21. However, once reached the 50Hz we can observe that as the frequency is increased the air-gap flux value decreases.
Torque-Speed Curve
The operation in steady state of the VFD is delimited by the nominal torque in the constant torque region. However, in the field wakening region for each value of the frequency we have a different maximal torque value The higher the frequency, the lower the maximal torque value, as illustrated in figure 23. The positive axis of the torque represents the VFD acting as motor and the negative axis represents the asynchronous machine acting as generator. In our application the operating region is the negative torque area. The diesel engine maximum torque in per-unit is 0,689. This value is inside the constant torque region. However, in the field weakening region the maximal frequency value that can achieve in steady state is 1,13 per-unit.
4.7 Electrical protections
In drive applications, the two fundamental parts that have to be protected are:
- The frequency converter. - The asynchronous machine.
4.7.1 Protections of the frequency converter
The following protections are internally integrated in frequency converters [14]:
- Motor overload protection: the asynchronous motor is protected against
overloads by limiting the current (around 1.5∙IN).
- Motor protection against short-circuits: the overcurrent caused by a short-circuit between phases
- Overheating of frequency converter electronic components: the drive is stopped when a temperature sensor detects that its limit has been exceeded.
- Overvoltage at the line supply power frequency.
- Line voltage dips.
- Loss of a phase: one of the three phases suddenly disappears.
- Motor winding over temperature.
- Braking resistor overload.
If any of these electrical faults occurred, the power supply is cut by opening the line contactor which is operated by a relay. This line contactor is integrated in the frequency converter. The motor is brought to a freewheel stop due to the disconnection from the power supply.
4.7.2 Protections upstream of the frequency converter
Upstream of the frequency converter the theoretical protections that have to be installed are:
- Residual Current Circuit Breaker. - Circuit breaker.
- Contactor. - Line choke. - EMC filter
