Electrical Engineer, Power Electronics

DC Motor Drive – Power Electronic

Introduction

A motor driver is a little current amplifier; the function of motor drivers is to take a low-current control signal and then turn it into a higher-current signal that can drive a motor.

A typical motor drive system is expected to have some of the system blocks indicated in Fig. 27.1. The load may be a conveyor system, a traction system, the rolls of a mill drive, the cutting tool of a numerically controlled machine tool, the compressor of an air conditioner, a ship propulsion system, a control valve for a boiler, a robotic arm, and so on.

null

The power electronic converter block may use diodes, MOSFETS, GTOs, IGBTs, or thyristors. The controllers may consist of several control loops, for regulating voltage, current, torque, flux, speed, position, tension, or other desirable conditions of the load. Each of these may have their limiting features purposely placed in order to protect the motor, the converter, or the load.

DC Motor Drives

Direct-current motors are extensively used in variable-speed drives and position-control systems where good dynamic response and steady-state performance are required. Examples are in robotic drives, printers, machine tools, process rolling mills, paper and textile industries, and many others. Control of a dc motor, especially of the separately excited type, is very straightforward, mainly because of the incorporation of the commutator within the motor. The commutator brush allows the motor-developed torque to be proportional to the armature current if the field current is held constant. Classical control theories are then easily applied to the design of the torque and other control loops of a drive system.

The mechanical commutator limits the maximum applicable voltage to about 1500 Vand the maximum power capacity to a few hundred kilowatts. Series or parallel combinations of more than one motor are used when dc motors are applied in applications that handle larger loads. The maximum armature current and its rate of change are also limited by the commutator.

Small servo-type dc motors normally have permanent magnet excitation for the field, whereas larger size motors tend to have separate field-supply Vf for excitation. The separately excited dc motors represented in Fig. 27.2a have fixed field excitation, and these motors are very easy to control via the armature current that is supplied from a power electronic converter.

Thyristor ac–dc converters with phase angle control are popular for the larger motors, whereas duty-cycle controlled pulse-width modulated switching dc–dc converters are popular for servo motor drives.

The series-excited dc motor has its field circuit in series with the armature circuit as shown in Fig. 27.2b. Such a connection gives high torque at low speed and low torque at high speed, a pseudo-constant-power-like characteristic that may match traction-type loads well.

We recall the block diagram for an armature-controlled DC motor:

null

Converters for dc Drives

Depending on application requirements, the power converter for a dc motor may be chosen from a number of topologies. For example, a half-controlled thyristor converter or a singlequadrant PWM switching converter may be adequate for a drive that does not require controlled deceleration with regenerative braking. On the other hand, a full four-quadrant thyristor or transistor converter for the armature circuit and a two-quadrant converter for the field circuit may be required for a high-performance drive with a wide speed range.

Thyristor

Thyristors are used to construct the first stage of an electric motor drive in order to vary the amplitude of the voltage waveform across the windings of the electrical motor as it is shown in Fig. 3.35.

null

An electronic controller controls the gate current of these thyristors. The rectifier and inverter sections can be thyristor circuits. A controlled rectifier is used in conjunction with a square wave or pulse-width modulated (PWM) voltage source inverter (VSI) to create the speed-torque controller system. Figure 3.36 shows a square-wave or PWM VSI with a controlled rectifier on the input side. The switch block inverter is made of thyristors (usually GTOs) for high power. Lowpower motor controllers often use IGBT inverters.

PWM

null

One of the basic functions in Power Electronic is Switching. Based on Figure 1.15, Switching Functions can be characterized completely with three parameters:

  1. The duty ratio D is the fraction of time during which the switch is on. For control purposes the pulse width can be adjusted to achieve a desired result. We can term this adjustment process as pulse-width modulation (PWM), perhaps the most important process for implementing control in power converters.
  2. The frequency fswitch =1/T (with radian frequency ω=2πfswitch) is most often constant, although not in all applications. For control purposes, frequency can be adjusted. This is unusual in power converters because the operating frequencies are often dictated by the application.
  3. The time delay t0 or phase Ø0=ωt0: Rectifiers often make use of phase control to provide a range of adjustment. A few specialized ac-ac converter applications use phase modulation

 

Source:

  1. Libro Rashid – Power Electronic Handbook

Literature Review by: Larry Francis Obando – Technical Specialist – Educational Content Writer.

Mentoring Académico / Emprendedores / Empresarial.

Copywriting, Content Marketing, Tesis, Monografías, Paper Académicos, White Papers (Español – Inglés)

Escuela de Ingeniería Eléctrica de la Universidad Central de Venezuela, Caracas.

Escuela de Ingeniería Electrónica de la Universidad Simón Bolívar, Valle de Sartenejas.

Escuela de Turismo de la Universidad Simón Bolívar, Núcleo Litoral.

Contact: Ecuador (Quito, Guayaquil, Cuenca) – telf 0998524011

WhatsApp: +593998524011

email: dademuchconnection@gmail.com

Anuncios
Ingeniería Eléctrica, Power Electronics

Driver de motor DC – Electrónica de potencia

Introducción

Un controlador (Driver) de motor es un pequeño amplificador de corriente; la función de los Drivers de motor es tomar una señal de control de baja corriente y luego convertirla en una señal de corriente más alta que pueda conducir un motor.

Un Driver es un sistema. Se espera que un Driver de motor típico, tenga un diagrama de bloques semejante al de la Figura 27.1. La carga puede ser un sistema de transporte, un sistema de tracción, los rodillos de accionamiento de un molino, la herramienta de corte de una máquina o herramienta numéricamente controlada, el compresor de un acondicionador de aire, un sistema de propulsión de barcos, una válvula de control para una caldera, el brazo de un robot, y así sucesivamente.

null

El bloque convertidor electrónico de potencia puede usar diodos, MOSFETS, GTO, IGBT o tiristores. Los controladores pueden constar de varios bucles de control para regular el voltaje, la corriente, el par, el flujo, la velocidad, la posición, la tensión u otras condiciones deseables de la carga. Cada uno de estos puede tener sus características limitantes intencionalmente colocados para proteger el motor, el convertidor o la carga.

DC Motor Drivers

Los motores de corriente continua (Motor DC) se utilizan ampliamente en sistemas de velocidad variable y sistemas de control de posición en los que se requiere una buena respuesta transitoria y un buen rendimiento en estado estable. Los ejemplos se encuentran en unidades robóticas, impresoras, máquinas-herramientas, laminadoras de procesos, industrias del papel y textiles, y muchos otros. El control de un motor de corriente continua es sencillo, principalmente debido a la incorporación del conmutador dentro del motor. El cepillo del conmutador permite que el par desarrollado por el motor sea proporcional a la corriente del inducido si la corriente de campo se mantiene constante. Por esto último, las teorías clásicas de control se aplican fácilmente al diseño del par.

El conmutador mecánico limita el voltaje máximo aplicable a aproximadamente 1500 vatios y la capacidad de potencia máxima a unos pocos cientos de kilovatios. Se utilizan combinaciones en serie o en paralelo de más de un motor cuando los motores DC se utilizan en aplicaciones que manejan cargas más grandes. La corriente de armadura máxima y su tasa de cambio también están limitados por el conmutador.

Los pequeños motores DC de tipo servo normalmente tienen excitación por imanes permanentes para el campo, mientras que los motores de mayor tamaño tienden a tener un suministro de campo por separado para la excitación. Los motores DC excitados por separado representados en la Figura 27.2(a) tienen excitación de campo fijo (corriente de campo if constante),  son muy fáciles de controlar a través de la corriente de armadura ia que se suministra desde un convertidor electrónico de potencia.

Los convertidores Thyristor de ac-dc con control de ángulo de fase son populares para los motores más grandes, mientras que los conmutadores de voltaje On-Off, también llamados convertidores dc-dc de conmutación modulada de ancho de pulso (PWM – Pulse Width Modulation por sus siglas en Inglés ) son populares para las unidades de servomotor.

El motor de corriente continua excitado en serie tiene su circuito de campo en serie con el circuito de armadura, como se muestra en la figura 27.2(b). Dicha conexión proporciona un alto par a baja velocidad y bajo par a alta velocidad, una característica que puede combinar bien con las cargas de tipo de tracción.

Es buen momento para recordar el diagrama de bloques típico para un motor DC controlado por armadura (para un repaso ver Dinámica de una Sistema Electromecánico con Motor DC):

null

Cuando un sistema Driver controla la corriente de armadura a través de un convertidor electrónico de potencia, lo que realmente controla es el nivel de la relación velocidad-torque del motor, la cual se muestra en la Figura 27.3(a):

null

Convertidores para DC Drives (Power Electronic Converters)

Dependiendo de los requisitos de la aplicación, el convertidor de potencia para un motor de corriente continua (que en el diagrama de bloques de la Figura 27.1 está identificado como Power Electronic Converter) puede elegirse entre una serie de topologías. Por ejemplo, un convertidor de tiristor semicontrolado o un convertidor de conmutación PWM de un solo componente puede ser adecuado para un drive que no requiere desaceleración controlada con frenado regenerativo. Por otro lado, un convertidor de tiristor o transistor de cuatro cuadrantes para el circuito de armadura y un convertidor de dos cuadrantes para el circuito de campo pueden ser necesarios para un variador de alto rendimiento con un amplio rango de velocidad.

Convertidor de tiristor

Los tiristores se utilizan para construir la primera etapa de un Driver para motor eléctrico con el fin de variar la amplitud de la forma de onda del voltaje  (Va en la Figura 27.2(a)) a través de los devanados del motor, como se muestra en la figura 3.35.

null

Las secciones del rectificador y del inversor pueden ser circuitos de tiristores. Se usa un rectificador controlado junto con un inversor de fuente de voltaje (VSI – Voltage Source Inverter) modulado por onda cuadrada o ancho de pulso (PWM) para crear el sistema controlador de velocidad-par. La Figura 3.36 muestra una onda cuadrada o PWM VSI con un rectificador controlado en el lado de entrada. 

El interruptor del bloque inversor está hecho de tiristores (generalmente GTO Gate Turn-Off Thyristor) para alta potencia. Los controladores de motores de baja potencia suelen utilizar inversores IGBT (Insulated-Gate Bipolar Transistor).

Pulse-Width Modulation

null

Una de las funciones básicas de la electrónica de potencia es la conmutación, el interruptor que apaga-enciende, conocido como Switcher. Podríamos entonces hablar de la Función Switching. Basándonos en la Figura 1.15, las funciones de Switching se pueden caracterizar por completo con tres parámetros:

  1. La relación de trabajo D (Duty Ratio): fracción de tiempo durante el cual el interruptor está en su posición de encendido. Para fines de control, el ancho del pulso se puede ajustar para lograr un resultado deseado. Podemos denominar este proceso de ajuste como modulación por ancho de pulso (PWM – Pulse Width Modulation), tal vez el proceso más importante para implementar el control en los convertidores de potencia, por lo cual será nuestro siguiente tema.
  2. La frecuencia fswitch :  suele ser constante, aunque no en todas las aplicaciones. Para fines de control, la frecuencia puede ajustarse. Esto es inusual en los convertidores de potencia porque las frecuencias de operación a menudo son dictadas por la aplicación. (ω=2πfswitch; fswitch=1/T).
  3. El tiempo de retardo t0 o la fase Ø0 = ωt0: los rectificadores a menudo hacen uso del control de fase para proporcionar un rango de ajuste. Algunas aplicaciones especializadas de convertidores ac-ac usan modulación de fase

 

Fuente:

  1. Libro Rashid – Power Electronic Handbook

 

Revisión literaria hecha por: Larry Francis Obando – Technical Specialist – Educational Content Writer.

Mentoring Académico / Emprendedores / Empresarial.

Copywriting, Content Marketing, Tesis, Monografías, Paper Académicos, White Papers (Español – Inglés)

Escuela de Ingeniería Eléctrica de la Universidad Central de Venezuela, Caracas.

Escuela de Ingeniería Electrónica de la Universidad Simón Bolívar, Valle de Sartenejas.

Escuela de Turismo de la Universidad Simón Bolívar, Núcleo Litoral.

Contact: Ecuador (Quito, Guayaquil, Cuenca) – telf 0998524011

WhatsApp: +593998524011

email: dademuchconnection@gmail.com

Electronic Engineer, Power Electronics

Power Electronics – Introduction

Introduction

In broad terms, the task of power electronics is to process and control the flow of electric energy by supplying voltages and currents in a form that is optimally suited for user loads.

Figure 1-1 shows a power electronic system in a block diagram form. The power input to this power processor is usually (but not always) from the electric utility at a line frequency of 60 or 50 Hz, single phase or three phases. The phase angle between the input voltage and the current depends on the topology and the control of the power processor. The processed output (voltage, current, frequency, and the number of phases) is as desired by the load. If the power processor’s output can be regarded as a voltage source, the output current and the phase angle relationship between the output voltage and the current depend on the load characteristic. Normally, a feedback controller compares the output of the power processor unit with a desired (or a reference) value, and the error between the two is minimized by the controller. The power flow through such systems may be reversible, thus interchanging the roles of the input and the output.

In recent years, the field of power electronics has experienced a large growth due to confluence of several factors. The controller in the block diagram of Fig. 1-1 consists of linear integrated circuits and/or digital signal processors. Revolutionary advances in microelectronics methods have led to the development of such controllers. Moreover, these advances in semiconductor fabrication technology have made it possible to significantly improve the voltage- and current-handling capabilities and the switching speeds of power semiconductor devices, which make up the power processor unit of Fig. 1-1. At the same time, the market for power electronics has significantly expanded. Electric utilities in the United States expect that by the year 2000 over 50% of the electrical load may be supplied through power electronic systems such as in Fig. 1-1.

Power Electronics Defined

It has been said that people do not use electricity, but rather they use communication, light, mechanical work, entertainment, and all the tangible benefits of both energy and electronics. In this sense, electrical engineering is a discipline very much involved in energy conversion and information. In the general world of electronics engineering, the circuits engineers design and use are intended to convert information, with energy merely a secondary consideration in most cases. In radio frequency applications, energy and information are sometimes on a more equal footing, but the main function of any circuit is that of information transfer.

What about the conversion and control of electrical energy itself? Electrical energy sources are varied and of many types. It is natural, then, to consider how electronic circuits and systems can be applied to the challenges of energy conversion and management. This is the framework of power electronics, a discipline that is defined in terms of electrical energy conversion, applications, and electronic devices. More specifically,

DEFINITION: Power electronics involves the study of electronic circuits intended to control the flow of electrical energy. These circuits handle power flow at levels much higher than the individual device ratings.

Power Electronics Vs Linear Electronics

In any power conversion process such as that shown by the block diagram in Fig. 1- 1, a small power loss and hence a high energy efficiency is important because of two reasons: the cost of the wasted energy and the difficulty in removing the heat generated due to dissipated energy.

Other important considerations are reduction in size, weight, and cost. The above objectives in most systems cannot be met by linear electronics where the semiconductor devices are operated in their linear (active) region and a line-frequency transformer is used for electrical isolation. As an example, consider the direct current (dc) power supply of Fig. 1-2a to provide a regulated output voltage V, to a load.

The utility input may be typically at 120 or 240 V and the output voltage may be, for example, 5 V. The output is required to be electrically isolated from the utility input. In the linear power supply, a line-frequency transformer is used to provide electrical isolation and for stepping down the line voltage. The rectifier converts the alternating current (ac) output of the transformer low-voltage winding into dc. The filter capacitor reduces the ripple in the dc voltage vd. Figure 1-2b shows the vd waveform, which depends on the utility voltage magnitude (normally in a t 10% range around its nominal value).

The transformer turns ratio must be chosen such that the minimum of the input voltage v, is greater than the desired output V. For the range of the input voltage waveforms shown in Fig. 1-2b, the transistor is controlled to absorb the voltage difference between v and V, thus providing a regulated output. The transistor operates in its active region as an adjustable resistor, resulting in a low energy efficiency. The line-frequency transformer is relatively large and heavy.

In power electronics, the above voltage regulation and the electrical isolation are achieved, for example, by means of a circuit shown in Fig. 1-3a.

In this system, the utility input is rectified into a dc voltage vd, without a line-frequency transformer. By operating the transistor as a switch (in a switch mode, either fully on or fully 0ff) at some high switching frequency f, for example at 300 kHz, the dc voltage vd is converted into an ac voltage at the switching frequency. This allows a high-frequency transformer to be used for stepping down the voltage and for providing the electrical isolation.

In order to simplify this circuit for analysis, we will begin with the dc voltage vd as the dc input and omit the transformer, resulting in an equivalent circuit shown in Fig. 1-3b.

Suffice it to say at this stage that the transistor diode combination can be represented by a hypothetical two-position switch shown in Fig. 1-4a (provided iL(t) > 0).

The switch is in position a during the interval t-on, when the transistor is on and in position b when the transistor is off during t-off. As a consequence, Voi equals Vd, and zero during t-on and t-off, respectively, as shown in Fig. 1-4b.

Let us define

where Voi is the average (dc) value of Voi-t, and the instantaneous ripple voltage V-ripple, which has a zero average value, is shown in Fig. 1-4c.

The L-C elements form a low-pass filter that reduces the ripple in the output voltage and passes the average of the input voltage, so that

where Vo, is the average output voltage. From the repetitive waveforms in Fig. 1-4b, it is easy to see that

As the input voltage Vd changes with time, Eq. 1-3 shows that it is possible to regulate Vo, at its desired value by controlling the ratio t-on/Ts which is called the duty ratio D of the transistor switch. Usually, Ts (= l/fs) is kept constant and t-on is adjusted.

There are several characteristics worth noting. Since the transistor operates as a switch, fully on or fully off, the power loss is minimized. Of course, there is an energy loss each time the transistor switches from one state to the other state through its active region. Therefore, the power loss due to switchings is linearly proportional to the switching frequency. This switching power loss is usually much lower than the power loss in linear regulated power supplies.

At high switching frequencies, the transformer and the filter components are very small in weight and size compared with line-frequency components.

Scope and Applications of Power Electronics 

The expanded market demand for power electronics has been due to several factors discussed below:

  • Switch-mode (dc) power supplies and uninterruptible power supplies. Advances in microelectronics fabrication technology have led to the development of computers, communication equipment, and consumer electronics, all of which require regulated dc power supplies and often uninterruptible power supplies.
  • Energy conservation. Increasing energy costs and the concern for the environment have combined to make energy conservation a priority. One such application of power electronics is in operating fluorescent lamps at high frequencies (e.g., above 20 kHz) for higher efficiency. Another opportunity for large energy conservation is in motor-driven pump and compressor systems. In a conventional pump system shown in Fig. 1-5a, the pump operates at essentially a constant speed, and the pump flow rate is controlled by adjusting the position of the throttling valve. This procedure results in significant power loss across the valve at reduced flow rates where the power drawn from the utility remains essentially the same as at the full flow rate. This power loss is eliminated in the system of Fig. 1-56, where an adjustable-speed motor drive adjusts the pump speed to a level appropriate to deliver the desired flow rate.

  • Process control and factory automation. There is a growing demand for the enhanced performance offered by adjustable-speed-driven pumps and compressors in process control. Robots in automated factories are powered by electric servo (adjustable-speed and position) drives. It should be noted that the availability of process computers is a significant factor in making process control and factory automation feasible.
  • Transportation. In many countries, electric trains have been in widespread use for a long time. Now, there is also a possibility of using electric vehicles in large metropolitan areas to reduce smog and pollution. Electric vehicles would also require battery chargers that utilize power electronics.
  • Electro-technical applications. These include equipment for welding, electroplating, and induction heating.
  • Utility-related applications. One such application is in transmission of power over high-voltage dc (HVDC) lines. At the sending end of the transmission line, line-frequency voltages and currents are converted into dc. This dc is converted back into the line-frequency ac at the receiving end of the line. Power electronics is also beginning to play a significant role as electric utilities attempt to utilize the existing transmission network to a higher capacity. Potentially, a large application is in the interconnection of photovoltaic and wind-electric systems to the utility grid.
Classification of Power Processors and Converters

For a systematic study of power electronics, it is useful to categorize the power processors, shown in the block diagram of Fig. 1-1, in terms of their input and output form or frequency.

In most power electronic systems, the input is from the electric utility source. Depending on the application, the output to the load may have any of the following forms:

  1. dc
    1. regulated (constant) magnitude
    2. adjustable magnitude
  2. ac
    1. constant frequency, adjustable magnitude
    2. adjustable frequency and adjustable magnitude

The utility and the ac load, independent of each other, may be single phase or three phase. The power flow is generally from the utility input to the output load.

The power processors of Fig. 1-1 usually consist of more than one power conversion stage (as shown in Fig. 1-6) where the operation of these stages is decoupled on an instantaneous basis by means of energy storage elements such as capacitors and inductors.

Therefore, the instantaneous power input does not have to equal the instantaneous power output. We will refer to each power conversion stage as a converter. Thus, a converter is a basic module (building block) of power electronic systems. It utilizes power semiconductor devices controlled by signal electronics (integrated circuits) and possibly energy storage elements such as inductors and capacitors. Based on the form (frequency) on the two sides, converters can be divided into the following broad categories:

  1. ac to dc
  2. dc to ac
  3. dc to dc
  4. ac to ac

We will use converter as a generic term to refer to a single power conversion stage that may perform any of the functions listed above. To be more specific, in ac-to-dc and dc-to-ac conversion, rectifier refers to a converter when the average power flow is from the ac to the dc side. Inverter refers to the converter when the average power flow is from the dc to the ac side.

Further insight can be gained by classifying converters according to how the devices within the converter are switched. There are three possibilities:

  1. Line frequency (naturally cornmutated) converters, where the utility line voltages present at one side of the converter facilitate the turn-off of the power semiconductor devices. Similarly, the devices are turned on, phase locked to the line voltage waveform. Therefore, the devices switch on and off at the line frequency of 50 or 60 Hz.
  2. Switching (forced-commutated) converters, where the controllable switches in the converter are turned on and off at frequencies that are high compared to the line frequency.
  3. Resonant and quasi-resonant converters, where the controllable switches turn on and/or turn off at zero voltage and/or zero current.

Interdisciplinary Nature of Power Electronics

The discussion in this introductory chapter shows that the study of power electronics encompasses many fields within electrical engineering, as illustrated by Fig. 1- 10.

Combining the knowledge of these diverse fields makes the study of power electronics challenging as well as interesting. There are many potential advances in all these fields that will improve the prospects for applying power electronics to new applications.

Sources:

  1. Power Electronic – Mohan
  2. Libro Rashid – Power Electronic Handbook

Literature Review by: Larry Francis Obando – Technical Specialist

Lunes 15 de noviembre, 11:08 am – Caracas, Quito, Guayaquil.

Escuela de Ingeniería Eléctrica de la Universidad Central de Venezuela, Caracas.

Escuela de Ingeniería Electrónica de la Universidad Simón Bolívar, Valle de Sartenejas.

Escuela de Turismo de la Universidad Simón Bolívar, Núcleo Litoral.

Contact: Ecuador (Quito, Guayaquil, Cuenca) telf. -593998524011

WhatsApp: 005998524011

email: dademuchconnection@gmail.com