Electrical Engg

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   1 INTRODUCTION TO ELECTRICAL DRIVES Drives are employed for systems that require motion control – e.g. transportation system, fans, robots, pumps, machine tools, etc. Prime movers are required in drive systems to provide the movement or motion and energy that is used to provide the motion can come from various sources: diesel engines, petrol engines, hydraulic motors, electric motors etc. Drives that use electric motors as the prime movers are known as electrical drives. It is estimated that about half of electricity generated is converted to mechanical energy and this conversion is performed using electric drives.  There are several advantages of electrical drives: a. Flexible control characteristic – This is particularly true when power electronic converters are employed where the dynamic and steady state characteristics of the motor can be controlled by controlling the applied voltage or current. b. Available in wide range of speed, torque and power; they are available in mW up to MW range. c. High efficiency, low noise, low maintenance requirements and cleaner operation d. Electric energy is easy to be transported over long distances. e. Adaptable to various operating conditions: explosive, submerged in liquid, various types of mounting, etc f. Can be started instantly and can be fully loaded immediately (no need to warm-up or re-fueling the motor) A typical conventional electric drive system for variable speed application employing multi-machine system is shown in Figure 1, which also known as the Ward-Leonard system. The variable speed of the load is obtained by varying the terminal voltage of the DC motor, which is fed by the DC generator. The AC motor is mechanically coupled to the DC generator and hence runs at constant speed. Subsequently, the field excitation of the DC generator is adjusted in order to provide the adjustable DC voltage to the DC machine. If the DC generator voltage is adjusted to be lower than the back EMF voltage of the DC motor, the ‘motor’ will be operated in regenerative braking mode. In other words, 4-quadrant operation is possible with this configuration. Obviously, the system is bulky, expensive, inflexible and require regular maintenance due to the present of the DC machines. In the past, for constant speed application, induction and synchronous motors were widely. An efficient variable speed operation of AC motors is only possible if both the frequency and magnitude of the 3-phase supply voltage are adjustable; unfortunately this is used to be almost impossible. Figure 1 Conventional variable speed electrical drive system With the advancement in power electronics, microprocessors and digital electronics, typical electric drive systems nowadays are becoming more compact, efficient, cheaper and versatile – this is shown in Figure 2. The voltage and current (magnitude and frequency) applied to the motor can be changed at will by employing power electronic converters. AC motor is no longer limited to applications where only AC source is available, however, it can also be used when the power source available is DC or vice versa AC motor DC generator variable DC DC motor variable speed Load fixed speed I f   I a     2 Figure 2 Modern Electric drive system employing power electronic converters Like other power electronic systems, modern electric drives is a multi-disciplinary field. The drive systems can be broken into several different research areas, as depicted in Figure 3. Figure 3 Multi-disciplinary nature of electric drive system Components of Electrical Drives  The main components of a modern electrical drive are the motor, power processor, control unit electrical source and the mechanical load. These components are different from one drive system to another drive system, depending upon the applications, cost, available electrical source, etc. These are briefly discussed below. a) Motors Motors obtain power from electrical sources. They convert energy from electrical to mechanical - therefore can be regarded as energy converters. In braking mode, the flow of power is reversed. Depending upon the type of power converters used, it is also possible for Power Source Control feedback Power Processor (Power electronic Converters) Control Unit Motor Load ã   Machine design ã   Speed sensorless ã   Machine theory ã   Non-linear control ã   Real-time control ã   DSP application ã   PFC ã   sensors ã   Speed sensorless ã   Power electronic converters ã   Utility interface ã   Renewable energy   3 the power to be fed back to the sources (regenerative braking) rather than dissipated as heat (dynamic braking). There are several types of motors used in electric drives – the choice of the type to be used depends on applications, cost, environmental factors and also on the type of sources available. Broadly, they can be classified as either DC or AC motors: DC motors (wound or permanent magnet) AC motors Induction motors – squirrel cage, wound rotor Synchronous motors – wound field, permanent magnet Brushless DC motor – require power electronic converters Stepper motors – require power electronic converters Synchronous reluctance motors or switched reluctance motor – require power electronic converters b) Power processor or power modulator  Typically, electrical sources are uncontrollable. For instance, if it is an AC source, the frequency and magnitude are fixed (from the utility company), or maybe both are varying randomly (such as obtained from wind generator). It is therefore necessary to provide an interface between the available electrical source and the motor so that the power flow between the source and the motor, hence the speed of the motor or the torque can be systematically regulated and controlled -– this is achieved by using power processor or power modulator. With controllable sources, the motor can be reversed, brake or can even be operated with variable speed. Conventional power processor (non- power electronics approached) used, for example, variable impedance or relays, to shape the voltage or current that is supplied to the motor – these methods are inflexible, inefficient and have limited control capability. In modern electric drive systems, power electronic converters are used to shape the desired voltage or current that is supplied to the motor. The power converters are commonly used to convert one form of electrical power to another (e.g. AC to DC, DC to AC, etc). The main advantage of using power electronic converters is because of their high efficiency. With power electronic converters, characteristic of the motors can be changed at will to adapt the load requirements. Power electronic converters have several advantages over classical methods of power conversion, such as : ã   More efficient – since ideally no losses occur in power electronic converters ã   Flexible – voltage and current can be shaped by simply controlling the switching functions of the power converter ã   Compact – smaller, compact and higher ratings solid–state power electronic devices are continuously being developed – the prices are getting cheaper. Power electronic converters are typically consists of power semiconductor devices and passive elements, such as inductors and/or capacitors. The losses in power semiconductor devices are minimized since they are always operated in either cut-off or saturated modes.  The conversion of electrical power from one form to another can be performed with either single-stage conversion or multiple-stage conversion. For example a conversion from AC to DC can be performed in two stages (AC ! DC ! DC) or it can be performed with single stage conversion, i.e. AC ! DC. The choice of which one to choose, in general, depends on application requirements, such as control bandwidth, output voltage or current ripples, cost, etc. AC to DC conversion Diode rectifier DC-DC converter control Controlled rectifier control AC DC AC AC DC   4 DC to AC DC to DC AC to AC c) Control Unit Control unit is used to generate the switching signals to the power switches of the power converters. The switching signals are generated depending on the control scheme adopted, which depends on the desired drive performance and the type of motors used. A controller can be as simple as few op-amps and/or a few digital ICs, or it can be as complex as the combinations of several ASICs and digital signal processors (DSPs). The controllers can be constructed from/using: ã   analog circuit - which is noisy, inflexible. However, analog circuit ideally can provide infinite control bandwidth. ã   digital circuit – immune to noise, configurable. The bandwidth is obviously limited that depends on the sampling frequency. Field Programmable Gate Arrays (FPGA) devices are gaining popularity due to their re-configurable features. However implementation of sophisticated control algorithm and observers can be very difficult and complex. ã   DSP/microprocessor – flexible, lower bandwidth compared to the digital circuit. DSPs perform faster operation than microprocessors (multiplication in a single cycle). With a DSP/microprocessor, complex estimations and observers can be easily implemented. Inverter (PWM) control DC-DC converter control Inverter (six-step) control DC-DC Converter control Controlled Rectifier control Inverter (six-step) control Diode Rectifier control Inverter (PWM) Matrix Converter control DC DC AC DC AC DC DC AC DC AC AC DC AC AC AC

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