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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 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 c. High efficiency, lower noise, low maintenance requirements and cleaner operation d. Electric energy is easy to be transported. A typical conventional electric drive system for variable speed application employing multi- machine system is shown in Figure 1. The system is obviously bulky, expensive, inflexible and require regular maintenance. In the past, induction and synchronous machines were used for constant speed applications – this was mainly because of the unavailability of variable frequency supply. Figure 1 Conventional variable speed electrical drive system With the advancement of 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 applied to the motor can be changed at will by employing power electronic converters. AC motor is no longer limited to application where only AC source is available, however, it can also be used when the power source available is DC or vice versa Figure 2 Modern Electric drive system employing power electronic converters AC motor DC generator variable DC DC motor variable speed Load fixed speed I f I a Power Source Control feedback Power Processor (Power electronic Converters) Control Unit Motor Load
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2 Electric drives is multi-disciplinary field. Various research areas can be sub-divided from electric drives as shown 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 motors, power processor, control unit and electrical source. 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 the power to be fed back to the sources rather than dissipated as heat.
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