Modern Variable Speed System

Electric driveElectrical drive is define as a form of machine equipment designed to convert electrical energy in to mechanical energy and provide electrical control of this process.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 1.

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Fig 1 modern electric drive system employing power electronics converter 2COM PONENTS OF ELECTRICAL DRIVESThe main components of a modern electrical drive are the load, motors, power processor, control unit and electrical source. These are briefly discussed below1.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 There are several types of motors used in electric drives choice of type used depends on applications, cost, environmental factors and also the type of sources available.. Broadly, they can be classified as either DC or AC motors they can be .Types of electric motors presently used for speed control applications are:

1.1.DC motors Shunt Series Compound Separately excited Switched reluctance motors

.1.2. AC motors Induction Wound rotor synchronous Permanent magnet synchronous Reluctance motors

.2. Power Converter Rectifiers Choppers Inverters Cycloconverters

.Conventional methods used, for example, variable impedance or relays, to shape the voltage or current that is supplied to the motor ,these methods however are inflexible and inefficient. Modern electric drives normally used power electronic converters to shape the desired voltage or current supplied to the motor. electronic converters have several advantages over classical methods of power conversion, such as 1)More efficient since ideally no losses occur in power electronic converters 2)Flexible voltage and current can be shaped by simply controlling switching functions of the power converter. 3) Compact smaller, compact and higher ratings solidstate power electronic devices are continuously being developed the prices are getting cheaper Converters are used to convert and possibly regulate (i.e. using closed-loop control) the available sources to sui t the load i.e. motors. These converters are efficient because the switches operate in either cut-off or saturation modes Several conversion are possible . 3.Controllers matching the motor and power converter to meet the load requirementsThe complexity of the control unit 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 types of the main controllers can be analog - which is noisy, inflexible. However analog circuit ideally has infinite bandwidth. digitalimmune to noise, configurable. The bandwidth is obviously smaller than the analog controller's depends on sampling frequency .

DSP/microprocessor flexible, lower bandwidth compared to above. DSPs perform faster operation than microprocessors (multiplication in single cycle). With DSP/microp., complex estimations and observers can be easily implemented. sourcesElectrical sources or power supplies provide the energy to the electrical motors. For high efficiency operation, the power obtained from the electrical sources need to be regulated using power electronic converters Power sources can be of AC or D C in nature and normally are uncontrollable, i.e. their magnitudes or frequencies are fixed or depend on the sources of energy such as solar or wind. AC source can be either three-phase or single-phase; 3-phase sources are normally for high power applications Advantages of Electrical DrivesA. 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 powerC. High efficiency, lower noise, low maintenance requirements and cleaner operationD. Electric energy is easy to be transported

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