POWER ELECTRONICS AND DRIVES

INTRODUCTION
Industrial loads require operation at any one of a wide range of speeds. Such loads are generally termed as variable speed drives. These drives demand precise adjustment of speed in a stepless continuous manner over the continuous speed range required. The loads may be constant torque loads, requiring constant torque at all speeds. The torque may also be a function of speed. These loads are driven by hydraulic pneumatic or electric motors. An industrial drive has some special features when driven by electric motors. The speed torque characteristics of the motor can be very easily modified to suit the load characteristics. The motor can be brought to operation without any warm up period, but with a soft starting.

One of the important features of an industrial drive is its four-quadrant operation in the speed-torque. Rapid acceleration of the drive and frequent reversing, if required, can be easily accomplished when an electric motor is employed.

Another feature of drives employing electric motors is smooth speed control over a wide range, e.g. the speed control of a dc motor in Ward Leonard control when a variable voltage is applied to the armature. This has not been possible with accumulator motors till the advent of thyristors .The development of compact thyristor power converters has made possible the smooth speed control of accumulator and dc motors .The efficiency of the drive in the complete speed range also improves. Electric motors have a good starting torque and can be started on load. Industrial drives demand very precise variation in speed. This can be easily accomplished by means of an electric motor, up to an accuracy of 1%.

POWER ELECTRONIC SYSTEMS
The task of the 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. The power input to this power processor is usually from the electric utility at a line frequency of 60 or 50 Hz, single phase or three phases. The processed output 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 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.

BASIC FEATURES OF AN ELECTRIC DRIVE

1. Electric drive offers energy transformations
2. The control components are used to limit the amplitude or rate of change of individual quantities
3. The finite inertia (element of energy storage) of the system does not allow the instantaneous speed changes there by resulting in a finite acceleration
4. The characteristics of almost all components are non-linear
5. The static and dynamic parameters of control components can be obtained only approximately or sometimes be estimated
6. When fed from thyristors power converters, the input voltage and current differ from conventional input to the motors
7. The power circuit of thyristor converter receives the thyristor control pulses from a control unit
8. The control of the drive system should be designed to protect the same from dangerous operating conditions or overloads
9. In many cases control of one quantity may depend on the control of the other e.g. speed control on voltage control etc.

1. Stable operation
2. The drive motor should have a good transient response

1. dc motors fed from dc supply
2. dc motors fed from ac supply
3. ac motors fed from ac supply

1. The rating of a motor selected should be in accordance with the mechanical work required by the driven machine
2. The motor selected should be capable of driving the load satisfactorily both under steady state as well as under transient conditions
3. When a motor is selected based on a given load diagram and is fully loaded it must not have excess temperature rise

SOLAR POWERED DRIVES
This part presents a review of photovoltaic solar powered drives. The photovoltaic source is a special power source with non-linear I-V characteristics. The characteristics vary with solar insolation and temperature. The main application of solar power drives has been for pumping water with DC or AC motors in remote and rural locations. It is important to note that the reflected I-V operating curve of the motor-pump system plays a dominant role in deciding the system performance. As the PV power is expensive, system efficiency can be increased (and the system cost can be reduced) only if the operating curve matches will with the locus of maximum power point of the PV source. In case, the direct coupling creates a mismatch, a dc-dc converter can be introduced and with the help of an appropriate maximum power point tracking algorithm, the load matching can be attained. DC motor drive is preferred for low powers (~ 500W) and AC motor drive is recommended for more than 1KW-power applications. With the increasing fear of depletion of fossil fuel, the PV power will have to be utilized for traction purposes also. A lot of R&D work is being carried in studying the possibilities of PV energy for automobiles.

TRENDS IN DRIVE TECHNOLOGY
With thyristor power converters achieving popularity in the area of adjustable speed drives, the operational problems of motors on these converters have started posing problems. The ac motor has a non-sinusoidal voltage or current while operating on inverters while a dc motor has a ripple superimposed on its current. These lead to additional losses and torque pulsations which are objectionable at low speeds. The design of a dc motor is being improved to provide a sufficiently good commutating capability, while that of inverters is being improved to provide voltage with least harmonic content.

Vector controlled induction motors and margin angle controlled synchronous motors are employed in high performance drives having precise speed and torque control.

Digital control using microprocessors is slowly replacing the analog control of drives. Modern control techniques to the implementation of sophisticated drives are possible, as are diagnosis, monitoring, warning etc.

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