Before applying advantageously the control principles to electric drive systems to adjust or improve their behaviour as a special case of control problem, it is necessary to have a knowledge of the specific Features of Electric Drive System, which can be summarized as follows with reference to its electrical characteristics.
1.Electrical drive offers energy transformations. These systems have reasonably high efficiency and are of special interest.
2.The control components are used to limit the amplitude or rate of change of variation of individual quantities, e.g., the armature current of a dc motor.
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, and the drive takes a definite time to follow the speed changes.
4.The characteristics of almost all the control components are more or less non-linear. Normally this non-linearity is introduced by saturation. These are approximated as linear elements or linearized about an operating point.
5.The static and dynamic parameters of control components can be obtained only approximately or sometimes be estimated. In many cases they change during operation.
6.When fed from thyristor power converters, the input voltage and current differ from the conventional input to the motors. In the case of dc motors the input is superimposed by ac components. In the case of ac motors the input is non-sinusoidal having harmonic components. If the measuring equipment of the control system is very much affected by these disturbances the control properties of the drive may not be satisfactory. Therefore care must be exercised to reduce or even eliminate, if possible, the effects of these disturbances on the measuring equipment.
7.The power circuit of thyristor converter receives the thyristor control pulses from a control unit. The operation of this may be disturbed by the power pulses created by the protective equipment of the drive. Necessary care must be exercised here also in designing the control unit to reduce these
8.The control of the drive system should be designed to protect the same from dangerous operating conditions or overloads. In the first case the system must be brought to standstill and in the second case the drive should be able to operate within its limits.
9.In many cases control of one quantity may depend on the control of the Speed control may depend on voltage control. The control must be simple. The system must be insensitive to parameter variations, re switching operations, etc.
The above discussion makes it very clear that many of the control circuits offered by the conventional theory of control may be used but with a caution in the area of Features of Electric Drive System, so that they give satisfactory dynamic and steady-state performance with simpler design, very little complexity, maximum reliability and safety, as demanded by the industrial user.