The purpose of a motor control system is to control one or more of the motor output parameters, that is, shaft speed, angular position, acceleration, shaft torque, and mechanical output power. The control of temperature at various points in the motor is also a frequent objective of motor control systems. Since it is the mechanical parameters of the motor that are being controlled by the input electrical parameters, the peculiar characteristic of the individual machine - that is, the characteristics that relate input electrical quantities to output mechanical quantities - are vitally important in the design and analysis of electronic control. It is therefore customary to treat the control of dc commentator motors separately from that of synchronous and induction motors. The nature of the load and power supply will also influence the nature of the control. The load box represents a very general concept of load and may be a pure inertia load. The power source box is also generalized and is meant to cover all power or excitation sources required by the motor, such as the field and armature power supply in a dc commentator motor.
The materials and structural features of power semiconductors used in motor control are much different from those of electromechanical devices, which, as would be expected, result in differing operating and environmental characteristics. It is essential to recognize those differences in the design and use of electronic motor control in which the two different types of components are used in a common environment and are subjected to the same voltage and current values. The principal differences may be summarized as follows:
Motors have large thermal capacities and can sustain thermal overloads for time period measured in minutes. Semiconductors have very short thermal time constants; lasting often less than 1 s. they also have poor natural thermal conduction paths and require heat sinks in more applications.
The current overload characteristics of the two types of devices are quite different, partly as a result of the thermal differences just noted. Semiconductor devices have relatively little over current capability.
Many semiconductors are limited by the rate of change of current, as will be explained for thyristors. There is no equivalent limitation in a motor.
Thyristors are also limited by a rate of change of voltage characteristics, which is not a factor in motor operation.
Semiconductor circuits are very susceptible to electromagnetic interference, both conductive and inductive, whereas motors are in no way similarly affected.
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