Induction motors for speed and position control From Electric Motor

On a constant-frequency supply, an induction motor is essentially a near-constant speed drive. Induction motors, however, can be used to provide accurate speed and position control in either direction of rotation by furnishing a controllable-voltage, controllable-frequency three-phase supply. This is done by means of an electronic inverter. Using semiconductor switches (e.g., transistors or thyristors), the utility supply is converted into a set of three near-sinusoidal inputs of controlled voltage and frequency to the stator winding. The speed of the motor will then approach the synchronous value of 120 f/p revolutions per minute for a controlled frequency of f cycles per second.

Reversal of the phase sequence from abc to acb reverses the direction of the torque. For accurate control of speed or of position, the speed of the shaft can be monitored by a tachometer or position sensor and compared with a signal representing the desired value. The difference is then used to control the inverter frequency. Generally, the voltage varies directly with the frequency to keep the magnitude of the magnetic field constant.

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72000 – Стандартизация и сертификация в машиностроении (The Standardization and Certification in Machine-Building)

STANDARDIZATION (7934 characters)

Standardization in industry is the development and application of standards that permit large production runs of component parts that can be readily fitted to other parts without adjustment. Standardization allows for clear communication between industry and its suppliers, relatively low cost, and manufacture on the basis of interchangeable parts.

A standard is that which has been selected as a model to which objects or actions may be compared. Standards for industry may be devices and instruments used to regulate colour, size, weight, and other product attributes, or they may be physical models. Standards may also be written mathematical or symbolical descriptions, drawings, or formulas setting forth the important features of objects to be produced or actions to be performed. Standards that are applied in an industrial setting include engineering standards, such as properties of materials, fits and tolerances, terminology, and drafting practices; and product standards intended to describe attributes and ingredients of manufactured items and embodied in drawings, formulas, materials lists, descriptions, or models.

Certain fundamental standards among firms are required to prevent conflict and duplication of effort. The standards activities of governmental departments, trade associations, and technical associations serve in part to meet national standards needs, but one specialized standardizing organization is needed to co-ordinate the diverse standardization activities of many different types of organizations and promote general acceptance of basic standards.

In the United States the American National Standards Institute (ANSI) performs this function. It does not initiate or write standards but provides the means by which national engineering, safety, and industrial standards can be co-ordinated. All interested groups may participate in the decision-making process, and compliance with the national standard is voluntary. The international body that serves this function is the International Organization for Standardization (ISO). Developing an international standard presents the greater challenge because of the breadth of representation and the diversity of needs and viewpoints that must be reconciled.