Electric Drive

electric drive

[i¦lek·trik ′drīv]
(mechanical engineering)
A mechanism which transmits motion from one shaft to another and controls the velocity ratio of the shafts by electrical means.

Electric Drive

 

a set of devices to convert electric energy into mechanical energy and to regulate the converted energy flux according to a specific law. It is the most common type of drive.

History. The first electric drive was devised in Russia in 1838, when B. S. Iakobi tested a DC electric motor supplied from a storage battery and used it to drive a screw propeller on a boat. However, adoption of the electric drive by industry was delayed for lack of a reliable source of electric energy. Even after the development of an industrial electric DC generator in 1870, efforts to introduce the electric drive were spontaneous and without practical significance. The widespread industrial use of electric drives stems from the discovery of the rotating magnetic field and the construction of a three-phase induction motor by M. O. Dolivo-Dobrovol’skii. During the 1890’s an electric drive incorporating an induction motor with a phase-wound rotor was used extensively in industrial enterprises to operate the working elements of machinery. In 1890 electric motors accounted for 5 percent of the total power of all types of prime movers used by industry; the figure had reached 75 percent by 1927 and nearly 100 percent by 1976. Today a substantial percentage of the electric drives in operation are used in transportation.

Principal types. Electric drives may be classified according to design characteristics into three types: single-motor, group, and multimotor. Single-motor electric drives are used in power tools, simple metalworking and woodworking machine tools, and household appliances. Group electric drives are almost never used in modern industry. Multimotor electric drives are used in multioperation metalworking machine tools and as individual electric traction drives for railroad transportation equipment. Electric drives may also be divided into reversible and nonreversible types (seeREVERSIBLE ELECTRIC DRIVE) and, depending on the possibility of controlling the flux of the converted mechanical energy, into uncontrolled and controlled-velocity types (including automated types with programmed control).

Primary components. All types of electric drives contain primary components that have the same functions: actuating components and control devices.

The actuating components of an electric drive usually consist of one or more electric motors (seeELECTRIC MOTOR) and a drive mechanism that transmits mechanical energy from the motor to a working element of the driven machine. AC motors are usually used in uncontrolled electric drives and are connected to the power supply through a contactor or circuit breaker, which serves as a protective device; in household electric drives the connection is made through a plug connector. The rotation speed of the rotor of the electric motor and, consequently, the speed of movement of the working mechanism coupled to the machine vary only with the load on the operating mechanism. High-power electric drives use induction motors. Starting reactors or autotransformer starters are connected between the motor and the power supply in order to limit starting currents; they are switched out after the motor has accelerated. Controlled-velocity electric drives usually use DC motors, because the rotation speed of the motor armatures can be varied continuously over a wide range with rather simple control devices.

The control devices of an electric drive include a push-button panel for starting and stopping the electric motor, contactors, locking contacts, frequency and voltage converters, safety devices, and an overload-protection unit for emergency conditions. AC sources typically supply electric drives used in industry and on electric railroad rolling stock whose motors are fed from an AC network, and either rotary converters or static converters (rectifiers) are used to convert the electric energy. When a DC source is used, as in independent electric power systems and electric railroad rolling stock whose motors are fed from a DC network, either relay-contactor systems or static converters are used (seeCONVERSION TECHNOLOGY). During the 1970’s there has been an increasing use of three-phase induction and synchronous motors in electric drives, with operating modes controlled by means of static, primarily semiconductor, frequency converters. Electric drives with static power converters based on mercury-arc or semiconductor rectifiers are sometimes called rectifier drives. The unit power of AC rectifier drives used in shaft crushers may reach 10 megawatts or more. The use of rectifier conversion devices solves the problem of returning energy from an electric motor to the power source in the most economical way (seeREGENERATIVE BRAKING).

One of the important indicators that determine the characteristics of control devices for a controlled-velocity electric drive is the smoothness with which the operating mechanism’s functioning is adjusted, which depends to a great extent on the smoothness and response speed of the control for the electric driving motor. Relay-contactor control devices with a relatively slow response speed provide stepped (discrete) control of the operating conditions; static systems with high response speeds provide continuous control.

In the simplest, relatively low-power electric drives, regulation of the functioning of working mechanisms is performed manually. A disadvantage of manual control is the inertia of the process and the resulting low productivity of the working mechanism as well as the impossibility of accurately reproducing recurring production processes, such as frequent starts. The functioning of the working mechanisms in an electric drive is usually regulated by automatic control devices. Such automatic electric drives are widely used in automatic control systems. In open-loop automatic control systems, a change in a disturbance, such as the load on the motor shaft, causes a change in the assigned operating condition of the electric drive. In closed-loop automatic control systems, as a result of the link between the system’s input and output, the assigned characteristics are automatically maintained under all operating conditions and can be controlled according to a specific law. Increasing use is being made of computers in such systems.

One type of automated electric drive is the servomechanism, in which an actuating member reproduces with a specific accuracy the movements of a working mechanism that are prescribed by the control member. Servomechanisms are classified according to the mode of operation as relay, or discrete, types and those having continuous control. Ranging in power from several watts to tens and hundreds of kilowatts, they find various applications in industry, military technology, and other fields.

During the 1960’s electric drives with numerical control began to be used in several areas of technology, such as multi-operation metal-cutting machine tools and automatic and semiautomatic transfer machines.

The creation of automated electric drives for handling individual production operations and processes is the basis of the integration of automation of production. Integrated automation will require electric drives with a wider range of power capacities and control functions, improved reliability, and optimum size and weight.

REFERENCES

Chilikin, M. G. Obshchii kurs elektroprivoda, 5th ed. Moscow, 1971.
Aven, O. I., and S. M. Domanitskii. Beskontaktnye ispolnitel’nye ustroistva promyshlennoi avtomatiki. Moscow-Leningrad, 1960.
Eletroprivod sistem upravleniia letatel’nykh apparatov. Moscow, 1973.
Osnovy avtomalizirovannogo elektroprivoda. Moscow, 1974.

IU. M. IN’KOV

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