Mechanical characteristics of production mechanisms - Electric drive

Mechanical characteristics of production mechanisms

By the mechanical characteristic of the production mechanism, we will understand the dependence of the moment of resistance mechanism on its angular velocity: . When displaying the mechanical characteristics of the engine and the production mechanism in one coordinate system, they are led to a single axis of rotation, usually to the motor shaft.

Despite the wide variety of production mechanisms that differ in terms of both power consumption and the principle of operation, their mechanical characteristics can be divided into five main types:

• The mechanical characteristic of the production mechanism independent of angular velocity. These mechanisms include those of them, in which the moment of frictional forces is the predominant moment: the mechanisms of feeding metal-cutting machines, the mechanisms of moving cranes, conveyors, piston pumps. Equation of mechanical characteristic

(2.8)

where is the moment of resistance from frictional forces in the moving parts of the production mechanism.

The mechanical characteristic of the production mechanism, graphically independent of the angular velocity, is shown in Fig. 2.4, the dependence 1.

• Linear-increasing mechanical characteristics of the production mechanism. Such a characteristic is possessed by direct current generators operating on a constant load, crimping rolls of rolling mills, ironing machines. The equation of the mechanical characteristic has the form

(2.9)

where - moment of resistance from frictional forces in the moving parts of the production mechanism; a - coefficient of proportionality.

The graph of the linearly increasing mechanical characteristic of the production mechanism is shown in Fig. 2.4, the dependence 2

• Nonlinearly increasing mechanical characteristics of the production mechanism. Such a characteristic has mechanisms with a centrifugal nature of the production process: fans, centrifugal pumps, centrifuges, propellers. The equation of the mechanical characteristic has the form

(2.10)

where - moment of resistance from frictional forces in the moving parts of the production mechanism; b - coefficient of proportionality; x is the exponent; at x = 2 - motion in the gaseous medium, with x = 3 - motion in the liquid.

The graph of the nonlinearly increasing mechanical characteristic of the production mechanism is shown in Fig. 2.4, the dependence 3.

• Nonlinear-decaying mechanical characteristic of the production mechanism. This characteristic is possessed by the main electric drives of processing machines: metalworking, plywood, etc. In such machines, the cutting moment varies inversely with the cutting speed. For example, a thin drill - a high speed of rotation of the cartridge, a large-diameter drill - a small speed of rotation of the cartridge. The equation of the mechanical characteristic has the form

(2.11)

where - moment of resistance from frictional forces in the moving parts of the production mechanism; c is the proportionality coefficient.

The graph of the non-linear-decreasing mechanical characteristic of the production mechanism is shown in Fig. 2.4, the dependence 4.

• Mechanical characteristic of the production mechanism with increased starting torque. This characteristic is possessed by mixers, some mechanisms of mixing liquid media, for example paints and others. In such mechanisms, after the entire liquid starts rotating in a confined space, the moment of resistance drops sharply. In some mechanisms, a large starting torque develops at the beginning of the starting, for example, in the electric drive of the main tramway, electric train, the mechanism of gaugovers when unscrewing the nuts. At such mechanisms, large intermolecular forces of attraction act at the beginning of the motion.

The graph of the mechanical characteristic of the production mechanism with an increased starting torque is shown in Fig. 2.4, the dependence 5.

As already noted, the operation of the electric drive in the steady state corresponds to the equality of the motor moments and the moment of resistance. In order to maintain the most stable speed of the electric drive with accidental changes in the moment of resistance of the production mechanism, primarily due to the misalignment and eccentricity of the coupling shafts, it is necessary to ensure the intersection of the mechanical characteristics of the engine and the mechanical characteristics of the mechanism brought to the motor shaft at the angle closest to the straight line.

Mechanical characteristics of production mechanisms

Fig. 2.4. Mechanical characteristics of production machinery

By the nature of the action, all kinds of static moments can be divided:

• reactive;

• Active.

Reactive moments have almost all the mechanisms - that is, those mechanisms in which the moment is created by friction forces. Such moments always hamper the movement and therefore change their sign with a change in direction of motion. The mechanical characteristics of production mechanisms with reactive moments are shown in Fig. 2.5, the dependencies . It also shows the mechanical characteristics of the DC motor and the speed of its steady motion.

A small group of mechanisms creates an active moment. Mechanisms with an active torque ns only impede movement, but under certain conditions themselves create the rotation of the electric drive. It is obvious that in order to ensure a constant speed, the electric machine goes into braking mode.

Mechanical characteristics of production mechanisms with reactive moments

Fig. 2.5. Mechanical characteristics of production mechanisms with reactive moments

Active moments retain their sign when the direction of rotation changes. A characteristic example of a mechanism with an active torque is the winch or the crane lifting mechanism. The mechanical characteristics of production mechanisms with active moments are shown in Fig. 2.6, the dependence . It also shows the mechanical characteristics of an induction motor and the speed of its steady motion.

Mechanical characteristic of the production mechanism with an active torque

Fig. 2.6. Mechanical characteristic of the production mechanism with an active torque

Note that in practice, mechanisms with an active torque in pure form do not exist, since friction mechanisms, and therefore reactive moment, are always present in the lifting mechanisms.

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