Saturday, February 9, 2013

Motor Drives

Motor Drives

General

A motor drive refers to an electromechanical system consisting of a drive motor (or motors), a driven machine, a connection from the drive motor to the driven machine, and associated switchgear and controls.

This topic will be mainly concerned with matters bearing on the selection of a drive motor for a particular application.

Historically, the form of driving that appeared first was group drive (or driving). In it, a single motor operated a line shaft, and this transmitted motion to the driven machines by means of ropes, V -belts, gears or some other members varying in number. Obviously, this form of drive was inevitably wasteful of power (in some cases as much as 50 % of the input power would be lost on its way from the drive motor to the driven machines). Also, the installation would ordinarily take up quite a lot of floor area.

With advances in drive-motor design, group driving gave way to individual drive - in this case each machine was equipped with a drive motor of its own, often coupled to each other directly.

The next step forward was the reduction in the power lost within the driven machine itself. With only one motor per driven machine, one has to provide gear trains, V-belts, eccentrics and similar ele­ments so that the machine could perform the various of its functions. With multimotor drive, each function (or, rather, each tool) of a driven machine is supplied by power by a separate drive motor. In other words, each tool has an individual drive of its own.

The advent of variable-speed drive motors has solved the problem of speed variation of the associated tool - now this can be done without resort to sophisticated mechanical devices (such as gear­boxes and the like).

Direct connection of a drive motor to a driven machine makes it advantageous to adapt one to the other, so that in a wide range of present-day equipment it is difficult, if at all possible, to draw a boundary between its electric and mechanical sections.

In simplified form, the history of drive for a radial drilling machine is shown in Fig 1 where the use of group driving is illus­trated at 1, improvements in individual drive at 2 through 4, and multimotor drive at 5.

A further advanced of motor drive for machine tools is that their control with the aid of clutches, flat and V-belts and the like may be replaced with purely electric controls. With electric control, the operator spends less time on the necessary adjustments, speeds up the production process, and turns out more products per unit time. In cases involving operations that follow one another at a high rate, it has become necessary in many cases to replace the human operator with automatic facilities in the control function.

Motor Drives

Automatic control is now applied to both simple machines such as squirrel-cage induction motors with ratings less than 1 kW, and to large units such as are used to drive blooming mills and totalling between them several megawatts of power. Automatic control may be limited to starting, stopping and reversing a motor. In other cases it may effect a sequence of operations in machining a part to a template. Automatic action is also used to protect driven machinery against overloads or breakdowns.