Direct drives

Definition and characteristics of a direct drive

A direct drive transmits motion to a system or object requiring actuation without the use of any further mechanical components. This means that there is no linking system, such as gearing or a motion transducer between the electric drive motor, which supplies and transforms the energy, and the so-called driven machine which absorbs the transformed energy. The drive motor and the driven machine are thus directly coupled to each other. The rotational speeds of the drive motor and the driven machine are coordinated, with the drive motor forming an adaptive unit. In this context, the driven machines used in direct drives, generally electric motors, are frequently of a special construction. It is the job of the motor to supply the torque, or the required force. The reason why direct drives are of such great interest is predominantly down to their primary characteristic of shortening the distance between electronic and mechanical elements to an absolute minimum.

Dynamic, compact and cost-efficient - the direct drive

Thanks to its principally light and compact construction and the low number of gears and other mechanical components incorporated in the system, servicing, maintenance and energy costs can be kept to a minimum, which at the same time facilitates the drive’s long-term use. Further characteristics of this drive construction are its high product quality, long-term availability, low degree of wear and tear, and low-noise operation. The direct link between the motor and the driven machine precludes unwanted attributes like play and elasticity. This is what gives the drive its extreme precision and reliability. Furthermore, the direct drive displays a high level of availability due to the small number of intermediate mechanical elements incorporated in the system and the high degree of rigidity enabled by the strong reinforcing properties of  the control loop.

The direct drive makes no use of any motion transformers, which significantly lowers the moment of inertia. Direct drives display greater dynamics in comparison with more traditional drive forms. This impacts positively on other characteristics of the drive unit, resulting in short latency times and a high final speed. All of these properties come together to create an enormously efficient and effective drive unit.

Direct drives in practice

In practice, direct drives have to be integrated in special drive systems to enable them to realise the characteristics described above. A pre-controller is incorporated in the system that allows targeted regulation of such variables as torque and speed of rotation, creating an optimum constructional solution for the benefit of users of the system. Due to the enormous power density associated with direct drives, it can occasionally happen that drive units become excessively hot; for this reason, specially adapted cooling systems are often employed to supply corrective cooling. Another factor to be taken into consideration in this type of system is that it does not display the customary self-locking effect. In some applications, braking mechanisms can be fitted to provide a solution to this problem. The direct drive generally displays larger dimensions and a greater weight when compared to a geared drive. Other disadvantages are the large control devices that it requires due to the high current consumption and very large current heat losses.

Direct drives based on linear and torque motors

Direction drives are often based on translational linear motors and rotational torque motors; moreover, they essentially apply the same action principles. The torque motor represents a specialised constructive form of the direct drive. It is a high-pole synchronous motor that is able to generate a higher rotational torque than a low-pole synchronous motor. The maximum possible rotational speed is, however, lower than with a low-pole synchronous motor. Torque motors are used in applications requiring a high torque level combined with a low rotational speed, for instance as a substitute for conventional constructions combining electric motor and gearing.

The advantages of the torque motor are similar to those of the direct drive. Torque motors are often equipped with an external water cooling unit to increase their power density. On the one hand this adds additional complexity to the system, but it does have the benefit of providing constant temperature conditions in the work space. Constant temperature conditions are a fundamental requirement of high-precision machine tools, such as multiple-axis milling machines.

Torque motors belong to the group of so-called slow runners, due to their high pole number and low speed of rotation. In terms of their performance data, fast runners, such as spindle motors, are the counterpart to slow runners. In contrast to rotational motors, linear motors do not move objects in a circular motion but position them along a linear or curved track. In addition to the respective motor build, both rotational and linear direct drives are generally fitted with a measuring system and a frequency converter.

The principles by which force and torque are generated in linear and torque motors vary greatly, giving rise, from a physical and technical perspective, to asynchronous motors, permanently excited synchronous motors and stepper motors. However, there is one aspect that all direct drives have in common, whatever their form and construction - they enable the direct transmission of motion.

Applications of direct drives

In respect of the applications in which direct drives can be found, considerable differences exist between slow runners on the one hand and fast runners on the other. High-pole slow runners, for instance, often have a relatively large diameter and accordingly are frequently used in correspondingly large-scale applications. Classic examples include hydroelectric power stations and wind generators, with which typical diameters are around 5 metres. The group of slow runners includes torque motors, which are particularly suitable for quick and precise positioning tasks that require the widest possible range of modification capability. They are found in the context of machine tools, servo-presses and valves, and inland shipping.

Fast-running direct drives, on the other hand, are mostly used in applications that require a large rotational speed. They are mounted directly inside the spindle of the spindle drive used in textile machines and similar equipment. Other devices to benefit from the distinctive features of fast runners include turbo molecular pumps, so-called vacuum pumps, and electric turbo chargers.

Other examples of applications for direct drives are centrifuges, which are primarily used in the context of chemical or medical processes. Furthermore, everyday items such as ventilators, mixers and vacuum cleaner fans are based on the operational principles of direct drive technology.

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