By eliminating the transmission in the drive train, its overall efficiency is better because there are no friction losses in the transmission. Furthermore, there are no inaccuracies in movement due to backlash in the gearbox, which is not desirable with positioning drives, and maintenance is simplified because there are fewer wearing parts. By eliminating the transmission, the drive train is more cost-effective and structurally simpler.
Compared to drives with gears, the direct drive has the disadvantage that the speed is limited by the drive machine. For electric drives that are supplied directly from the 50 Hz national grid, the maximum speed is limited to 3000 min −1 for technical reasons , higher speeds can only be achieved with the aid of gearboxes or frequency converters . Many turbomachines such. B. Compressors , but can be built simpler and smaller at higher speeds.
With a direct drive, the drive machine must generate the drive torque of the driven machine. By installing a gearbox, the drive torque of the motor can be selected to be smaller, which makes the motor smaller, lighter and more cost-effective. If the speed is high, not only can the transmission be saved, but the mass of the motor is also lower, since the output power increases with increasing speed with the same torque. If, on the other hand, a low speed is required, a suitable direct drive motor is often heavier than a gear motor (reduction gear + high-speed motor).
Direct drives can only be made variable-speed by changing the speed of the drive machine. In electric drives may be caused by frequency be accomplished (FU), which can supply the engine is higher than 50 Hz and at frequencies with which the 3000 min -1 constraint will be invisible for the maximum speed of the drive train. Another possibility for speed control is the use of pole-changing motors or Dahlander motors , but where the maximum speed is still 3000 min −1 .
The direct drives include all electric linear and torque motors . The physical-technical principles for generating force or torque can be very different. They range from asynchronous motors and stepper motors to permanent magnet synchronous motors (PMS) of various types. What all direct drives have in common is the direct coupling of a moving motor element with the mass to be moved, for example a linear or rotary table or a swivel bridge in mechanical engineering .
The elimination of motion converters, for example ball screws or gears , leads to a reduction in mass or moment of inertia . This makes direct drives much more dynamic than conventional drives. Either shorter reaction times, higher track accuracy in mechanical engineering, higher top speeds or higher speeds can be achieved. The direct coupling between the motor and the moving object avoids play and elasticity. This increases the accuracy. For positioning drives, the increased control rigidity and contour accuracy. Ultimately, positioning resolutions in the micrometer and submicrometer range can be achieved.
Their structure is simpler, more compact and ultimately more reliable. When using direct drives, however, some special features must be observed, such as the lack of self-locking or self-locking . As a result, many applications require brakes or buffers. When moving against gravity , counterbalancing measures may also be necessary.
Since they are connected directly to the machine without a gearbox, direct drives are of course also more load-dependent than conventional drives; and due to their direct mass coupling, they are also more susceptible to vibrations. This must be taken into account both when designing the motors and when parameterizing the control structures. Ultimately, the entire construction is to be designed to be very rigid and suitable for direct drive. The forces of linear motors range in size depending on some Newton up to the range of 20 k N in torque motors from a few Nm up to 100 Nm. The elimination of power or torque transmission means that there are limits to direct drives in the heavy-duty range.
Linear motors and torque motors basically work according to the same operating principles. Torque motors can be viewed as wound linear motors, the latter as unwound torque motors with two ends. Force and torque should be as uniform as possible during movement. The remaining ripple ( cogging , load pulsation) significantly determines the quality of a direct drive. Cogging is caused by magnets and occurs even when there is no current. When the motor is energized, the load pulsation is added depending on the current. Various engine principles produce these effects to a greater or lesser extent or suppress them entirely.
For the highest accuracy requirements, the direct drive must generate a very current-proportional force in connection with a high-resolution measuring system and integrated into a control structure, i.e. have the most linear behavior possible. This is the only way to achieve the finest infeeds or a constant speed and speed. The power of a linear motor and the torque of a torque motor arise in the working air gap, i.e. the active interface between the primary and secondary part. With permanent magnet motors , this is the area above the permanent magnets .
- Differences in structure
- Linear motors usually consist of an energized primary part, the laminated core with coil system, and a passive secondary part. Depending on the underlying principle, the secondary part is either designed as a toothed rail, equipped with induction loops or with permanent magnets. In the widely used PMS linear motors, the permanent magnets with alternating polarity (NSNS ...) are located on the surface of the secondary part, on a magnetic rail. When the primary part, which is usually three-phase , is energized, a current-dependent force arises between the primary and secondary part. In combination with linear guides and a measuring system for position detection, linear tables, for example, can be implemented for machine tools in a relatively simple manner .
- Torque motors also consist of an energized primary part, the stator, and a passive secondary part, also called a rotor. The PMS, which are also widely used, have permanent magnets on a mostly cylindrical rotor ring, also alternately polarized and pointing towards the working air gap. In special cases the magnets are embedded. Depending on whether the rotor is internal or external, one speaks of an internal rotor or external rotor motor . If the rotor is coupled to a suitably mounted turntable, a relatively simple rotary or swivel axis structure is possible in combination with an angle measuring system.
Depending on the motor topology and the structure of the stator and rotor, there are various properties and adaptation options. As a result, there is no linear motor or torque motor, but a multitude of different types, series and customer-specific adaptations. The degree of freedom in design is much greater than with conventional drives.
Advantages and disadvantages
- There is no gearbox, which means lower costs, less maintenance and better efficiency
- less wear and tear and noise
- with high-speed direct drives lower volume, lower mass, high power density
- higher system rigidity, no play
- more compact design
- high dynamics
- For the purpose of direct drive, slow-running motors are heavier with the same power or they have only a low torque.
- Direct drives often require specially designed motors that not only generate special speeds, but also often have to have more robust bearings to accommodate additional bearing forces or imbalances. Therefore, they can often only be used for products that are manufactured in large numbers.
Examples of direct electric drives are vacuum cleaner fans ( universal motors ), mixers, centrifuges (laboratory centrifuges, fruit juice centrifuges), fans and spin dryers . Then there are stationary grinding machines (bench grinder ) or water pumps , which are mostly operated with asynchronous motors.
The speeds are often a distinguishing feature:
With a high number of poles, the speed is significantly reduced with the existing supply frequency. While a standard motor is usually 2 or 4-pole and therefore has a speed of 3000 or 1500 min −1 at 50 Hz , a 30-pole machine has a nominal speed of 200 min −1 .
Slow runners with a high number of poles have a large diameter, which can be several meters.
A classic application of low-speed motors are, for example, generators in hydropower plants: for example with a speed of 65.2 min −1 and a number of poles of 92.
The speed of synchronous generators or asynchronous motors can vary if they are connected via a frequency converter .
Slow runners are often permanently excited DC motors or (partly electronically commutated) multi-pole synchronous motors.
- Use in certain wind power plants : typical data: approx. 3 MW, frequency approx. 15 Hz, speed approx. 17 min −1 , number of poles approx. 100, diameter approx. 5 m, power supply by means of a full converter
- Cableway or conveyor belt drive: direct drive sheave is only by Leitner AG offered
- Propeller nacelle drive (ships): directly driven propeller
- Platter direct drive (higher quality turntables )
- Bobbins or mandrels (higher quality tape recorders )
- Drives for rotary kilns , which are usually built around the kiln
- Direct drives in vehicles, mostly as external rotor built
- Axle motors in railway vehicles, used in the beginning and recently again at Syntegra with permanent magnet motor
- Ship drive: direct drive from the ship's diesel engine to the shaft system ; to change the direction of rotation, the engine must be stopped and the control of the cylinders changed by shifting the camshaft so that the machine runs backwards after restart
- Steam locomotive , drives with steam engines
- Aircraft engines without propeller gears , such as radial engines , sometimes also built as rotary engines .
High-speed runners run significantly faster than standard motors. There are engines that over 100,000 min -1 achieve, for example when used in electric turbochargers . This is achieved through a supply with frequency converters with a supply frequency of several hundred Hz to over 1000 Hz. The motor is smaller than a standard motor with the same power. The rotating parts have to withstand considerable radial accelerations (centrifugal force).
- Spindle drive for textile machines: integrated directly into the spindle, highly dynamic
- Motor spindle in machine tools
- Turbo molecular pump (vacuum pump), approx. Up to 100,000 min −1
- Electrical turbocharger : Speed 130,000 min -1
- Joachim Klement: Technology of electrical direct drives . expert verlag, 2009, ISBN 978-3-8169-2822-5 ( google.de ).
- gearbox vs. Direct drive . November 13, 2018 ( aradex.de ).
- Direct drives selected appropriately. In: Detmar Zimmer, Joachim Böcker, Alexander Schmidt, Bernd Schulz, University of Paderborn. February 2005, accessed July 23, 2019 .
- DirectDrive. LEITNER ropeways, accessed on December 2, 2018 .
- Günter Mau: Manual Diesel Engines in Power Plant and Ship Operation . Springer-Verlag, 2013, ISBN 978-3-322-90621-2 ( google.de ).