Torque motor

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A torque motor ( torque (Engl.) = Torque ) is a multi-pole , electrical direct drive from the group of slow-speed . Torque motors have very high torques at relatively low speeds.

construction

Simplified structure of a torque motor (permanent-magnet brushless direct current motor in external rotor design)
Internal rotor torque motor with wave winding

In simplified terms, a torque motor can be viewed as a large servo motor with a hollow shaft that is optimized for high torques . Torque motors are usually built as brushless DC motors . In some cases, however, switched reluctance motors are also referred to as torque motors. Since there is no conceptual standardization, the term torque (or high torque ) is often used colloquially for other forms of low-speed or high -torque drives.

Torque motors can be manufactured as external rotors ( stator inside, rotor ("rotor") outside) and as conventional internal rotors (rotor inside, stator outside). External rotors are more common in torque motors because, due to the relationships shown below, they have a greater torque available for the same size.

Special forms are, for example, swivel motors that do not perform a 360 ° rotation. The linear motor , which is a development of a torque motor and is used for translatory drives, can be viewed as a modification of the torque motor .

Mode of action

In its basic principle, the torque motor corresponds to conventional electric motors . The calculation bases for multi-pole servomotors largely apply . The torque motor receives the increased torque primarily from the high number of pole pairs and the resulting short paths for the magnetic return path and winding overhangs, which results in low copper losses and a low mass-to-torque ratio.

In addition, torque motors have a large diameter. Because the torque is with a given force  F over the relationship

depends on the radius  r . With the electric motor, the force is created in the air gap between the rotor and stator. Because of the larger air gap radius , external rotors can therefore generate higher torques than internal rotors with the same size.

properties

The high drive torque of torque motors enables high accelerations and leads to a high dynamic of the system. Torque motors have a higher drive rigidity than classic motor-gear units and no torsional backlash . This results in better control properties due to reduced disturbance variables (see:  cascade control ) and increased repeatability.

Powerful torque motors sometimes have very large dimensions, but they are compact due to the lack of mechanical elements such as toothed belts and gears . The motor is arranged coaxially to the shaft to be driven.

Due to the elimination of mechanical elements, torque motors are low-noise and low-maintenance. There is practically no wear in the engine . The heat development is greater than with conventional servomotors, an exposed location of this heat source is often not possible due to the omitted gearbox. In many cases, this means that the motor must be water- cooled.

Torque drives are more expensive to buy - by a factor of 1.5 to 2.5 compared to conventional motor-gear units. There are cost savings in maintenance and energy consumption. Although torque motors usually have a lower maximum efficiency than high-speed motors, the overall system can be made more efficient by eliminating gears and transmission elements.

Applications

Today, in many areas, for example machine tools, there are requirements that can only be satisfied with modern drives. Motors with gears are often at a disadvantage here, as gears can only be built with a lot of backlash and the effective mass moment of inertia of a high- speed motor is transferred to the slow side in the square of the gear ratio .

Torque motors are used for fast and precise movement and positioning tasks and are used wherever highly dynamic, low-backlash or high-torque drives are required.

  • In machine tools , rotary tables , rotary indexing tables and other swivel drives are equipped with torque motors in order to achieve high positioning accuracy and high dynamics.
  • In servo presses , torque motors replace the conventional motor-flywheel combination in order to be able to dynamically control the speed via the stroke.
  • Pivoting torque motors are used in servo valves for controlled opening and closing.
  • In inland shipping , torque motors have been part of diesel-electric drive systems since 2010 and are installed directly on the propeller shaft.

The company Oswald Elektromotoren GmbH received the German Environment Prize in 2017 for the commercialization of torque motors as an energy-efficient replacement for hydraulic drives .

Web links

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  1. Rückle: Turning Rotary Table. (PDF; 325 kB) March 16, 2010, accessed on May 31, 2011 .
  2. Fibro: Directly driven NC rotary tables. (PDF; 305 kB) November 15, 2006, accessed October 8, 2012 .
  3. Schuler: Servo presses in monoblock design. (No longer available online.) February 23, 2011, formerly in the original ; Retrieved May 31, 2011 .  ( Page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. @1@ 2Template: Dead Link / www.schulergroup.com  
  4. Joachim Venghaus: laboratory hydraulics. Script for the course, September 1, 2008, Stralsund University of Applied Sciences, viewed on May 31, 2011.
  5. Bonapart.de - Inland Shipping News: Diesel-electric motor ship Goblin goes into motion. September 13, 2013, accessed November 15, 2013 .