12 January 2010
The direct-drive, permanent-magnet (PM) torque motor is characterised by a large diameter-to-length ratio, which gives it the characteristic ‘flat’ look.
Another feature of this type of motor is, to optimise torque production, they are usually built with a large number of magnetic poles. Large numbers of magnet pole pairs in the rotor aid high torque generation.
Torque motors normally operate at relatively low speeds, usually under 1,000 rpm. In fact most users of Baumüller’s DST torque motors run them at 100 to 200 rpm, says Joachim Weissenberger, sales director of exports.
High-torque motors are also characterised by a high overload capability and reduced noise emissions.
With IP54 protection, these motors can be used successfully in harsh industrial environments. Typically, direct drives are used as alternatives to geared and hydraulic motors in the plastics and printing industries.
The advantage of direct drive
But the most important aspect of the torque motor is that it is a direct drive motor, meaning that there are no power transmission elements between motor and driven load.
This brings advantages of high dynamic motion with essentially no backlash and excellent static / dynamic load stiffness that allows precise motion control.
Mechanical power transmission devices such as gearboxes have components that can eventually wear down, degrading performance, and, as they deteriorate, introduce backlash — gaps between mating components — into the system. Because they are mechanical, they may require lubrication and adjustment.
‘Users will typically replace gearboxes every three to four years,’ said Mr. Weissenberger. Another negative factor about gearboxes is their high noise.
And, in these energy-conscious times, it must be admitted that even the best mechanical transmission systems, that is, the most efficient ones, waste power in the form of heat and friction. Noise from gearboxes is, in effect, another form of wasted energy.
Mr. Weissenberger estimates a direct drive motor has energy savings of about 30% when compared to DC motors, and 10 to 15% when compared to asynchronous motors.
As the use of direct drive motors eliminates mechanical problems, it is often the case that a smaller motor can be used to drive the load when direct drive is introduced to replace the older transmission systems.
Baumüller’s DST torque motor line
Baumüller has been a major player on the direct drive technology market since 1991.
Because, as mentioned above, the gearboxes and other mechanical transmission elements are absent, the motors are said to offer zero backlash. This enables high control effectiveness.
This attribute makes it possible to draw conclusions about the quality of the connected process by monitoring motor torque and speed, explains product manager Marcel Möller. Operating parameter changes, such as changes in lubricant viscosity, are correlated in the controller based on software programs, resulting in better system control and product quality.
‘As a rule, direct drives also improve overall system efficiency and lead to energy savings,’ he adds.
Baumüller integrates water cooling into its DST torque motors as a recognised need for top torque performance. ‘Only this way is it possible to achieve the high torque density and simultaneous high overload capacity,’ states Mr. Möller. ‘Integrated water cooling furthermore enables a higher protection class IP54, which helps DST motors meet harsh conditions found in an industrial setting.’
Besides more cooling capacity, a further—yet perhaps counter-intuitive—advantage of water cooling is reduced noise emission. Water-cooled torque motors run quieter than their fan-cooled counterparts.
Mr. Weissenberger says that more and more customers asking for direct drive solutions instead of conventional motor-gearbox combinations, and it is for this reason that Baumüller has extended its range of DST torque motors.
These synchronous motors offer a higher power density, in addition to a wider range of available speeds and torques. For example, the 400 mm diameter size, the largest, means that, for the first time, rated torques up to 20,000 Nm and maximum torques up to 32,000 Nm are possible.
The new product line also requires less installation space, thereby reducing the contact surface for machines.
Because torque motors have practically no residual torque, the customer can improve the smooth running characteristics.
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