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`Direct drive` ousts gears and encoders

01 July, 2001

`Direct drive` ousts gears and encoders

ABB has launched "a new generation of drive technology" which it says will enable "entirely new mechanical configurations, including drive systems without gearboxes or pulse encoders, to improve overall machine efficiency".

The technology, called DriveIT Direct Drive, was launched at a pulp and paper exhibition in Finland in June. It is aimed at low-speed drive applications above 300 rpm that would traditionally require mechanical reducers to step down the speed from a 1,500 rpm motor (shown on left in diagram below).

The new approach (shown on the right) does away with these reduction gears, allowing the drive motor to be coupled directly to the load. Normally this would require a large motor but ABB is suggesting using a synchronous motor with a radial flux construction and a permanent magnet rotor. This 400/690V AC motor -which could be air- or water-cooled - will come in ratings from 27-700kW, with nominal operating speeds between 300 and 850 rpm. In most cases, the motor will be the same size or smaller than an equivalent induction machine.

Because the motor is synchronous, there is no rotor slip, resulting in accurate static rotor speeds, and a dynamic control that is enhanced by eliminating the need for slip compensation.

The way that the motor is constructed and cooled is said to result in lower noise levels than conventional motors. The PM rotor stays cool, and versions with water-cooled frames can be designed with high power densities and protection classes. This will improve reliability, especially in harsh environments, says ABB.

The potential drawback of using a synchronous machine of this type is that it needs a dedicated control system, designed especially for permanent magnet flux control. However, ABB has been able to adapt its Direct Torque Control (DTC) technology, originally developed for use with induction motors, to control permanent magnet motors.

The controller is based on ABB`s ACS 600 frequency converter and uses the same water- or air-cooled hardware as would be used for induction motors. But the company has developed new control software that allows the hardware to drive synchronous motors.

Eliminating the conventional gears and mechanical linkages will, claims ABB, result in simpler plants that are easier to install, make better use of floorspace, reduce the need for spares and be more reliable. Machine availability will be higher, there will be fewer production interruptions, and less materials wastage. When it is needed, maintenance will be quicker.

Because the drive can run without a pulse encoder, fewer components are needed, further reducing maintenance, and producing a robust drive, according to ABB.

Other claimed attractions of the new direct drive technology include better machine "runnability", because of the absence of shaft and gearplay resonances, and lower energy costs, because of reduced losses.

Initially, ABB appears to be focusing on applications on papermaking machinery, such as pulp driers, and coating and finishing machines.

At the recent EMO 2001 show, Siemens launched a similar direct-drive "torque motor" for machine tool applications. The stator of the synchronous machine has an iron core with a three-phase winding; the permanent magnet rotor has a cylindrical iron core. The motor can deliver a torque of up to 4,100N.

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