ELECTRICAL AND ELECTRONICS ENGINEERING..!!: Constructional Features and Principle Operation of linear Induction Motor

Linear motors

Since linear motors do not have any gear unit it is more simple converting motion in electrical drives. Combined with magnet floating technology an absolutely contact-less and so a wear resistant passenger traffic or non-abrasive transport of goods is possible. Using this technology usually should enable high speed. So Transrapid uses a combination of synchronous linear drive and electromagnetic floating. Linear direct drives combined with magnet floating technology are also useful for non-abrasive and exact transport of persons and goods in fields as transportation technology, construction technology and machine tool design. Suitable combinations of driving, carrying and leading open new perspectives for drive technology.

9.5.1 Technology of linear motors

In the following function, design, characteristic features, advantages and disadvantages are demonstrated shortly. In principle solutions based on all electrical types of machines are possible unrolling stator and rotor into the plane.

Linear motor then corresponds to an unrolled induction motor with short circuit rotor or to permanent-magnet synchronous motor. DC machines with brushes or switched reluctance machines are used more rarely. Depending on fields of usage linear motors are constructed as solenoid, single-comb or double-comb versions in short stator or long stator implementation. It is an advantage of long stator implementations that no power has to be transmitted to passive, moved secondary part, while short stator implementations need the drive energy to be transmitted to the moved active part. For that reason an inductive power transmission has to be used to design a contact-less system. In contrast to rotating machines in single-comb versions the normal force between stator and rotor must be compensated by suitable leading systems or double-comb versions must be used instead. This normal force usually is one order of magnitude above feed force. In three-phase windings of synchronous or induction machines a moving field is generated instead of three-phase field. This moving field moves at synchronous speed.

As in three-phase machines force is generated by voltage induction in the squirrel-cage rotor of the induction machine or by interaction with permanent-magnet field of the synchronous machine.

Three-phase machine supply is made field-oriented by frequency converters to achieve high dynamic behavior. For that induction machines need flux model and speed sensor, but synchronous machines just need a position sensor. For positioning jobs high dynamic servo drives with cascade control consisting of position control with lower-level speed and current control loop are used. This control structure is usual in rotating machines. Depending on the place the position measurement is installed a distinction is made between direct and indirect position control. Since many movements in production and transportation systems are translatory, linear drives are useful in these fields. In such motors linear movements are generated directly, so that gear units such as spindle/bolt, gear rack/pinion, belt/chain systems are unnecessary. As a result from that rubbing, elasticity and play are dropped, which is positive for servo drives with high positioning precision and dynamic. In opposition to that there are disadvantages such as lower feed forces, no self-catch and higher costs.

High speed applications

In the magnet high-speed train Transrapid wheels and rail are replaced by a contact-less
working electromagnetic float and drive system. The floating system is based on attractive forces of the electromagnet in the vehicle and on the ferro-magnetic reaction rails in the railway. Bearing magnets pull the vehicle from below to the railway, guide magnets keep it on its way. An electronic control system makes sure, that the vehicle always floats in the same distance to the railway. Transrapid motor is a long-stator linear motor. Stators with moving field windings are installed on both sides along the railway. Supplied three-phase current generates an electromagnetic moving field within windings. The bearing magnets, and so also the vehicle are pulled by this field. Long-stator linear motor is divided into several sections. The section, in which the vehicle is located, is switched on. Sections, that make high demands on thrust, motor power is increased as necessary. Drive integrated in the railway and cancelling of mechanical components make magnet high-speed vehicles technical easier and safer. Transrapid consists of two light weight constructed elements. Capacity of the vehicles can be adjusted to certain requirements. Operating speed is between 300 and 500 km/h. A linear alternator supplies floating vehicle with required power. Advantages of magnet highspeed train are effective in all speed areas. After driving only 5 km Transrapid reaches a speed of 300 km/h in contrast to modern trains needing at least a distance of 30 km. Comfort is not interfered with jolts and vibrations. Since vehicle surrounds the railway Transrapid is absolutely safe from derailment. Magnet high-speed train makes less noise than conventional railway systems because there is no rolling noise. Also energy consumption is reduced
compared with modern trains. This high-speed system is tested in continues operation at a
testing plant in Emsland in Germany and some commercial routes in Germany are planned. A high-speed train route is currently under construction in Shanghai, China, further projects areeither in progress or under review.

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