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Principle of Linear Electric Motor

What is a Linear Motor?

Linear motor is a kind of transmission device that directly converts electric energy into mechanical energy of linear motion without any intermediate conversion mechanism. We also call it linear motor, linear motor, linear motor, push rod motor. The most commonly used types of linear motor are flat linear motor, U-slot linear motor and cylindrical linear motor. It has the advantages of simple structure, less wear and tear, low noise, strong combination and convenient maintenance. There are almost all kinds of linear motors corresponding to the varieties of rotary motors.

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Structure Composition of Linear Motor



The structure of the straight line mainly includes three parts: the stator, the mover and the supporting wheel of the linear motion. In order to ensure good electromagnetic field coupling between stator and stator in the range of travel, the length of stator and stator core is different. Stator can be made into short stator and long stator. Because of the high cost of long stator structure and operation cost, it is seldom used. Like the rotating magnetic field, the stator core is made of silicon steel sheets with slots on the surface. There are three-phase, two-phase or single-phase windings embedded in the slots. The single-phase linear asynchronous motor can be made into a cover-pole type or phase-shifted by capacitance.

Linear motor is the primary expansion and straightening of traditional cylindrical motor, changing the primary closed magnetic field into open magnetic field, while the stator part of rotary motor becomes the primary of linear motor, and the rotor part of rotary motor becomes the secondary of linear motor. If the primary is fixed, the secondary can move in a straight line in the direction of the traveling wave magnetic field. It can realize the feeding mode of direct drive by linear motor of high-speed machine tool, and install the primary and secondary of linear motor directly on the worktable and bed of high-speed machine tool respectively. Because the transmission chain of this feed transmission mode is shortened to 0, it is called "zero transmission" of machine tool feed system.

Working Principle of Linear Motor

In linear motors, the stator of a rotary motor is called the primary stage, and the rotor of a rotary motor is called the secondary stage. The secondary stage moves along the primary line under the action of electromagnetic force. At this time, the primary stage needs to be long, extending to the position needed for movement, while the secondary stage does not need to be so long. In fact, linear motors can either make the primary stage very long, or make the secondary stage very long; they can be primary fixed, secondary mobile, or secondary fixed, primary mobile.

Linear induction motors are evolved from rotary motors. When the three-phase (or multi-phase) winding on the primary side enters the symmetrical sinusoidal AC current, the air gap magnetic field will be generated. When the longitudinal edge effect caused by the breaking of the two ends of the core is not considered, the distribution of the air gap magnetic field is similar to that of the rotary motor, and it distributes sinusoidally along the straight line.

Main Classification of Linear Motor
Linear motors can be divided into DC, asynchronous, synchronous and stepping motors according to their working principles; linear motors can be divided into single-sided flat type, double-sided flat type, disc type, cylinder type (or tube type) according to their structural forms. The most commonly used types of linear motor are flat linear motor, U-slot linear motor and cylindrical linear motor. Voice coil motor is named for its structure similar to the voice coil of the speaker. It has the characteristics of high frequency response and high precision. This kind of motor can be divided into cylindrical voice coil motor and swing voice coil motor. That is the so-called cylindrical linear motor.

Flat-plate linear motor
The iron core of the flat linear motor is installed on the steel laminated structure and then on the aluminium backplane. The iron laminated structure is used to guide the magnetic field and increase the thrust. The suction between the magnetic track and the motor is proportional to the thrust generated by the motor. The laminated structure leads to the joint force. Care must be taken to install the movers on the magnetic rails so that the suction between them does not cause harm.

U-slot linear motor
U-groove linear motor has two parallel magnetic rails between metal plates and both of them are facing the coil actuator. The mover is supported by the guide rail system in the middle of the two magnetic rails. The actuator is non-steel, which means there is no suction and no interference between the magnetic track and the thrust coil. Non-steel coil assembly has low inertia and allows very high acceleration. Coils are generally three-phase, brushless commutation. The motor can be cooled by air cooling to improve its performance. There are also water cooling methods. This design can reduce flux leakage better because the magnet is installed face to face in the U-shaped guide groove. This design also minimizes the damage caused by strong magnetic attraction. This design allows combination of tracks to increase travel length, limited to the operational length of the cable management system, the length of the encoder, and the ability of the mechanically constructed large and flat structure.

Cylindrical linear motor
Cylindrical moving magnet linear motor actuator is cylindrical structure. Moving along a cylinder with a fixed magnetic field. This motor is the first commercial application discovered, but it can not be used in flat plate and U-groove linear motors which require space saving. The magnetic circuit of the linear motor with cylindrical moving magnet is similar to that of the dynamic magnetic actuator. The difference is that coils can be replicated to increase travel. Typical coil windings are composed of three phases, and the Hall device is used to realize brushless commutation. The thrust coil is cylindrical and moves up and down the magnetic rod. This structure is not suitable for applications sensitive to flux leakage. Care must be taken to ensure that the finger is not stuck between the magnetic rod and the attractive side. A potential problem in the design of tubular linear motors occurs when the stroke increases, because the motor is completely cylindrical and moves up and down along the magnetic rod, the only support point is at both ends. The length of the magnet contact thrust coil is always limited to ensure that the radial deviation of the magnet is not caused.

Advantages of Linear Motor
1. Simple structure. The linear motor does not need to pass through the intermediate conversion mechanism to directly generate linear motion, which greatly simplifies the structure, reduces the inertia of motion, greatly improves the dynamic response performance and positioning accuracy, and also improves the reliability, saves the cost, and makes the manufacture and maintenance more simple. Its primary level can directly become a part of the organization, this unique combination makes this advantage further manifested.
2. High acceleration. This is a significant advantage of linear motor drive over other screw, synchronous belt and rack-and-pinion drive.
3. Suitable for high-speed linear motion. Because there is no constraint of centrifugal force, ordinary materials can also achieve higher speed. Moreover, if air cushion or magnetic cushion is used to preserve the gap between the primary and secondary stages, there will be no mechanical contact during movement, so there will be no friction and noise in the moving part. In this way, the transmission parts are not worn out, which can greatly reduce the mechanical loss, avoid the noise caused by towing cables, cables, gears and pulleys, and thus improve the overall efficiency.
4. High utilization rate of primary winding. In the tubular linear induction motor, the primary winding is cake type and there is no end winding, so the utilization rate of winding is high.
5. There is no lateral edge effect. The transverse effect refers to the weakening of the magnetic field at the boundary caused by transverse interruption, while the cylindrical linear motor has no transverse interruption, so the magnetic field distributes uniformly along the circumference.
6. It is easy to overcome the problem of unilateral magnetic pull. Radial tension cancels each other, and there is basically no problem of unilateral magnetic tension.
7. Easy to adjust and control. By adjusting voltage or frequency or replacing secondary materials, different speed and electromagnetic thrust can be obtained, which is suitable for low-speed reciprocating operation.
8. Strong adaptability. The primary iron core of linear motor can be sealed with epoxy resin as a whole, which has good anti-corrosion and moisture-proof performance, and is easy to use in humid, dust and harmful gas environment; moreover, it can be designed into a variety of structural forms to meet the needs of different situations.

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