Generally, the motor is rotating when working, but the means of transportation driven by rotating motors (such as electric locomotives and trams in the city) need to do linear motion, and some parts of the machine driven by rotating motors also need to do linear motion, which requires adding a set of devices to change the rotational motion into linear motion, whether it can be directly driven by the linear motion motor, so as to save this set, people have raised this question, and now a linear motor with linear motion has been made, that is, a linear motor.

What is a linear motor?

Linear motors are also known as linear motors, linear motors, linear motors, and actuator motors. The most commonly used types of linear motors are flat and U-groove, and tube. The typical composition of the coil is three-phase, with Hall elements for brushless commutation.

Modern advanced drive technology is mainly divided into two categories: one is electromagnetic, and the other is non-electromagnetic.

The modern advanced drive technology of electromagnetic is mainly modernElectromagnetic drives are composed of modern control systems, and their drives include traditional improved electromagnetic drives and newly developed electromagnetic drives. Among them are rotating, straight, maglev, electromagnetic emitting, etc.

A linear motor is a transmission device that directly converts electrical energy into linear motion mechanical energy without the need for any intermediate conversion mechanism. It can be seen as a rotating motor split radially and unfolded into a plane.

Linear motor is a novel motor that directly converts electrical energy into linear motion mechanical energy without passing through any conversion device, which has the advantages of simple system structure, less wear, low noise, strong combinability, and convenient maintenance. There are almost all varieties of rotary motors and linear motors.

The schematic diagram of the linear motor structure is shown in the figure below. The linear motor is to straighten the primary of the traditional cylindrical motor, change the closed magnetic field of the primary to the open magnetic field, and the stator part of the rotary motor becomes the primary of the linear motor, and the rotor part of the rotary motor becomes the secondary of the linear motor.

DC motor structure diagram

If the primary is stationary, the secondary can move in a straight line in the direction of the traveling wave magnetic field. It can realize the feed mode of direct drive of the linear motor of the high-speed machine tool, and the primary and secondary stages of the linear motor are directly installed on the worktable and bed of the high-speed machine tool respectively. Because the transmission chain of this feed transmission method is shortened to 0, it is called the "zero transmission" of the machine tool feed system.

How linear motors work

Generally, the motor rotates when it works. However, vehicles driven by rotating motors (such as electric locomotives and trams in cities) need to move in a straight line, and some parts of the machine driven by rotating motors also need to move in a linear line. This requires adding a set of devices that turn rotational motion into linear motion. Can you directly use a linear motor to drive, so as to save this set? This question was raised decades ago. Now a linear motor has been made, that is, a linear motor.

The principle of a linear motor is not complicated. Imagine that a rotating motion induction motor is split along the radius and flattened, which becomes a linear induction motor. In linear motors, those equivalent to the stator of rotating motors are called primary; It is equivalent to rotating the rotor of the motor, called the secondary, the primary is communicated through the alternating current, and the secondary is under the action of electromagnetic force along the primary to make a linear motion.

At this time, the primary should be made very long, extending to the position that the movement needs to reach, while the secondary stage does not need to be so long, in fact, the linear motor can make both the primary and the secondary very long; It can be both primary fixed and secondary moving, as well as secondary fixed and primary moving.

Linear induction motors evolved from rotary motors. When a three-phase (or multi-phase) winding on the primary side is passed through a symmetrical sinusoidal AC current, an air-gapped magnetic field is generated. When the longitudinal edge effect caused by the breakage of the two ends of the iron core is not taken into account, the distribution of this air-gap magnetic field is similar to that of a rotating motor, and is distributed in a sinusoidal direction in a straight line.

But instead of rotating, it translates along a straight line, known as the traveling-wave magnetic field (as shown in the 6 curve in Figure 1). Obviously, the moving speed of the traveling wave magnetic field is the same as the linear speed of the rotating magnetic field on the inner circular surface of the stator, and the speed at which the traveling wave magnetic field moves is called the synchronous velocity.

(1: Traveling wave magnetic field, 2: Secondary side, 3: Primary side)

Where:

D - the diameter of the circumference of the rotating motor in the fixed hand;

t—polar distance, t=πD/2p;

P - polar logarithm;

f1 – The frequency of the power supply.

The traveling wave magnetic field cuts the secondary side guide bar, which will generate induced electromotive force and current in the guide bar, and the current of the guide bar interacts with the air gap magnetic field to generate tangential electromagnetic force. If the primary side is fixed, the secondary side will move in a straight line along the direction of the traveling wave magnetic field under the action of this electromagnetic force. If the speed of the secondary side movement is expressed by v and the rotation rate is expressed by s, there is

In the electric state, the s is between 0~1.

The secondary side movement speed is:

It can be seen that changing the pole distance or power supply frequency can change the speed of the secondary side movement; Changing the energized phase sequence in the primary winding can change the direction of movement of the secondary side.

Classification and structure of linear motors

There are three main types of linear induction motors: flat, cylindrical and circle type, among which the flat type is the most widely used.

1. Flat type

Flat motors can be seen as direct evolution of ordinary rotary asynchronous motors. The left figure of Figure 1 shows a rotating induction motor, which is envisaged to be split radially, and the stator and rotor circumference are stretched into a straight line, as shown in the right figure of Figure 1, which obtains the simplest flat linear induction motor. In rotating motors, the rotor rotates around the shaft, see the green arrow line; In linear motors, the mover moves in a straight line, see the green arrow line.

Figure 1 - Rotary motor vs. linear motor

There is a three-phase winding embedded on one side corresponding to the stator of the rotating motor, called the primary (stator); The side corresponding to the rotating motor rotor is called the secondary (mover or slider). The movement mode of linear motor can be fixed primary, so that the secondary moves, which is called dynamic secondary; On the contrary, if the secondary is fixed and the primary movement is made, it is called the dynamic primary.

Obviously, the primary and secondary lengths cannot operate normally, and the actual flat linear induction motor primary length and slider length are not equal. The figure below in Figure 2 shows the long primary and short secondary structures.

Figure 2 - Flat linear motor

In order to offset the unilateral magnetic attraction of the stator magnetic field to the mover, the flat plate linear induction motor usually adopts a bilateral structure, that is, the structure of using two stators to clamp the mover in the middle.

Figure 3 - Bilateral flat linear motor

The core of the secondary side of the flat linear induction motor is made of silicon steel sheets, and the side opposite the secondary side is grooved, and the winding is placed in the groove. Winding can be single-phase, two-phase, three-phase, or multi-phase. There are two types of structure on the secondary side: one is the grid structure, with a groove on the iron core and a guide bar placed in the groove; And connect all the guide strips in the groove with end guides; The other is a solid structure, which uses a single uniform metal material, which can be divided into non-magnetic secondary side and steel secondary side. The conductivity of the non-magnetic secondary side is good, generally copper or aluminum.

2. Cylindrical type

The cylindrical linear motor is also called the tubular linear motor, which is rolled into a cylindrical shape by rolling the flat linear motor in the direction perpendicular to the linear motion, forming a cylindrical linear motor, see the figure below.

Cylindrical linear motor

The flat linear induction motor shown in the figure A in the above figure is rolled into a cylindrical shape along the direction perpendicular to the linear motion, and a cylindrical linear induction motor is formed (as shown in Fig. 4, the evolution of the cylindrical linear induction motor). In special occasions, this motor can also be made into a rotating linear motor with both rotary and linear motion. The moving body of a rotating line can be either primary or secondary.

The mover of the linear induction motor is generally a mild steel plate covered with copper plate or inlaid with copper strips, or a metal plate with good conductivity (copper plate or aluminum plate); Cylindrical linear motor movers mostly use thick-walled steel pipes, and cover the outer wall of the pipe with copper pipes or aluminum tubes 1 to mm thick.

If the mover is made of permanent magnet material, it forms a linear synchronous motor.

3. Disc type

The secondary (rotor) of the disc linear motor is made into a flat disc shape, which can rotate freely around the axis through the center of the circle: two primaries are placed on the plane of the outer edge of the disc, so that the disc is subjected to tangential force for rotational movement. Because its operating principle and design method are the same as those of flat plate linear induction motors, they are still linear motors.

Disc type linear motor

As shown in the figure above, the secondary side of the disc is made into a flat disc shape, which can rotate freely around the axis through the center of the circle: the primary side is placed on the plane of the outer edge of the secondary side of the disc, so that the disc is rotated by tangential force. However, its operating principle and design method are the same as those of flat linear induction motors, so they still belong to the category of linear motors. Compared with ordinary rotary motors, it has the following advantages:

a) Torque and rotation speed can be adjusted by combining multiple primary sides or by the radial position of the primary side on the disc.

b) Lower speed can be obtained without going through the gear reduction box, so the vibration and noise of the motor are minimal.

The development of linear motors

In 1840

Wheatsone began to propose and build a slightly prototype linear motor.

In 1905

Two people have suggested using a linear motor as the propulsion mechanism of the train, one suggesting that the primary be placed on the track and the other that the primary should be placed at the bottom of the vehicle. These suggestions were undoubtedly a stimulant for researchers in the field of linear motor research at that time, so that researchers in many countries devoted themselves to these research work. In 1917, the first cylindrical linear motor appeared, in fact it was a DC reluctance motor with a switched primary coil, and people tried to use it as a missile launcher, but its development did not go beyond the model stage.

1940-1955

Researchers in some developed countries in the world have carried out some experimental application work on the basis of experiments. In 1945, Westinghouse Electric Company of the United States first successfully developed an electric traction aircraft catapult, which was powered by a 7400kW linear motor, and successfully used 4.1s to accelerate a jet aircraft weighing 4535kg from a standstill in a stroke of 165m at a speed of 188km/h. For the needs of nuclear power. In 1954, the Royal Aircraft Manufacturing Company made a device for launching missiles using bilateral flat DC linear motors, which could reach speeds of up to 1600km/h. At this stage, it is particularly worth mentioning that linear motors as the driving device of high-speed trains have been highly valued and planned to be implemented by various countries.

In 1965

With the significant improvement of control technology and material properties, practical equipment using linear motors has been gradually developed, such as MHD pumps using linear motors, automatic plotters, head positioning drives, record players, sewing machines, air compressors, conveying devices, etc.

1971 to present

From the beginning of 1971 to the current stage, linear motors have finally entered the independent application era, in this era, the application of various linear motors has been rapidly promoted, and many devices and products with practical value have been made, such as linear motor-driven steel pipe conveyors, coal transporters, cranes, air compressors, stamping machines, stretching machines, various electric doors, electric windows, electric textile machines and so on. What is particularly gratifying is that the maglev train driven by linear motors has a speed of more than 500km/h, which is close to the flight speed of aviation, and the cumulative test travel has reached hundreds of thousands of kilometers.

The main parameters and characteristics of linear motors

Linear motor parameters:

1. Max. voltage ——— the maximum power supply voltage or continuous power supply peak voltage, which is mainly related to the selection and process of motor enameled wire and motor insulation materials.

2. Peak Force ——— the maximum thrust of the motor, in a short period of time (a few seconds), depending on the safety limit capability of the motor's electromagnetic structure (closely related to the enameled wire material of the motor); Unit: N

3. Peak current ——— the maximum working current, corresponding to the maximum thrust, which is lower than the demagnetization current of the motor (working under the peak theoretical current of the motor for a long time will cause the motor to heat up, which will cause great damage to the life of the motor, and more seriously will lead to the demagnetization of the magnet inside the motor. );

4. Continuous power (Peak power) - Under the condition of continuous temperature rise and heat dissipation, the heat loss of the continuous operation of the motor reflects the thermal design level of the motor;

5. Max. Continuous Power Loss ——— determine the upper limit of heat loss that the motor can operate continuously under temperature rise and heat dissipation conditions, reflecting the thermal design level of the motor.

7. Maximum speed ——— the maximum operating speed under the determination of the supply voltage, which depends on the number of inverse potential lines of the motor and reflects the result of the electromagnetic design of the motor.

6. Motor Force Constant ——— the thrust-to-current ratio of the motor, in units N/A or KN/A, reflecting the result of the electromagnetic design of the motor, and in a sense, it can also reflect the level of electromagnetic design;

7. Back EMF ——— the back potential (coefficient) of the motor, in units Vs/m, which reflects the result of the electromagnetic design of the motor and affects the maximum running speed of the motor under the determined power supply voltage. (Reflecting the design parameters of the motor)

8. Motor Constant ——— the ratio of the square root of motor thrust to power consumption, unit N/√W, which is a comprehensive embodiment of the electromagnetic design and thermal design level of the motor.

9. Magnetic pole Pitch NN (Magnet Pitch) ———— the magnetic pole spacing distance of the secondary permanent magnet of the motor, which basically does not reflect the design level of the motor, and the driver needs to calculate the motor electrical angle required for vector control by the feedback system resolution accordingly.

10. Winding resistance/Resistance per phase ——— phase resistance of the motor, which is often given as a line resistance, that is, Ph-Ph, which has a great relationship with the heat generation of the motor, and can reflect the electromagnetic design level in the sense;

11. Induction per phase ——— the phase inductance of the motor, and the line inductance, that is, Ph-Ph, is related to the back potential of the motor, which can reflect the electromagnetic design level in the sense;

12. Electrical time constant ——— the ratio of motor inductance to resistance, L/R;

13. Thermal resistance ——— related to the heat dissipation capacity of the motor, reflecting the heat dissipation design level of the motor;

14. Motor attraction force ——— flat plate type linear motor with iron core structure, especially permanent magnet motor, the normal attraction of the secondary permanent magnet to the primary iron core is an order of magnitude higher than the rated thrust of the motor, which directly determines the bearing capacity and selection of the support guide rail of the linear motion axis of the linear motor.

Features of linear motors:

Before the advent of practical and affordable linear motors, all linear motion had to be converted from rotating machinery by using ball or roller screws or belts or pulleys. For many applications, such as when encountering large loads and the drive shaft is vertical. These methods are still the best. However, linear motors have many unique advantages over mechanical systems, such as very high and very low speeds, high acceleration, almost zero maintenance (no contact parts), high precision, and no empty back. Linear motion is achieved by simply removing the motor without the need for gears, couplings or pulleys, which makes sense for many applications by removing unnecessary parts that reduce performance and reduce mechanical life.

1) Simple structure. The tubular linear motor does not need to go through the intermediate conversion mechanism to directly produce linear motion, which greatly simplifies the structure, reduces the moment of inertia, and greatly improves the dynamic response performance and positioning accuracy. It also improves reliability, saves costs, and makes manufacturing and maintenance easier. Its primary level can be directly part of the institution, and this unique combination makes this advantage even more evident.

2) Suitable for high-speed linear motion. Because there is no centrifugal force constraint, ordinary materials can also reach high speeds. Moreover, if the gap is preserved by air cushions or magnetic pads between the primary and secondary stages, there is no mechanical contact during movement, so there is no friction and noise in the moving part. In this way, the transmission parts are not worn, which can greatly reduce mechanical losses and avoid noise caused by tow cables, steel cables, gears and pulleys, etc., thereby improving the overall efficiency.

3) High utilization rate of primary winding. In tubular linear induction motors, the primary winding is cake-type and has no end winding, so the winding utilization rate is high.

4) No lateral edge effect. The lateral effect refers to the weakening of the magnetic field at the boundary caused by the lateral break, while the cylindrical linear motor has no lateral breaking, so the magnetic field is evenly distributed along the circumference.

5) It is easy to overcome the problem of unilateral magnetic tension. The radial tensile force cancels each other out, and there is basically no problem of unilateral magnetic tension.

6) Easy to adjust and control. By adjusting the voltage or frequency, or replacing the secondary material, different speeds and electromagnetic thrusts can be obtained, which is suitable for low-speed reciprocating operation.

High-speed machining centers replace high-speed milling machine machining

Cincinnati Milacron of the United States has produced a HyperMach large-scale high-speed machining center for the aviation industry, with a spindle speed of 60,000r/min and a main motor power of 80kW. The linear feed adopts a linear motor, its shaft travel is up to 46m, the table has a fast stroke of 100m/min, and the acceleration is up to 2g.

It only takes 30 minutes to machine a large thin-walled aircraft part on such a machine; The same parts are processed on a general high-speed milling machine, which takes 3 hours; It takes 8 hours to process on an ordinary CNC milling machine, which is quite obvious.

7) Adaptable. The primary iron core of the linear motor can be sealed with epoxy resin, which has good anti-corrosion and moisture-proof performance, and is easy to use in the environment of moisture, dust and harmful gases. And it can be designed into a variety of structural forms to meet the needs of different situations.

8) High acceleration. This is a linear motor drive and is a significant advantage over other lead screws, timing belts and rack and pinion drives.

9) Accuracy: linear motors are higher than those of "rotary servo motor ball screw" due to the simple transmission mechanism, positioning accuracy and repeatability accuracy, and are easy to achieve through position detection and feedback control. The positioning accuracy of linear motors can reach ±2μm or even higher. The "rotary servo motor ball screw" can only reach a maximum of 10μm.

10) Speed: linear motor has considerable advantages, when the speed of linear motor reaches 5m/s, the acceleration reaches 10g; When the ball screw speed is 2m/s, the acceleration is only 1.5g. In terms of speed and acceleration, linear motors have considerable advantages, and the speed of linear motors will be further increased after successfully solving the heating problem, while the "rotary servo motor ball screw" is limited in speed and difficult to improve more.

11) Lifespan: linear motor due to the installation gap between moving parts and fixed parts, no contact, will not wear due to the high-speed reciprocating movement of the mover, long-term use has no change in motion positioning accuracy, suitable for high-precision occasions. Ball screws cannot ensure accuracy in high-speed reciprocating motion, due to high-speed friction, it will cause wear of the lead screw nut, affecting the accuracy requirements of the movement. The demand for high precision cannot be met on occasions.

Conclusion

Linear motor is a new type of motor that has become increasingly widely used in recent years. It is mainly used in three aspects: first, it is applied to automatic control systems, which have many applications; secondly, it is a drive motor for long-term continuous operation; and third, it is used in devices that need to provide huge linear motion energy in a short time and short distance.

High-speed maglev train Maglev train is the most typical example of the practical application of linear motors, at present, the United States, Britain, Japan, France, Germany, Canada and other countries are developing linear levitation trains, of which Japan is making the fastest progress.

Elevator driven by linear motor The world's first elevator driven by linear motor was installed in April 1990 in the Mansei Building in Guam, Tokyo, Japan, with a load of 600kg, a speed of 105m/min, and a lifting height of 22.9m. Since the linear motor-driven elevator does not have a traction unit, the machine room on the roof of the building can be omitted.

If the height of the building increases to about 1,000 meters, it is necessary to use a wireless elevator, which is driven by a linear motor with high-temperature superconducting technology, the coil is installed in the hoistway, and the car is equipped with high-performance permanent magnet material, just like a maglev train, controlled by radio waves or light control technology.

When the ultra-high-speed motor rotates beyond a certain limit, the motor with rolling bearings will be sintered and damaged. To this end, in recent years, a linear suspension motor (electromagnetic bearing) has been developed abroad, which uses suspension technology to make the mover of the motor suspended in the air, eliminating the mechanical contact and frictional resistance between the mover and the stator, and its rotation speed can reach more than 25000~100000r/min, so it has been widely used in high-speed motors and high-speed spindle components.

For example, the newly developed 5-axis controllable electromagnetic high-speed spindle for multi-process automatic CNC lathes by Yaskawa Company in Japan uses two warppet-oriented electromagnetic bearings and one axial thrust electromagnetic bearing, which can bear the load of the machine tool in any direction. In the middle of the shaft, in addition to the high-speed motor, it is also equipped with an automatic tool exchange mechanism suitable for multi-process automatic CNC lathes.

In China, linear motors have also been gradually promoted and applied. Although the principle of linear motor is not complicated, it has its own characteristics in design and manufacturing, and the product is not as mature as the rotary motor, which needs to be further studied and improved.