General motors are rotated when working. But traffic driven by a rotating motor Tools (such as electric cars and trams in the city) need to be linear, and some parts of the machine driven by the rotating motor also need to move in a straight line. This requires a device that increases the rotational motion into a linear motion. It is not possible to directly use a linear motion motor to drive, so that this set is eliminated, and the problem has been raised. Now, a linear motion motor, that is, a linear motor, has been fabricated.
What is a linear motor
straight line The motor is also called a linear motor, a linear motor, a linear motor, and a push rod motor. The most common types of linear motors are flat and U-groove, and tubular. The typical composition of the coil is three-phase.There are Hall elements for brushless commutation.
Modern advanced drive technology is divided into two main categories: one is electromagnetic and the other is non-electromagnetic.
Modern advanced drive technology of electromagnetic type is mainly composed of modern electromagnetic drive and modern control system. Its drive includes traditional improved electromagnetic drive and new development. Type electromagnetic drive. Among them are rotating, linear, maglev, electromagnetic emission, and the like.
Linear motor is a transmission that converts electrical energy directly into linear motion without the need for any intermediate conversion mechanism. It can be seen as a rotating motor that is cut radially and flattened.
Linear motor is a novel motor that converts electrical energy directly into linear motion mechanical energy without having to pass any switching device.The utility model has the advantages of simple system structure, less wear, low noise, strong combination and convenient maintenance. Rotating motors have almost the same variety of linear motors.
The schematic diagram of the linear motor is shown below. The linear motor straightens the primary development of the conventional cylindrical motor, and the closed magnetic field that becomes the primary is an open magnetic field, and the stator portion of the rotating electrical machine becomes the primary of the linear motor, and the rotor portion of the rotating electrical machine becomes the secondary of the linear motor.
DC motor structure diagram
If the primary is stationary,The secondary can move in a straight line along the direction of the traveling wave magnetic field. The feed mode of the direct drive of the linear motor of the high-speed machine tool can be realized, and the primary and secondary of the linear motor are directly mounted on the work table and the bed of the high-speed machine tool respectively. Since the transmission chain of this feed transmission mode is shortened to zero, it is called the "zero transmission" of the machine tool feed system.
How linear motors work
General motor operation It is all turning. However, vehicles driven by rotating electric motors (such as electric motors and electric trains in the city) need to be linearly moved, and some parts of the machine driven by the rotating electric motor also have to move linearly. This requires a device that increases the rotational motion into a linear motion. Can you directly use a linear motion motor to drive this, thus eliminating the need? This problem was raised decades ago. A linear motion motor, a linear motor, has now been produced.
The principle of linear motor is not complicated. It is envisaged that a rotating motion induction motor is cut along the radius and flattened, which becomes a linear induction motor. In a linear motor, it is equivalent to a rotating motor. The stator is called the primary; the equivalent of the rotor of the rotating electrical machine is called the secondary, the primary is connected to the alternating current, and the secondary is linearly moved along the primary under the action of the electromagnetic force.
At this time, the primary is going to be long and extends to the position that the movement needs to reach, while the secondary does not need to be so long. In fact, the linear motor can either make the primary very long or The secondary is very long; it can be primary fixed, secondary moving, secondary fixed, primary moving.
Linear induction motor is evolved by rotating electric motor When the three-phase (or multi-phase) winding of the primary side is supplied with a symmetrical sinusoidal alternating current, an air gap magnetic field is generated. When the longitudinal direction of the core is broken, the longitudinal direction is not considered. At the edge effect, the distribution of this air gap magnetic field is similar to that of a rotating motor. Distribution sinusoidally linear direction.
But it is not a rotation but a translation along a straight line, called a traveling wave magnetic field (as shown by the curve in Figure 6). It is obvious that the moving speed of the traveling wave magnetic field is the same as the linear velocity of the rotating magnetic field on the inner circular surface of the stator, and the moving speed of the traveling wave magnetic field is called the synchronous speed.
(1: traveling wave magnetic field, 2: secondary side, 3: primary side)
In the formula:
D——the diameter of the inner circumference of the rotating motor;
t——pole distance, t=πD/2p;
f1 - the frequency of the power supply.
The traveling wave magnetic field cuts the secondary side bar, which will generate induced electromotive force and current in the bar. The current of the bar interacts with the air gap magnetic field to generate tangential electromagnetic force. If the primary side is not fixed, the secondary side is under the action of this electromagnetic force.Linear motion is performed along the moving direction of the traveling wave magnetic field. If the speed of the secondary side movement is represented by v and the slip rate is represented by s, then there are
When the motor is in the state, s is Between 0~1.
Second side movement speed is:
Visible, changing the pole pitch or power frequency can change the speed of the secondary side movement; changing the power phase sequence in the primary winding can change two The direction of the secondary movement.
Classification and structure of linear motors
Linear induction motors are mainly available in three types: flat, cylindrical and ring type, of which flat type is the most widely used.
1. Flat type
The flat motor can be seen as a direct evolution from the ordinary rotary asynchronous motor. The left figure of Figure 1 shows a rotating induction motor, which is supposed to be It is cut along the radial direction, and the circumference of the stator and the rotor are straight, as shown in the right figure of Figure 1, which gives the simplest flat-type linear induction motor. In the rotating machine, the rotor rotates around the axis.See the green arrow line; in a linear motor, the mover moves in a straight line, see the green arrow line.
Fig. 1 - Rotating motor and linear motor
The three-phase winding is embedded in one side of the stator of the rotating motor, called the primary (stator); the side corresponding to the rotor of the rotating motor is called the secondary (mover or slider). The linear motor can be moved in a fixed manner, so that the secondary motion is called a dynamic secondary; on the contrary, the primary can be fixed and the primary motion can be called a primary motion.
Obviously the primary and secondary lengths are not working properly. The actual length of the flat linear induction motor is not equal to the length of the slider, as shown in Figure 2. The figure is a short primary long secondary structure; the lower diagram of Figure 2 is a long primary short secondary structure.
Fig. 2—flat linear motor
In order to offset the single-sided 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 in which the two stators sandwich the mover in the middle.
Figure 3 - Bilateral flat linear motor
The flat-type linear induction motor—the secondary side core is laminated with silicon steel sheets, and the side opposite to the secondary side is grooved, and the winding is placed in the groove. The windings can be single phase, two phase, three phase or multiple phases. There are two types of structure on the secondary side: one is a grid structure, the iron core is grooved, the guide bar is placed in the groove; and the end bar is used to connect the guide bars in all the grooves; the other is a solid structure, which is a monolithic structure. Uniform metal material,Can be divided into non-magnetic secondary side and steel secondary side. The electrical properties of the non-magnetic secondary side are good, generally copper or aluminum.
2. Cylindrical type
Cylindrical linear motor is also called tubular straight line In the motor, the flat linear motor is rolled into a cylindrical shape in a direction perpendicular to the linear motion, and a cylindrical linear motor is formed, as shown in the following figure.
Cylinder linear motor
The flat line induction shown in a above The motor is rolled into a cylindrical shape in a direction perpendicular to the linear motion to form a cylindrical linear induction motor (the evolution of the cylindrical linear induction motor shown in Fig. 4). In special occasions, the motor can also be used. A rotary linear motor having both a rotary motion and a linear motion is produced. The moving body of the rotary straight line may be the primary side or the secondary side.
Line The moving motor of the induction motor is usually a low carbon steel plate or a copper strip.It is also possible to use a metal plate (copper plate or aluminum plate) with good electrical conductivity; a cylindrical linear motor mover is often made of a thick-walled steel pipe, and a copper pipe or an aluminum pipe of 1 to mm thickness is covered on the outer wall of the pipe.
If the mover is made of permanent magnet material, it constitutes a linear synchronous motor.
3. Disc type
Second (rotor) of disc type linear motor Made into a flat disc shape, it can rotate freely around the axis of the center: the two primarys are placed on the plane of the outer edge of the disc, so that the disc is rotated by the tangential force. Since its operating principle and design method are the same as those of the flat type linear induction motor, it is still a linear motor.
Disc linear motor
The disc-shaped linear induction motor is as shown in the figure above, and its secondary side is flat. The shape of the disc is free to rotate about the axis passing through the center of the circle: the primary side is placed on the plane of the outer edge of the secondary side disc,The disc is subjected to a rotational motion by a tangential force. However, its operating principle and design method are the same as flat linear induction motors, so it is still in the category of linear motors. Compared with ordinary rotating electric machines, it has the following advantages:
a) Torque and rotational speed can be combined through multiple primary sides or through the primary side in the disc Adjust the radial position above.
b) A lower speed can be achieved without going through the gear reducer, so the motor's vibration and noise are small.
Development of Linear Motors
1840, Wheatsone began to propose and make a slightly linear motor.
1905,Two people have suggested using a linear motor as the propulsion mechanism for the train. One suggestion is to place the primary on the track. Another suggestion is to place the primary at the bottom of the vehicle. . These suggestions were undoubtedly a stimulant for the researchers in the field of linear motor research at that time, so that researchers in many countries have invested in these research work. The first cylindrical linear motor appeared in 1917. In fact, it was a DC reluctance motor with a switching primary coil. It was tried as a missile launcher, but its development did not go beyond the model stage.
1940-1955, National developed country researchers, experimental On the basis of this, some experimental applications have been carried out. In 1945, Westinghouse Electric Company first developed a successful electric traction aircraft catapult. It was powered by a linear motor of 7400 kW, and successfully used a 4,450 kg weight for a period of 4.1s.The jet was accelerated by a static speed of 188km/h during the 165m stroke. Its success in testing and the reliability of the linear motor have received due attention. Subsequently, the United States made use of linear motors. It is used as an electromagnetic pump for pumping liquid metals such as potassium and sodium for nuclear power. In 1954, the Royal Aircraft Manufacturing Company used a bilateral flat-type DC linear motor to make a missile-launching device with a speed of 1600 km/h. In this stage, it is worth mentioning that linear motors, as the driving devices for high-speed trains, have received great attention from all countries and are planned to be implemented.
1965, With the significant improvement in control technology and material performance, apply straight lines Practical equipment for motors has been gradually developed, such as MHD pumps with linear motors, automatic plotters, head positioning drives, record players, sewing machines, air compressors, conveyors, etc.
From 1971 to the present, From 1971 to the current stage, linear motors Finally entered the era of independent application. In this era, the application of various types of linear motors has been rapidly promoted, and many practical devices and products have been made, such as linear motor driven steel pipe conveyors, coal conveyors, cranes. , air compressors, punching machines, stretching machines, various electric doors, power windows, electric textile machines, etc. Particularly gratifying is the maglev train driven by linear motors, which has a speed of over 500km/h, which is close to the flight speed of aviation, and the test journey has reached hundreds of thousands of kilometers.
Main parameters and characteristics of linear motors
Linear motor parameters:
1. Maximum voltage (max. voltage) ———Maximum supply voltage or continuous supply peak voltage, mainly related to motor enameled wire, motor insulation material selection and process Related;
2. Peak Force—The maximum thrust of the motor, in a short time (a few seconds), depending on the electromagnetic structure of the motor Safety limit capability (related to the motor's enameled wire material); unit: N
3. Peak current - maximum operating current, and maximum thrust I want to correspond, lower than the demagnetization current of the motor (long-term operation at the peak theoretical current of the motor will cause the motor to heat up, which will cause great damage to the life of the motor, and more serious will cause the magnetic steel inside the motor to demagnetize.);
4.Peak power — The heat loss of the continuous operation of the motor under continuous temperature rise conditions and heat dissipation conditions, reflecting the thermal design level of the motor;
5. Maximum continuous power consumption (Max. Continuous Power Loss) — Determine the upper limit heating loss of the motor that can be continuously operated under temperature rise conditions and heat dissipation conditions, reflecting the thermal design level of the motor;
7. Maximum speed (-maximum speed) - the maximum operating speed under the determination of the supply voltage, depending on the motor's back EMF line, reflecting the motor electromagnetic The result of the design;
6. Motor Force Constant - the motor's thrust current ratio, in units of N/A or KN/A, reflected The result of the electromagnetic design of the motor can also reflect the level of electromagnetic design in a certain sense;
7. Back EMF (- Back EMF) --- Motor back EMF (coefficient), unit Vs / m, reflecting the results of the electromagnetic design of the motor, affecting the motor Determine the maximum operating speed at the supply voltage; (reflects the design parameters of the motor)
8. Motor Constant - Motor Thrust and Power Consumption The square root ratio, in N/√W, is a comprehensive representation of the electromagnetic design and thermal design level of the motor;
9. Magnetic pole pitch NN (Magnet PItch) ————The magnetic pole separation distance of the secondary permanent magnet of the motor does not reflect the motor design level. The driver needs to calculate the motor electric angle required by the vector control according to the resolution of the feedback system;
10.Winding resistance / each phase (Resistance per phase) - the phase resistance of the motor, the line resistance is given below, that is, Ph-Ph, and the relationship with the motor heat is large, in the sense that Reflects the level of electromagnetic design;
11.Inductance/Induction per phase ———The phase inductance of the motor, under The line inductance is often given, that is, Ph-Ph, which is related to the motor back EMF, which can reflect the electromagnetic design level in the sense;
12. Electrical time Constant (Electrical time constant) ———The ratio of motor inductance to resistance, L/R;
13. Thermal impedance(Thermal Resistance) --- related to the heat dissipation capability of the motor, reflecting the heat dissipation design level of the motor;
14. Motor Attraction Force —--plate type linear motor with iron core structure, especially permanent magnet motor, secondary pole permanent magnet to primary The normal attraction of the core is one order of magnitude higher than the rated thrust of the motor, which directly determines the bearing capacity and selection of the support rail of the linear motion shaft of the linear motor.
Characteristics of linear motors:
in practical and Before the advent of 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 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 speed and very low speed, high acceleration, almost zero maintenance (no contact parts), high precision, no air return.Complete linear motion requires only the motor without gears, couplings or pulleys, which makes sense for many applications, removing those parts that are unnecessary, reduce performance and shorten mechanical life.
1) Simple structure. The tubular linear motor does not need to go through the intermediate conversion mechanism to directly generate linear motion, which greatly simplifies the structure, reduces the motion inertia, greatly improves the dynamic response performance and positioning accuracy, and also improves the reliability, saves the cost, and makes the manufacturing and maintenance more Simple. Its primary and secondary can be directly part of the organization, and this unique combination makes this advantage even more apparent.
2) Suitable for high-speed linear motion. Because there is no constraint on centrifugal force, ordinary materials can also achieve higher speeds. Moreover, if the air gap or the magnetic pad is used to store the gap between the primary and secondary, there is no mechanical contact during the movement, and thus the moving part has no friction and noise. In this way, the transmission components are not worn, which can greatly reduce the mechanical loss and avoid the noise caused by the streamers, cables, gears and pulleys, thereby improving the overall efficiency.
3) High primary winding utilization. In a tubular linear induction motor, the primary winding is pie-shaped and has no end windings, so the winding utilization is high.
4) No lateral edge effects. The lateral effect refers to the weakening of the magnetic field at the boundary due to the lateral breaking, and the cylindrical linear motor has no lateral break, so the magnetic field is evenly distributed along the circumference.
5) It is easy to overcome the problem of single-sided magnetic pull. The radial pulling forces cancel each other out, and there is basically no problem of single-sided magnetic pulling force.
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 applications.
High-speed machining center replaces high-speed milling machine processing
CincinnatiMI lacron produces a HyperMach large-scale high-speed machining center for the aviation industry The spindle speed is 60000r/min and the main motor power is 80kW. The linear feed adopts linear motor with a shaft stroke of 46m, the table has a fast stroke of 100m/min and an acceleration of 2g.
It takes only 30 minutes to machine a large thin-walled aircraft part on this machine; the same part is processed on a general high-speed milling machine, which takes 3 hours; on a normal CNC milling machine, it takes 8 hours. The advantage is quite obvious.
7) The adaptability is strong. The primary core of the linear motor can be sealed with a epoxy resin Good anti-corrosion and moisture-proof properties, easy to use in humid, dust and harmful gas environment; and can be designed into various structural forms.Meet the needs of different situations.
8) High acceleration. This is a linear motor drive that offers a significant advantage over other lead screws, timing belts and rack and pinion drives.
9) Accuracy: Linear motor is simpler than the transmission mechanism, positioning accuracy, repeatability, and position detection feedback control will be more "rotating servo motor ball screw" High and easy to implement. Linear motor positioning accuracy can reach ± 2μm or even higher. The "rotary servo motor ball screw" can only reach up to 10μm.
10) Speed: Linear motors have considerable advantages. When the linear motor speed reaches 5m/s, the acceleration reaches 10g; and the ball screw speed For 2m/s, the acceleration is only 1.5G. From the comparison of speed and acceleration, the linear motor has considerable advantages, and the speed of the linear motor will be solved after successfully solving the heating problem. Further improvement, while the "rotary servo motor ball screw" is limited in speed, it is difficult to increase more.
11) Life expectancy:The linear motor has a mounting gap between the moving part and the fixed part, has no contact, and is not worn by the high-speed reciprocating motion of the mover. The long-term use has no change in the positioning accuracy of the motion, and is suitable for high-precision occasions. Ball screw can not guarantee the accuracy in high-speed reciprocating motion, because of high-speed friction, it will cause the wear of the screw nut, affecting the accuracy requirements of the motion. The need for high precision is not met.
Linear motor is a new type of motor, which has been widely used in recent years. It is mainly used in three aspects:First, it is applied to automatic control systems, and there are many such applications; secondly, it is used as a long-term continuous running motor; thirdly, it is used in a device that needs a large linear motion energy in a short time and a short distance.
High-speed maglev train maglev train is the most typical example of practical application of linear motor. At present, the United States, Britain, Japan, France, Germany, Canada and other countries are developing Linear suspension trains, with Japan making the fastest progress.
Linear motor-driven elevator The world's first elevator driven by linear motor was installed in Wanshi Building, Guam, Tokyo, Japan, in April 1990. The load is 600kg, the speed is 105m/min, and the lifting height is 22.9m. Since the linear motor-driven elevator does not have a traction unit, the machine room at the top of the building can be omitted.
If the height of the building is increased to about 1000 meters, a wire ropeless elevator must be used. This type of elevator 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 the maglev train, it is controlled by radio waves or light control technology.
When a super high-speed motor rotates beyond a certain limit, the motor with the rolling bearing will be sintered and damaged. In recent years, a linear suspension motor (electromagnetic bearing) has been developed abroad. The suspension technology is used to suspend the motor's mover in the air, eliminating the mechanical contact and frictional resistance between the mover and the stator. The rotational speed can reach 25000. ～100000r/min or more, it is widely used in high-speed motors and high-speed spindle components.
The 5-axis controllable electromagnetic high-speed spindle for multi-process automatic CNC lathes newly developed by Yaskawa Corporation of Japan uses two meridional electromagnetic bearings and one axial thrust The electromagnetic bearing can withstand the load of the machine in any direction. In the middle of the shaft, in addition to the high-speed motor, it is equipped with a tool automatic switch system that is compatible with the multi-process automatic CNC lathe.
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 manufacture. The product is not as mature as rotary motor, and needs further research and improvement.