A linear induction motor for improving drive performance over a wide speed range

Abstract

The invention relates to a linear induction motor wherein drive performance and power density are increased over a wide speed range, providing torque production at significant values when high speeds are reached.

Description

[0001] A LINEAR INDUCTION MOTOR FOR IMPROVING DRIVE PERFORMANCE OVER A WIDE SPEED RANGE Technical Field

[0002] The invention relates to a linear induction motor wherein drive performance and power density are increased over a wide speed range, providing torque production at significant values when high speeds are reached.

[0003] Prior Art

[0004] Speed control of induction motors can be achieved by various methods applied from the stator side or the rotor side.

[0005] Methods applied from the stator side are as follows:

[0006] Voltage control method: In this method, the supply voltage is changed using an autotransformer. In practice, the rated voltage cannot be exceeded since it increases insulation stress and may cause insulation failure.

[0007] Frequency control method: The magnetic flux density of the stator core is inversely proportional to the applied frequency. To reduce core losses and to ensure the motor performs better, the maximum magnetic flux density must be kept constant. Therefore, it is necessary to change the voltage and the frequency to keep the maximum magnetic flux density constant. However, this method cannot be applied for frequencies above the rated frequency because the voltage also needs to be increased and this is not possible due to insulation constraints. This method requires variable voltage and variable frequency converters, which makes the method costly. However, this method offers a wide speed control range without affecting the efficiency of the motor.

[0008] Pole changing speed control: It is a method applied in the speed control of induction motors and is applicable only to squirrel cage induction motors. The number of poles in the rotor of a slip ring induction motor is fixed, however, a cage rotor canbe designed suitable for any number of poles. The poles of the induction motor can be changed in two ways:

[0009] 1) Multiple winding sets (Dahlander): In this method, multiple stator winding sets designed for different pole sets are used. During operation, any one can be connected according to the speed requirement of the user, the other sets remain open. The speed decreases as the number of poles increases. This method can only make step-by-step speed change and is costly since it contains more than one stator winding.

[0010] 2) Consequent pole changing method: In this method, another set of poles can be obtained by reversing the coils. This method can provide only two different speed sets.

[0011] Stator resistance control method: In this method, three resistors must be connected to each phase of the stator winding to achieve the desired speed. Although there is some power loss due to rheostats, this method is generally preferred for short-term uses in small power machines. This method is more advantageous in starting rather than speed control.

[0012] Methods applied from the rotor side are as follows:

[0013] Rotor resistance control method: This induction motor speed control method is possible only for slip ring induction motors because electrical access cannot be provided to the rotor of a squirrel cage induction motor. This method is realized in a manner where external resistance is connected to the rotor of the motor via slip rings and brushes, and this situation causes a decrease in torque.

[0014] There are also significant disadvantages:

[0015] • It is not possible to operate at a higher speed than normal.

[0016] • Large speed changes require large resistance values; however, adding such large values will cause significant copper loss and a decrease in circuit efficiency.

[0017] • More losses occur due to the presence of resistance.Squirrel cage induction motor cannot be operated with this method.

[0018] Slip power recovery method: It is a method used for the purpose of obtaining steady state torque at a slip frequency where an external voltage is injected into the rotor via slip rings and brushes. This method can be realized in two ways: In the first method, the rotor voltage is increased, which leads to an increase in speed for a specific load. When the rotor voltage increases, torque increases; consequently, rotor speed increases and slip decreases. In the second method, the rotor voltage is decreased, then torque decreases, speed increases and slip decreases.

[0019] Cascade connection (or) tandem connection: In this method, two induction motors are needed; the first one essentially must be a slip ring induction motor, the other can be any type of induction motor. Both machines are mechanically connected to each other; the first motor feeds the second motor via slip rings. In this system, the slip rings of the primary motor are used to feed and control the secondary motor. The speed of the first motor is controlled by affecting the speed of the second motor. This method is generally used in applications where a wide speed range is needed. Because it provides a wider speed control since two different types of motors are used.

[0020] High-speed linear induction motors are preferred in applications of high-speed linear drive systems due to providing a contactless thrust, having high starting torque, and being easy to produce. In terms of durability, the fact that its rotor is in a solid structure enables a robust structure. However, it does not possess good electromagnetic properties compared to other rotor structures. Eddy currents occur on its surface due to the rotor being solid. Due to these eddy currents, the motor has a high slip value and a low power factor value. The effects of these disadvantages increase further at high speeds. For this reason, the mechanical power transfer of the linear induction motor decreases significantly at high speeds. In order to correct this situation, the motor needs to have high power density at high speeds as well.

[0021] In the patent document numbered GB1285557A located in the prior art, a linear motor speed control system is mentioned. When the system is considered in general terms, it is seen that the model is a feedback speed control system for a vehiclemoving with a linear motor. Speed control is performed by remaining within the electrical and mechanical characteristic limits of the linear motor. It is evident that the main target of the control system is to keep the speed of the vehicle in the desired value range. In other words, the designed control system does not have any effect on the limits of the torque value that the linear motor will produce depending on the frequency while at the levels of rated current and rated voltage.

[0022] In the patent document numbered CN113420521 A located in the prior art, a realtime simulation modeling method for the segmented power supply switching process of the three-phase linear induction motor is mentioned.

[0023] In the patent document numbered CN105591520A located in the prior art, a linear motor and an integrated control chip are mentioned.

[0024] When the studies existing in the prior art are examined, a need has been arisen for the development of a linear induction motor wherein drive performance and power density are increased, providing torque production at significant values when high speeds are reached.

[0025] Objectives of the Invention

[0026] The object of this invention is to develop a linear induction motor wherein drive performance and power density are increased, providing torque production at significant values when high speeds are reached.

[0027] Another object of this invention is to develop a linear induction motor that will provide a solution to the problem of the linear motor not being developed at a sufficient level for a full-scale hyperloop project.

[0028] Another object of this invention is to develop a linear induction motor capable of operating over a very wide speed range.Another object of this invention is to develop a linear induction motor wherein the rated drive torque can be maintained together with the rated current value at frequencies higher than the rated voltage and rated frequency.

[0029] Detailed Description of the Invention

[0030] The linear induction motor developed to achieve the object of this invention is shown in the attached figures.

[0031] These figures;

[0032] Figure la: A schematic view of the primary of a known concentric wound linear induction motor.

[0033] Figure lb: An exploded schematic view of the primary of a known concentric wound linear induction motor.

[0034] Figure 2a: A schematic view of the primary of the linear induction motor subject of the invention.

[0035] Figure 2b: An exploded schematic view of the primary of the linear induction motor subject of the invention.

[0036] Figure 3: A schematic view of the switching mechanism located in the linear induction motor subject of the invention.

[0037] Figure 4: A view of the single phase equivalent circuit of the linear induction motor.

[0038] Figure 5: A graph of the force variation depending on frequency.

[0039] The parts located in the figures are individually numbered and the equivalents of these numbers are given below.

[0040] 1. Main winding

[0041] 2. Additional winding

[0042] 3. Linear motor core

[0043] 4. Motor driver

[0044] 5. Switching mechani sm

[0045] 6. Additional modulea; Output terminal connecting the winding to star connection

[0046] b; Output terminal connecting the winding in series to the next winding

[0047] X, Y and Z; additional winding (2) output terminals

[0048] U, V and W; last additional winding (2) input terminals

[0049] A linear induction motor wherein drive performance and power density are increased, providing torque production at significant values when high speeds are reached, comprising;

[0050] • motor driver (4) located within the linear induction motor and enabling the operation of the motor,

[0051] • main winding (1) connected to the motor driver (4) and positioned on the linear motor core (3),

[0052] • additional windings (2) connected in series to the main winding (1),

[0053] • switching mechanisms (5) connected to the additional windings (2), enabling the additional windings (2) to be made active or passive,

[0054] • additional module (6) containing additional winding (2) and switching mechanism (5).

[0055] In the primary (stator) part of the linear motor subject of the invention, there are main winding (1) directly connected to the motor driver (4) and additional windings (2) becoming active or passive connected in series to the main winding (1) by means of switching mechanisms (5).

[0056] The term “winding” refers to the structure formed by connecting all coils in the same phase to each other in series. With the term winding, windings belonging to all three phases are referred to. A coil is a structure consisting of a plurality of turns placed on a stator tooth.

[0057] The function of the additional windings (2) will be explained again below over the equivalent circuit. The control mechanism briefly operates as follows: At the determined lowest frequency, the motor starts operating with the main winding (1)and all additional windings (2) connected in series to each other. In order for all windings to be connected in series to each other, all of the switches in the switching mechanism (5) shown in figure 3 are brought to position “a”. Thereby, the main winding (1) and all additional windings (2) become series connected. Since there is no switching mechanism (5) after the last additional winding (2), as can be seen in figure 3, the motor becomes star connected (if desired, delta connected by changing the connection type). Then, as the motor speeds up, that is, as the supply frequency increases, the connection of the additional windings (2) is disconnected one by one at determined frequencies, starting with the last additional winding (2). For this process, the switches in the switching mechanism (5) in the module where the additional winding (2) desired to be disconnected is located are taken from position “a” to position “b”. This process, which optimizes the force produced by means of changing the electromagnetic characteristic of the motor against the increase in frequency, can be considered exactly like the gear shifting process in cars.

[0058] The primary structure of the concentric wound linear induction motor defined in the literature and the simple switching mechanism (5) designed for the new linear induction motor developed to achieve the object aimed in this work are shown in the attached figures. As can be seen in Figure 1, the linear induction motor in the literature consists of a linear motor core (3) consisting of silicon steel sheets and three-phase windings wound with copper wires. Generally, as in other motors known in the literature, the linear induction motor also contains 3 -phase windings having a single number of turns determined in accordance with the rated operating values. In Figure 1, a schematic view of the 3 -phase windings having a single number of turns placed in the slots of the linear motor core (3) is presented. In Figure 2, a schematic representation of the motor subject of the invention consisting of a plurality of 3-phase windings is presented. In the figure, for ease of illustration, a linear motor primary consisting of 3 different 3-phase windings is shown, however, this number may increase or decrease depending on the requirements of the motor. The number of turns of each winding is also determined according to the motor requirements. The main object in the invention is to be able to obtain more than one motor characteristic in one motor. Again, for ease of illustration, a motordesign consisting of concentric windings is presented in the figures, but the multi -winding motor design subject of the invention can be applied in distributed winding.

[0059] The additional module (6) located in the motor subject of the invention consists of an additional winding (2) and a switching mechanism (5). In the figure, one additional module (6) is shown, but the number of additional windings (2) can be increased by connecting more additional modules (6) between the terminals X, Y, Z and U, V, W.

[0060] The part expressed as additional module (6) is the part containing an additional winding (2) and a switching mechanism (5). It can also be considered as the number of gears of the motor. For example, for a motor having 2 additional modules (6): The main winding (1), the winding in the 1st additional module (6) and the winding in the 2nd additional module (6) are made series connected by means of taking all switches to position “b”, forming the 1st gear. Then, the windings in the 2nd additional module (6) are made passive by means of taking the switches of the 2nd additional module (6) from position “b” to position “a”. In this case, only the main winding (1) and the windings in the 1st additional module (6) are active. This forms the 2nd gear. Finally, the switches in the 1st additional module (6) are also taken from position “b” to position “a”, and the windings in this module also become passive. Only the main winding (1) is active, which forms the 3rd gear. According to the desired application, the number of additional modules (6), in other words, the number of gears, can be increased or decreased.

[0061] This model has been proposed to serve as a kind of gear in the linear motor for Hyperloop technology, where it is desired to start with very high capacity loads and to reach very high speeds without any transmission organ (gear, cog, belt-pulley, etc.) (There is no rotating part in the linear motor, the secondary is the rail itself, therefore the thrust power is directly transferred to the rail).

[0062] Currently, the Linear Induction Motor (LIM) technology, which is used in the prototypes of Hyperloop technology (a rail transportation system aiming for transportation and shipping at supersonic speeds) and is expected to be used for the propulsion system in full-scale Hyperloop transportation in the future, cannotproduce torque at significant values when high speeds are reached. At the same time, due to the fact that transmission organs that would adjust speed and force during force transmission cannot be used due to the nature of the linear motor generating force directly on the rail, linear motors do not yet possess a sufficient level of development for a full-scale hyperloop project.

[0063] To examine the above-mentioned problems in more detail; the variation of the thrust force produced by the LIM when operating above the rated frequency will be explained using the equivalent circuit model.

[0064] The single-phase equivalent circuit of the double-sided linear induction motor is as shown in Figure 4.

[0065] Calculation of primary winding resistance (Ri): J represents the primary side current density, a represents half of the stack length, lcerepresents the end winding length, IPh represents the phase current, NPh represents the number of turns in each phase, and oCu represents the conductivity value of copper.

[0066] 7(4a+2ice)jVgft

[0067] Ri=— - (Formula I)

[0068]

[0069] (acu^'phlph)

[0070] The primary side leakage reactance Xi is calculated in the form of formula II: 1 represents the total leakage permeance, fi represents the source frequency, q represents the number of slots per pole per phase, and p represents the number of pole pairs.

[0071] 15,8( / 1NA2aA) „

[0072] Xi= J”1— - (Formula II)

[0073]

[0074] 1003qpv 7

[0075] The magnetizing reactance Xmis calculated using the primary side number of phases mi, the synchronous speed vs, the winding factor kwi, and the equivalent air gap ge.X„= (Formula III)

[0076]

[0077] p^e7T

[0078] Birincil The value of the secondary side resistance (R2') referred to the primary side is calculated in the form of formula IV. OAI represents the conductivity of aluminum, d represents the secondary thickness, and T represents the pole pitch.

[0079] (Fonnu|a IV)

[0080]

[0081] ^AldPT

[0082] The equivalent circuit is solved for the rated voltage Ui and the rated current Ii and the secondary side current referred to the primary side is found with the aid of formula V and formula VI. Krand Crvalues are coefficient values resulting from the end effect.

[0083] U1

[0084] Ii= (Formula V)

[0085] Ri +7X1 +-j— — —

[0086] R2' ' Jxm

[0087] Ih-I^+jX^

[0088] (Formula VI)

[0089]

[0090] Son olarak, Finally, the thrust force "F" is calculated with formula VII.

[0091] F=m^KrCr(Formula VII)

[0092]

[0093] VsS

[0094] The approximate value of the thrust force to be generated by the motor can be calculated using the equivalent circuit approach and equivalent circuit parameters.

[0095] As is clearly seen from the formulas, the increase in frequency (operating the motor above the rated frequency) will increase the inductive reactance and therefore will lead to a decrease in the current drawn from the motor driver (4). This situation will also cause a decrease in the produced force. The important point to be noticed is the following: At high frequencies, a current below the rated current is drawn from themotor driver (4), which shows that a motor having a lower inductive reactance can produce a greater force by drawing more current under the same frequency using the same motor driver (4). As will be seen, it is not possible for a linear induction motor, which is desired to operate in a very wide speed range (0-1200 km / h) such as Hyperloop technology and does not contain any transmission organ, to meet the requirements by being designed in a single motor characteristic. In contrast, the motor subject of the invention offers a solution to this problem by containing the characteristics of a plurality of motors. The motor having a higher number of turns at low frequencies (while both the main winding (1) and the additional windings (2) are active) is capable of producing high starting force, but when the same motor reaches high frequencies (high speeds), it draws less power and produces smaller forces compared to a motor having a lower number of turns due to the increasing inductive reactance. In contrast, a motor having the same core and a lower number of turns (while only the main winding (1) is active) produces greater force at high frequencies. This situation is shown in figure 5.

[0096] Figure 5 belongs to an example of the linear motor subject of the invention consisting of a main winding (1) and an additional winding (2). In this design, the main winding (1) consists of 3-phase windings with 80 turns each, and the additional winding (2) consists of 3-phase windings with 40 turns each. The dashed line in the graph shows the variation of the force produced depending on frequency in the case where both the main winding (1) and the additional winding (2) are active. The solid line, on the other hand, shows the variation of force depending on frequency in the case where only the main winding (1) is active and the additional winding (2) is passive. From the graph, it is observed that the force / frequency characteristic of the motor to which the dashed line belongs is superior compared to the solid line up to a frequency of 60 Hz, however, the force / frequency characteristic of the solid line exhibits better performance after the frequency of 60 Hz. Based on this situation, it is seen that the invention is a linear induction motor design capable of producing the necessary force for every speed value without being bound to rated operating values for hyperloop technology.

Claims

CLAIMS1. A linear induction motor in which drive performance and power density are increased, enabling the production of torque at significant values when high speeds are reached, characterized in that it comprises;a motor driver (4) located within the linear induction motor and providing the operation of the motor,a main winding (1) connected to the motor driver (4) and positioned on the linear motor core (3),- additional windings (2) connected in series to the main winding (1), switching mechanisms (5) connected to the additional windings (2), enabling the additional windings (2) to be made active or passive, an additional module (6) containing an additional winding (2) and a switching mechanism (5).

Images