What is Linear Induction  Motor : Design & Its Working

In the period 1840’s itself, the development of linear induction motor has started by Charles Wheatstone at London, but this seems to be impractical. Whereas in the year 1935, the operating model was brought into development by Hermann Kemper, and the full-size operating version was introduced by Eric in the year 1940. Then after, this device was employed in many applications across many industries. This article gives clearly explains Linear Induction Motor, its working principle, performance, design, construction, advantages and disadvantages and major applications. Let us dive into the concept.

What is a Linear Induction Motor?

Linear Induction Motor is abbreviated as LIM and this is the enhanced version of the rotary induction motor where the output is linear translational motion in the place of rotating motion. This device generates linear movement and force other than the rotating torque. The design and the functionality of the linear induction motor can be shown in the below figure by creating a radically shaped cut in the revolving induction and thus leveling the section.

The output is a leveled stator or the upper side having iron-plated laminations where these carry three-phase multiple poles winding having conductors which are in 90⁰ angles to the motion direction. It also consists of squirrel enclosed type of winding whereas it is generally included with an endless aluminum or copper made sheet which is kept on solid plated iron support.

Regardless of the device name, not all the linear induction motors generate linear movement, few of the device generate are utilized for delivering revolutions having great diameters and the utilization of the endless primary sections is more costly.

Design

The fundamental construction and linear induction motor design almost correspond the same as three-phase induction motor, even though it does not appear like that of a normal induction motor. When a cut is formed in the stator section of the polyphase induction motor and placed on a flat surface, then this creates the primary section of the linear induction motor. In the same way, when a cut os formed in the rotor section of the polyphase induction motor and placed on a flat surface, then this creates the secondary section of the linear induction motor.

In addition to this, there exists another model of the linear induction motor that is utilized for the enhancement of performance and this called DLIM which is Double-Sided Linear Induction Motor. This model has a primary section that is placed on another end of the secondary section. This design is used for enhancing the utilization of flux on both the primary and secondary sides. This is the construction of a linear induction motor.

Working Principle of Linear Induction Motor

The below section provides a clear explanation of the working of linear induction motor.

Here, when the primary section of the motor is energized by using a balanced three-phase power, then there will be flux movement all across the length of the primary section. This linear movement of the magnetic field is equal to the revolving magnetic field in the stator section of the three-phase induction motor.

With this, there will be the induction of electric current in the conductors of the secondary winding because of comparative movement in between the conductor and flux movement. The current that is induced gets in connection with the flux movement to generate either linear thrust of force and this is shown by

Vs = 2tfs m/sec

When the primary section is made to be constant and the second section has movement, then the force pulls the secondary section in its direction itself and this results in the generation of necessary rectilinear movement. When a power supply is provided to the system, the generated field will provide a linear moving field where the velocity is represented as per the above-mentioned equation.

In the equation, ‘fs’ corresponds to the amount of supply frequency measure in Hz

‘Vs’ corresponds to the linear moving field measured in m/sec

‘t’ corresponds to the pitch of the linear pole which means the distance between pole to pole measured in meters

V = (1-s)Vs

In correspondence to the same justification, in the condition of induction motor, the secondary runner does not hold the same speed as the speed value of the magnetic field. Because of this, there generates a slip.

The linear induction motor diagram is shown as follows:

Characteristics of Linear Induction Motor

A few of the LIM characteristics are:

End Effect

Dissimilar to the circular induction type of motor, LIM has a characteristic called “End Effect”. The end effect consists of efficiency and performance losses which are the consequence of magnetic energy that is carried away and dropped at the ending of the primary section through the relative motion of the primary and secondary sections.

Only with the secondary section, the functionality of the device seems to be the same as the rotary machine, required that it is nearly two poles apart but having a minimal primary reduction in the thrust which happens at low slip still it is either 8 or more poles longer. With the existence of end effects, LIM devices do not hold the ability to run light, whereas the general kind of induction motors holds this ability to operate the motor having a closer synchronous field under minimal load circumstances. Opposing this, the end effect generates corresponding losses having linear motors.

Thrust

The drive that is caused by the LIM devices is almost the same as that of general induction motors. These drive forces represent an approximately same characteristic curve the same as slip, even though modulated by the end effects. This is also termed as a Tractive effort. It is shown by

F = Pg/Vs measured in Newtons

Levitation

Furthermore, in contrast to the rotary motor, LIM devices have electrodynamic levitation force which has zero reading at ‘0’ slip and this generates an approximately fixed amount of gap when the slip enhances in either of the directions. This takes place only in single-sided motors and this characteristic will not generally happen when an iron supporting plate is utilized for the secondary section because this creates an attraction that overcomes the lifting pressure.

Transverse Edge Effect

Linear Induction Motors also exhibit a Transverse edge effect which is that the current paths that are in the same direction of movement develop losses and because of these paths, there will be a reduction in the effective thrust. As because of this transverse edge effect takes place.

Performance

The performance of the linear induction motor can be known by the below-explained theory where the synchronous speed of the moving wave is represented by

Vs = 2f (pith of the linear pole)……..m/s

‘f’ corresponds to supplied frequency measured in Hertz

In the case of a rotary induction motor, the speed of the secondary section in the LIM is less than that of the synchronous speed and is given by

Vr = Vs (1-s), ‘s’ is the LIM slip and it is

S = (Vs – Vr)/Vs

The linear force is given by

F = power of the air gap/Vs

The thrust velocity curve shape of LIM is almost identical to that of the speed v/s torque curve of the rotary induction motor. When there is a comparison between LIM and rotary induction motor, the linear induction motor needs an increased air gap and because of this, there will be increased magnetizing current and the factors like performance and power factor will be minimal.

In the case of RIM, the area of the stator and rotor sections is similar, whereas in LIM one is shorter than the other section. At constant speed, the shorter section will have continuous passage than that of the other.

Advantages and Disadvantages

The advantages of the linear induction motor are:

The crucial benefits of LIM are:

• There exist no magnetic attractional forces at the time of assembly. For the reason that LIM devices have no permanent magnets, there exists no attractional force at the time of system assembly.
• Linear induction motors also have the advantage of traveling long lengths. These devices are mainly implemented for long length applications because secondary sections are not included with permanent magnets. The non-existence of magnets in the second section allows these devices to be not expensive because the price of the device crucially lies in the development of a magnetic track.
• Effectively useful for heavy-duty purposes. Linear induction motors are primarily used in high-pressure linear motor conditions where they are present with steady force ratings of nearly 25gms of accelerations and some hundreds of pounds.

The disadvantages of linear induction motor are:

• The construction of LIM devices is somewhat complicated as they require sophisticated control algorithms.
• These have increased attractional forces at the time of operation.
• Shows no force at the time of standstill.
• The enhanced physical size of the device means that the packaging size is more.
• Requires more power for functionality. When compared with permanent magnets linear motors, the efficiency is less and generates more heat. This further needs water cooling devices to be included in the construction.

Applications of Linear Induction Motor

The exclusive utilization of linear induction motors can be found in applications like

• Metallic conveyor belts
• Mechanical controlling equipment’s
• Actuators for high-velocity circuit breakers
• Shuttle boosting applications

On the whole, this is all about the concept of Linear Induction Motors. This article has provided a clear explanation of linear induction motor principles, design, working, uses, benefits, and drawbacks. It is further necessary to know how the speed v/s pole pitch characteristics in linear induction motor perform?