An induction motor is practically a constant speed motor, which means, for the entire loading range, change in speed of the motor is quite small. Speed of a DC shunt motor can be varied very easily with good efficiency, but in case of Induction motors, speed reduction is accompanied by a corresponding loss of efficiency and poor power factor. As induction motors are widely being used, their speed control may be required in many applications. There are different methods of controlling the speed of induction motor, but this work focuses on controlling the speed, forward and backward direction of induction motor using star/delta stater.

 

 

 

CHAPTER ONE
1.0                                                        INTRODUCTION
The AC induction motor (ACIM) is the most popular motor used in consumer and industrial applications, and represented the "muscle" behind the industrial revolution. The concept of this "sparkless" motor was first conceived by Nicola Tesla in the late nineteenth century as a polyphase structure consisting of two stator phases in an orthogonal relationship. It has since been modified to the more common three phase structure, which results in balanced operation of the motor voltages and currents.
The motor does not have a brush/commutator structure like a brush DC motor has, which eliminates all the problems associated with sparking; such as electrical noise, brush wear, high friction, and poor reliability. The absence of magnets in the rotor and stator structures further enhances reliability, and also makes it very economical to manufacture. In high horsepower applications (such as 500 HP and higher), the AC induction motor is one of the most efficient motors in existence, where efficiency ratings of 97% or higher are possible. However, under light load conditions, the quadrature magnetizing current required to produce the rotor flux represents a large portion of the stator current, which results in reduced efficiency and poor Power Factor operation.
AC induction motor performs best when they are driven with sinusoidal voltages and currents. One of the advantages of ACIMs is the incredibly smooth operation they can provide as a result of low torque ripple. To achieve this, most ACIMs consist of a slotted stator structure where the windings are placed in the slots with a sinusoidal winding distribution, resulting in a sinusoidal flux distribution in the airgap. This flux also links the rotor circuit, which consists of copper or aluminum bars shorted at each end, and mounted on a stacked laminate structure comprised of soft iron, or other ferrous material. In most cases, motor efficiency can be increased by decreasing the rotor bar resistance. As the flux cuts across these conductors, a d-flux/dt voltage is impressed across the rotor bars, which results in current flow in the rotor. In other words, current is induced in the rotor circuit from the stator circuit; much the same way that secondary current is induced from the primary coil in a standard transformer. This rotor current produces its own flux, which interacts with the stator mmF to produce torque. However, in order to achieve this d-flux/dt effect on the rotor bars, the rotor cannot rotate at the same speed as the rotating stator field. As a result, induction motors are classified as asynchronous motors. The difference in rotational speed between the stator flux vector and the rotor is called slip. As more torque is required from the motor shaft, the slip frequency increases. In conclusion, the motor speed is a function of the number of stator poles, the motor torque (and consequently motor slip), and the frequency of the AC input voltage.
The three phase topology represents an ideal choice for variable-speed applications. Three phase inverters are commonly used as shown in the diagram, where motor speed can be controlled by simply varying the voltage and frequency of the applied waveform (open-loop V/Hz or scalar control). Alternately, speed can be controlled by wrapping a speed loop around a torque loop incorporating Field Oriented Control (FOC). The former can be easily achieved with an economical device such as an MSP430, but FOC is more suitable to a powerful 32-bit processor such as TI's C2000 processors.
AC induction motors are also available in single-phase versions. Most single phase versions actually have two phases, where one phase is used to help get the motor started. Once the motor reaches a certain speed, this phase can be disconnected, resulting in the motor operating on just one phase.

1.2                                             OBJECTIVE OF THE PROJECT
This is a starting method that reduces the starting current and starting torque of a motor. The objective of this work is to wire a star-delta electric motor, using it to control the speed and direction of an induction motor.

1.3                                                 SCOPE OF THE PROJECT
The Star Delta starting method is a motor starting mechanism that minimizes the large amount of starting current that motors draw in. The Star Delta, as the name suggests basically involves feeding the motor with 1/sq.root3 (58%) of the full load current until it attains speed then applying the full load current. This method is commonly referred to as "Soft Starting" the motor, For this to work the whole set-up requires 3 contactor i.e The Star Contactor, The Delta Contactor and The Main Contactor. However for the motor to be started in Star Delta, its internal connection at the terminal box has to be wired in Delta-giving it capability of receiving the full-load current at any instant.
Traditionally, in many regions there was a requirement that all motor connections be fitted with a reduced voltage starter for motors greater than 4KW 5HP.This was to curb the high inrush of starting currents associated with starting induction motors.

1.4                                         SIGNIFICANCE OF THE PROJECT

• The operation of the star-delta method is simple and rugged
• It is relatively cheap compared to other reduced voltage methods.
• Good Torque/Current Performance.
• It draws 2 times starting current of the full load ampere of the motor connected
  
  

1.5                                           LIMITATION OF THE PROJECT

• Low Starting Torque (Torque = (Square of Voltage) is also reduce).
• Break In Supply – Possible Transients
• Six Terminal Motor Required (Delta Connected).
• It requires 2 set of cables from starter to motor.
• It provides only 33% starting torque and if the load connected to the subject motor requires higher starting torque at the time of starting than very heavy transients and stresses are produced while changing from star to delta connections, and because of these transients and stresses many electrical and mechanical break-down occurs.
• In this method of starting initially motor is connected in star and then after change over the motor is connected in delta. The delta of motor is formed in starter and not on motor terminals.
• High transmission and current peaks: When starting up pumps and fans for example, the load torque is low at the beginning of the start and increases with the square of the speed. When reaching approx. 80-85 % of the motor rated speed the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch over to delta position is necessary, and this will very often result in high transmission and current peaks.
• Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta starter.
• Low Starting Torque: The star-delta (wye-delta) starting method controls whether the lead connections from the motor are configured in a star or delta electrical connection. The initial connection should be in the star pattern that results in a reduction of the line voltage by a factor of 1/√3 (57.7%) to the motor and the current is reduced to 1/3 of the current at full voltage, but the starting torque is also reduced 1/3 to 1/5 of the DOL starting torque.
• The transition from star to delta transition usually occurs once nominal speed is reached, but is sometimes performed as low as 50% of nominal speed which make transient Sparks.

1.6                                      FEATURES OF STAR-DELTA MOTOR
For low- to high-power three-phase motors.

• Reduced starting current
• Six connection cables
• Reduced starting torque
• Current peak on changeover from star to delta
• Mechanical load on changeover from star to delta

1.7                                  APPLICATION OF STAR-DELTA MOTOR
The star-delta method is usually only applied to low to medium voltage and light starting Torque motors.
The received starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque. This starting method only works when the application is light loaded during the start.
If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.

1.8                                              PROBLEM OF THE PROJECT
The most basic feature of an Induction motor is its self starting mechanism. Due to the rotating magnetic field, an emf is induced in the rotor, because of which current starts flowing in the rotor. As per the Lenz law, the rotor will start rotating in a direction so as to oppose the flow of electric current and this gives a torque to the motor. Thus the motor gets self started.
During this self starting period, as torque increases, a large amount of current flows in the rotor. To achieve this the stator draws a large amount of current and by the time the motor reaches its full speed, a large amount of current is drawn and coils get heated up, damaging the motor. Hence there is a need to control the motor starting. One way is to reduce the applied voltage, which in turn reduces the torque.

 

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