Analysis Of Controlling Induction Motor Using Star –Delta

Description

ABSTRACT

This work is on analysis of controlling induction motor using star and delta connection, which is the reduced voltage starting method on motor. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding. This also reduces the torque by a factor of three. After a period of time the winding are reconfigured as delta and the motor runs

Star/Delta motor are probably the most common reduced voltage of the motor. They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply.

The Star/Delta (or Wye/Delta) starter is one of the lowest cost electromechanical reduced voltage starters that can be using to control induction motor.

CHAPTER ONE

1.1                                                        INTRODUCTION

An induction motor converts electrical energy into mechanical energy. The reverse of this would be the conversion of mechanical energy into electrical energy and is done by an electric generator.

Induction motors are used to produce linear or rotary force (torque), and should be distinguished from devices such as magnetic solenoids and loudspeakers that convert electricity into motion but do not generate usable mechanical powers, which are respectively referred to as actuators and transducers.

Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges. To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed.

Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding. This also reduces the torque by a factor of three.

The Star/Delta starter is manufactured from three contactors, a timer and a thermal overload. The contactors are smaller than the single contactor used in a Direct on Line starter as they are controlling winding currents only. The currents through the winding are 1/root 3 (58%) of the current in the line.

There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star current while the motor is connected in star.

The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.

Three phase star-delta motors are constructed similarly to a direct online single phase motor, but the terminals for each winding are not terminated within the motor, instead they are brought out of the  motor for control wiring to connect to.

Three phase star-delta motors are used for maximum talk, where the motor will try to start under a heavy load. A star-delta motor starts in two stages, controlled by equipment connected to it.

The motor is required to start. To provide the maximum available torque, the motor is started in ‘star’, which provides a supply of 230volts to each of the windings (providing high current to each winding).

Once the motor is running, it is running in an inefficient mode, due to the use of a high current supply, which will (if left) cause the motor to overheat.

Three phase induction motors, also called asynchronous motors, are most extensively used motors in industries because of certain advantages like self starting, robust design, simple construction, less maintenance, efficient and comparatively low cost, though there is a problem of peak starting current associated with these motors. Peak starting current can be up to 5 to 7 times of full load current (flc) and sometimes it may become as high as 10 times of flc. However, the problem of peak starting current remains only for few seconds till the motor attains its speed, this problem may become severe especially with the motors above 10 HP. To deal with the problem of peak starting current or inrush current associated with three phase induction motors, many different starters having different mechanism and principle of operation are used. Star-delta starters are used for motors ranges from 5 hp or 3.5 kW. Star delta starters first configure windings of 3 phase motors in star thereby reduce voltage across each winding and then after few seconds these starters configure windings in delta and motors start run at full load voltage without any difficulty.

1.2                                             OBJECTIVE OF THE PROJECT

The objective of this work is to analyze the controlling of an induction motor using star-delta connection. This connection reduces the starting current and starting torque of a 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

1. Low Starting Torque (Torque = (Square of Voltage) is also reduce).
2. Break In Supply – Possible Transients
3. Six Terminal Motor Required (Delta Connected).
4. It requires 2 set of cables from starter to motor.
5. 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.
6. 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.
7. 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. In some cases the current peak can reach a value that is even bigger than for a D.O.L start.
8. Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta starter.
9. 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.
10. 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 CONNECTION

For low- to high-power three-phase motors.

1. Reduced starting current
2. Six connection cables
3. Reduced starting torque
4. Current peak on changeover from star to delta
5. 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                                 COMPONENTS OF STAR-DELTA MOTOR

Star-delta motor Consists following units

1. Contactors (Main, star and delta contactors) 3 No’s (For Open State Starter) or 4 No’s (Close Transient Starter).
2. Time relay (pull-in delayed) 1 No.
3. Three-pole thermal over-current release
4. Fuse elements or automatic cut-outs for the main circuit
5. Fuse element or automatic cut-out for the control circuit