Design And Construction Of Star And Delta, Forward And Backward Electric Motor

ABSTRACT

This work is on the forward reverse star delta electric motor control using switches. Moving ac motor clockwise and anticlockwise has many applications in the present technology, such as in our garage doors. Some garage door motors are easily reversed AC motors and geared down. Forwarding and reversing of an ac motor is done using a push button switches.

Depending on the choice of the operator, starting the circuit is selected via the forward or the reverse push button switches. When the forward push button switch is selected, the command is transferred to the relevant forward function of the circuit which activates the forward main coil (blue). The relevant auxiliary contacts of the forward main coil (blue) would then be actuated opposite its normal state, that is, a normally open contact would eventually switch to a close contact whereas a normally close contact would switch to an open contact.

CHAPTER ONE

1.1                                                        INTRODUCTION

An AC 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.

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 AC 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 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.

The forward and backward of an AC motor control is very important because of both local and industrial application. This work discus how three phase Motor will rotate in two directions viz forward and Reverse.

1.2                                             OBJECTIVE OF THE PROJECT

The objective of this work is to design the Forward and Reverse controlling of an ac motor using star-delta connection for three phase 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.

1.4                                         SIGNIFICANCE OF THE PROJECT

• With these work, a three phase motor can move Forward and Reverse direction
• 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. Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta starter.
8. 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.
9. 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 THE PROJECT

Industries are using forward and reverse ac motor solutions for process, control of devices such as ac conveyor

Consumer products including electronic toys, home security, gate and garage door openers also make use of this device.

1.8                                 COMPONENTS OF STAR-DELTA MOTOR

Star-delta motor Consists following units

1. Contactors (Main, star and delta contactors)
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