Starting of Induction Motor

A three phase Induction Motor is Self Starting. When the supply is connected to the stator of a three-phase induction motor, a rotating magnetic field is produced, and the rotor starts rotating and the induction motor starts. At the time of starting, the motor slip is unity, and the starting current is very large.

The purpose of a starter is not to just start the motor, but it performs the two main functions. They are as follows.

To reduce the heavy starting current
To provide overload and under voltage protection.

The three phase induction motor may be started by connecting the motor directly to the full voltage of the supply. The motor can also be started by applying a reduced voltage to the motor when the motor is started.

The torque of the induction motor is proportional to the square of the applied voltage.Thus, a greater torque is exerted by a motor when it is started on full voltage than when it is started on the reduced voltage.

There are three main methods of Starting of Cage Induction Motor. They are as follows.

Direct On Line Starter

The direct on line starter method, of an induction motor is simple and economical. In this method, the starter is connected directly to supply voltage. By this method small motors up to 5 kW rating is started to avoid the supply voltage fluctuation.

Star Delta Starter

The star delta starter method of starting three phase induction motors is very common and widely used among all the methods. In this method, the motor runs at delta connected stator windings.

Auto Transformer Starter

The Auto transformer is used in both the type of the connections, i.e., either star connected or delta connected. The auto transformer is used to limit the starting current of the induction motor.

The above three starters are used for cage rotor induction motor.

Slip Ring Induction Motor Starter Method of Starting Induction Motor

In the Slip Ring Induction Motor starter, the full supply voltage is connected across the starter. The connection diagram of the slip ring starter induction motor is shown below.

Slip Ring Induction Motor

Full starting resistance is connected and thus the supply current to the stator is reduced. The rotor begins to rotate, and the rotor resistances are gradually cut out as the speed of the motor increases. When the motor is running at its rated full load speed, the starting resistances are cut out completely, and the slip rings are short-circuited.

Comments

MOSFET Symbol

MOSFET stands for Metal Oxide Silicon Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor. This is also called as IGFET meaning Insulated Gate Field Effect Transistor. The FET is operated in both depletion and enhancement modes of operation. The following figure shows how a practical MOSFET looks like. Construction of a MOSFET The construction of a MOSFET is a bit similar to the FET. An oxide layer is deposited on the substrate to which the gate terminal is connected. This oxide layer acts as an insulator (sio 2 insulates from the substrate), and hence the MOSFET has another name as IGFET. In the construction of MOSFET , a lightly doped substrate, is diffused with a heavily doped region. Depending upon the substrate used, they are called as P-type and N-type MOSFETs . The following figure shows the construction of a MOSFET . The voltage at gate controls the operation of the MOSFET . In this case, both positive and n...

Different Types of Resistors

Different Types of Resistors : Apart from coming in various shapes and sizes, resistors are also classified according to what the active material is made of. 1. Carbon Resistors : Carbon Film Resistors The resistive material in these resistors is made of carbon or graphite dust. Since carbon compounds burn easily, these resistors can handle only low amounts of dissipated powers. Also since the material is a powder, they are not very accurate and have loose tolerances.shown 2. Metal Film Resistors : Metal Film Resistors Just like their name suggests, the resistive material is a metal film. Since the metal film can be made or calibrated to very specific dimensions, the resistance can be controlled finely and as a result these resistors are very accurate. 3. Wire Wound Resistors : Wire Wound Resistors The resistive material is made of a wire. Since these wires can be as thick as one likes, these resistors can be made to handle very high powers and are...

what is electric current

Electric Current Definition : Electric current is defined as the rate of flow of negative charges of the conductor. In other words, the continuous flow of electrons in an electric circuit is called an electric current.The conducting material consists a large number of free electrons which move from one atom to the other at random. Unit of Current : Since the charge is measured in coulombs and time in seconds, so the unit of electric current is coulomb/Sec ( C/s ) or amperes ( A ). The amperes is the SI unit of the conductor. The I is the symbolic representation of the current. Current Folw Q= 1 Coulomb; t = 1 Second; then I = 1A Thus, a wire is said to carry a current of one ampere when charge flows through it at the rate of one coulomb per second. When an electrical potential difference is applied across the metallic wire, the loosely attached free electrons start moving towards the positive terminal of the cell shown in the figure bel...

Electronic Hub

Search This Blog