Starting of induction and cage motors starters/Engineering Tutorials ~ MONU TUTORIALS ACADEMY

STARTING OF INDUCTION AND CAGE MOTORS

Starting induction motors

When the supply is connected to the stat-or of a three phase induction motor a rotating magnetic flux is produce and the rotor start rotating. Thus a three phase induction motor is self starting motor. At the time of starting the motor slip is unity and the starting current is very large. The starter of the motor performed main two function.

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

Three phase induction motor may be started either by connecting the motor directly the full voltage of the supply or by applying a reduced voltage to the motor during starting period.

Starting of cage motors

The following are the commonly used starter for cage motors:

Direct on-line starter
Star-delta starter
Auto transformer starter

Direct on-line Starter :-

In the direct on-line starter method of starting cage motors, the motor is connected by means of a stater across the full supply voltage. The connection for one type of the direct-on-line [D.O.L] starter. It consist of coil-operated contact-or C controlled by start and stop push button which may be installed at convenient places remote from the starter. On pressing the start push the button S 1 the contact-or coil C is energized from two line conductor L 1 and L 2 The three main contacts M and the auxiliary contact A close and the terminal a and b are short circuited. The motor is thus connected to the supply. When the pressure on S 1 is released, It moves back under spring action. Even then the coil C remain energized through ab. thus,the main contact M remain closed and the motor continues to get supply. For this reason , contact A is called hold-on-contact.

When the STOP push button S 2 is pressed the supply through the contact-or coil C is disconnected. Since the coil C is d-energized the main contact M and auxiliary contact A are opened. The supply to motor is disconnected and the motor stops.

Star-Delta Starter :-

This is a very common type of starter and extensively used, compared to the other types of the stater. A star-delta starter is used for a cage motor designed to run normally on Delta-connected stat-or winding. Fig shown the connections of a three phase induction motor with the star-delta starter. When the switch S is in the start position, the stat-or winding are connected in star When the motor picked up speed, say 80% of its rated value. the changeover switch S is thrown quickly to the run position which the connect stat-or winding in Delta. By connecting the stat-or winding, First in star and then in delta, the line current drawn by the motor at starting is reduced to one-third as compared to starting current with the winding connected in Delta.

Auto-Transformer Starter :-

At auto-transformer starter is suitable for both star and delta-connected motors. In this method, the starting current is limited by using a three-phase auto-transformer to reduce the initial stat-or applied voltage. Figure shown the motor with the auto-transformer starter. The auto-transformer is provided with a number of tapping.

In practice, The starter is connected to one particular tapping to obtain the most suitable starting voltage. A double throw switch S is used to connect the auto-transformer in the circuit for starting. When the handle H of the switched S in the Start position, the primary of the auto-transformer is connected to the supply line and the motor is connected to the secondary of the auto-transformer, When the motor pick-up the speed, say to about 80% of its rated value. The handle H is quickly moved to the RUN position. The auto-transformer is disconnected from the circuit and the motor is directly connected to the line and gets its full rated voltage. The handle is held in the run position by the under-voltage relay. In case the supply voltage fails or falls below a certain value, the handle is released and return to the off position. Overload protection is provided by thermal overloads relays.

Slip Ring Induction Motor Starter

The connection of three phase slip ring induction motor with a stater. Full supply voltage is connected across the stat-or. Full starting resistance are connected, and thus the supply current to the stat-or is reduced. The rotor begins to rotate and the rotor resistance are gradually cut out as the motor speed up. When the motor is running at its full speed, the starting resistance are cut out completely and the slip rings are short-circuited.

About Admin MC3

This is dummy text. It is not meant to be read. Accordingly, it is difficult to figure out when to end it. But then, this is dummy text. It is not meant to be read. Period.