In this article we will learn about the types of starters and their working. We will also learn that how a induction motor start form a starter. We learn main types of starters which are used to starting of squirrel cage induction motor.
Types of starters
Following are the different types of starters used for starting of induction motors.
For squirrel cage induction motors various types of starters used as.
1. DOL (Direct-On-line) starter.
2. Star-Delta Starter
3. Autotransformer Starter
1. DOL Starter ( it is low capacity types of starters,direct online starter).
In case of small capacity motors having rating less than 5 HP, the starting current is not very high and these motors can withstand such starting current without any starter. Thus, there is no need to reduce applied voltage to control the starting current. Such motors use a type of starter which is used to connect stator directly to the supply lines without any reduction in voltage. Hence, the starter is known as Direct-on-Line starter.
Though, this starter does not reduce the applied voltage, it is used because it protects the motor from various severe abnormal conditions like overloading, low voltage, single phasing etc. hence it is one of the types of starters which is used at below 5 HP motor.
The NO contact is normally open and NC is normally closed. At start, NO is pushed for fraction of second due to which, coil gets energized and attracts the contactor. So, stator directly gets supply. The additional contact provided, ensures that as long as supply is ON, the coil gets supply and keeps contactor in ON position. When NC is pressed, the coil circuit gets opened due to which coil gets de-energized and motor gets switched OFF from the supply. Under cover load condition, current drawn by the motor increases due to which there is an excessive heat produced, which increases temperature beyond limit. Thermal relays get opened due to high temperature, protecting the motor from overload conditions. It is main types of starter in squirrel cage induction motor.
2. Star-Delta Starter
If the stator winding of the motor is directly connected with the supply, it will draw a high current. To control this high current, star-Delta starter is used. A star-Delta starter is used for a cage motor designed to run normally on delta connecting stator winding. A star-delta starter is used with a squirrel cage induction motor having a capacity up to 5HP. It is operated by a handle. It is manual types of starters.
In start position, downwards, the stator winding are connected in star. The voltage on each winding will be equal to the line voltage under root three, that is 57.7% of line voltage. When the rotor gains speed, the starter is quickly changed to run position that is upwards, thereby connecting the stator winding in delta. In delta, the phase voltage is equal to the line voltage, therefore, full line voltage is applied to the stator winding and the motor will run at its normal speed. Hence it is also one of the types of starters which used at 5 HP.
3. Autotransformer starter
These types of starters are used for starting of the squirrel cage induction motors, having a capacity of 25 or more than 25HP. A three phase autotransformer, with many tapings in each winding is used in it as shown in figure.
Three leads are taken out of the winding of the motor and connected with stator. At the time of starting, reduced voltage is applied to the stator winding of motor through these tapings.
The motor runs at a low speed, so that the current drawn by the motor reduces. As the motor attains about 75% of the speed, the handle is put in run position and motor autotransformer is out of circuit. This starter is provided with no-volt and over load coils. Hence it is one of the types of starter which is used for high rating motor.
Hence it is the main types of starters which is used to start of squirrel cage induction motor. If you will find any in correct above please comment below in comment box.
For knowing more about the types of starters you must watch this video.