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A three-phase electric motor can be connected in two ways, star and delta. it is a process, which regulates the power input during the start up of motors. When the motor is starting up it needs less torque and current than, when it is on full rotation, the star connection will start the rotation of the motor with only one third of current and torque and when motor runs on speed after few seconds it converted to delta for reduction of more amperes, this procedure reduces expense of electricity.
Star- Delta connection is especially used for HV Motors to reduce the starting current of the motor.
Star-Delta starters are widely used due to their relatively low price.There are no limits to the number of times they can be operated.The components require very little space.
The starting current is reduced to approximately one-third.
But the main drawbacks are:
The starter can only be applied to motors where the six leads or terminals can be accessed.The supply voltage must be the same as the rated motor voltage for Delta connection. Because the starting current is reduced to approximately one-third of the rated current, the starting torque is also reduced to one-third.If the motor does not reach at least90% of its rated speed at the time of switching from Star to Delta the current peak will be as high as in a D.O.L. start, thus causing harmful effects to the contacts of the contactors and the connection system brings no advantage to the electrical system.
The high starting currents often result in a voltage drop
unpleasant and couples require high start-mechanical elements
resistant to overloads. This is why electricity distributors
set limits for motor starting current values, to reduce the current and couples use etoil triangle., and also not to toast the engines they need or types of coils
Aa method of connecting polyphrase circuits in which one end of each phase line is connected to a common neutral point that may be connected to the earth as protection against lightning or to a wire to which all the other neutral points of the system are connected and a connection used in a three-phase electrical system in which three elements in series form a triangle, the supply being input and output at the three junctions.
Basically3 Phase Motors are started using soft starters.....most of the HV Motors are started using star-delta starters ,because to reduce the starting current and increasing the torque .....
