Selection of Motor Starter

this is depends on their requirment.if they need higher staring torque,and the direct starting is not making any voltage dip in the system its no problem.

Various types of reduced voltage motor starters (part winding, star-delta, stepped resistance, autotransformer, solid state) can reduce the motor starting current. This minimizes voltage dip in the network.

A robust network can support DOL starting of large motors, but smaller networks might not be able to do this; in which case they may require reduced voltage starting for motors above a specified size.

The stipulation of limiting the power rating of motors that ca be started DOL applies only to those customers who draw power from LT Lines. This is because, if an LV customer starts his high capacity motor DOL, the line voltage drop caused in the main LV lines would affect the neighbouring LV customer connected to the same LV mains. And, this could be a flip-flop action. So, to avoid this social problem, regulatory authorities have imposed that DOL starting be limited to motors of capacity upto, say, 7.5HP or 10HP.

But, if a customer is drawing power from a HT mains, he would have a transformer and as such any voltage drop created by starting any of his high capacity motors would not affect his neighbouring HT customer. May be, in his own plant some smaller motors may get affected, but, then that is his "own" problem and NOT a social problem. So, for a HT customer, there are no stipulations as to the size of the motor that can be started DOL. The decision is made based on starting torque requirements, which is good with DOL and, of course, economic considerations.

Star delta starter are for motors that require high Starting torque to more a stationary machine, then once running the requirement for the high torque, high current is reduced, mainly because some machines have the (inbuilt) kinetic energy to overcome that little bit of extra load, for example, car crushers/shredders, conveyor belts.[/p]

Am I right in assuming that you have just got this job straight out of school?

don't try to apply all you've been taught in one afternoon... apply logic and study the principals not the absolute theory.

Ask the <...they...>.