elpee:- if the motor is supplied with the same voltage as in delta when connected in star then you would only develope a third of the power and a quarter of the torque (ie 100 hp delta 30 hp star).the power factor and efficency would also be down.the d.o.l.locked rotor current would be reduced to around 3.5 times full load current.
if the motor is connected to a centrifugal pump or fan then full speed would not be achived the rotor would sit in the pull up part of the speed torque curve and the stator would slowly burn out.
kind regds.

elpeef the motor connected in star &applied full rated voltage,we know that two windings(phases) are connected in series between two phases. Incase of delta each winding is connected between two phases.  
         That's why the current passes through star connetion is less (i.e 1/3) as compared with delta connection, the power consumed inthis circuit is
reduced by three times, effiency is also decreased, pf& tarque aiso decreased    

while measuring insulation resistance using Megger at first it shows some what lesser value of IR and then it comes to a steady value some higher value than the initial. why?

If a motor is rated to operate in star connection at it's rated voltage, then you must not connect in delta at that same voltage. Changing the conection between star and delta effectively changes the voltage rating of the motor. When connected in star, the voltage accros one winding is the voltage between phase and neutral, or in other words, the line voltage divided by the square root of 3. (1.732) When connected in delta, the voltage across a winding is the full line voltage.
 The motor is designed to operate a a particular flux in the iron and this is a function of the voltage acroos a winding. The voltage rating of a delta connected motor increases by root 3 (1.732) when connected in star, and the voltage rating of a star connected motor reduces by the same proportion when connected in delta.

If a motor is operated above it's design voltage, the iron becomes overfluxed, the current rises dramatically and the motor is damaged.

If a motor is operated below its design voltage, the effective power rating of the motor is reduced.

A Delta designed motor, started in star connection will draw one third of the Delta current and produce one third of the delta torque while starting.

Do not operate a motor connected in delta at the star rated voltage!!

Mark Empson
http://www.lmphotonics.com