HI ..

first i would like to thank you for all your explanation and hard work.I have a small question..
what is meant by y-delta connection (joining y and delta in the same motor) ??
i heard that this connection is good for starting because of the lower currant in delta-configuration than in Y-configuration

thank you .

 

http://www.allaboutcircuits.com/vol_2/chpt_10/5.html

 

Your supply and driven device dictates this more than anything. I'm pretty much totally 480V Y at work, in other words I've got 480V between the phases but can grab 277V to neutral from any one to feed lighting. Can't say that I've got a motor that even has a neutral on it, just earth ground through the conduit or, if I've added it, a separate earth ground wire in the conduit.

There's a reason for transmitting power in three phase, that's why you usually see three wires on the poles above you.

 

Wye-Delta starting is used when it is desirable to reduce the inrush current on a 3 phase motor to minimize line disturbances, and sometimes to provide a soft start. The motor has at least 6 leads brought out and the windings are connected via a special multi-contactor starter in a wye configuration to start, then after a time delay, connected in delta for running at full horsepower. Starting torqe and starting current are both reduced by a factor of 3.