Waht is different bwtween using star delta and Transformers self
and when we must to use the Transformers self with Motor

Hello Nader

Have a look at http://www.lmphotonics.com/m_start.htm for some basic information on the star/deta starter and the autotransformer starter.
There is more information on the star/delta transformer at http://www.lmphotonics.com/star_delta.htm

The star/delta starter is the cheapest option, but has only one "reduced voltage" option, star connection. The autotransformer starter has several output voltage tappings allowing for far better control than with the star/delta starter. The star/delta starter creates a major current and torque transient at the transition from star to delta. The autotransformer starter can be designed to eliminate the switching transient (Kordorfer method) and is therefore a much better solution than the star/delta starter, but at a higher price.

Best regards,

One more issue that is only briefly mentioned in Marke's first paper is very important in my opinion. Y-Delta motors are specially designed for that purpose as he said. This means they also require careful connection in the field, because if the 6 wires from the starter are not connected to the proper 6 leads in the motor (12 leads in the US with Dual Voltage motors), the motor will not work and damage may occur to both the motor and the starter. This can be a serious problem in applications where the equipment may be worked on by technicians without that knowledge. Autotransformers use 3 leads from the starter to the motor so they connect the same as DOL starters and they work on any AC induction motor.

Better yet however, use a soft starter. Smaller, cheaper, much more flexible and fewer moving parts, maybe none if a bypass contactor is not necessary.

Thank you very mush for help