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Star? Delta?


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I'm trying to learn about motors and I have some questions

Q1. Is it normal to run a 3phase squirrel cage motor wired in delta configuration?

Q2. Under what circumstances (if any)would one wire it to run in star?

Q3. Am I correct to believe that it is normal to start some motors in star (wye) because the starting current is reduced and then to switch over to delta because the motor runs more efficiently in delta?


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Hello solas


Welcome to the forum.


Q1. You must connect the motor in the format for which it was designed. If the motor is designed to be delta connected, then that is how you use it.

The motor is designed to have a flux level in the iron and this flux level is dependent on the voltage across the stator windings. If you connect the motor in star or in delta, the winding voltage must remain the same. In the case of a 400 volt 50Hz motor designed for delta operation, you would need to connect to 692volts in star connection to keep the flux in the iron correct. In delta, the voltage across a winding is equal to the line voltage. In star, the voltage across a winding is equal to the phase - neutral voltage.


Q2. You connect the motor according to the manufacturers instructions. If the motor was designed to operate in star at the voltage you are using, then you connect it in star.


Q3. Connecting a delta designed motor in star at the normal delta line voltage, reduces the voltage across the windings by 1/rt3. This reduces the start current to one third and the start torque to one third. This is used in the star delta starter as an attempt to reduce the start curent. There are a number of issues with this form of starter, but it is very common in some regions. see http://www.lmphotonics.com/star_delta.htm


Best regards,

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