I would like to ask a question about how to start 220/380V 3 phases induction motor by star-delta method with 3phases 380V power source. Practically I think that it is impossible to be able to do it without step-down transformer by convert the power source from 380V to 220V. But, my boss tell me that I need to be able to makes it happen without using transformer. So, is there any suggestion to solve this problem? I just start working so any help will be appreciated. Thanks
How to start 220/380V 3 phases induction motor by star-delta method with 3 phases 380V power source?
3 Answers
That's simple. First the motor is wired in star (Y) and the voltage is distributed over two windings. Each winding gets Vline/sqrt(3) if you calculate from 380V you get 220V. The current is also reduced by factor 1/sqrt(3), while the power is reduced by factor 1/sqrt(3)*sqrt(3) = 1/3.
Once the motor reaches the rpms, it is switched to delta connection, now you get full voltage on windings, current and nominal power.
If the motor is designed to run in star a 380V 3-phase power supply, then it cannot be connected in delta on the 'same' supply.
This is similar to applying 380 volt to 220 v windings so clearly the motor would fail.
The solution is either to get a 3 phase step down transformer to get 220 3 phase voltage and you need to calculate the ratings if the KVA of the transformer based on the load.
OR get an inverter, simply provide a 220V single phase (Line and neutral of the 380V supply) to it and get a 220V 3 phase.
Hope the answer is useful and clear
There are better options than using a step-down transformer. You can use the 380 V motor connection with an autotransformer starter that reduces the voltage during starting and then switches to full voltage. You could also use an electronic starter that does the same thing. Search "reduced-voltage motor starters."