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What Will Happen if Delta Connected Motor in STAR
Motor winding heated due to wrong connection?

we are using 3 hp motor in tray separator. motor is delta connected as per name plate but we ran the motor in STAR. now motor one coil has burnt. explain me the reason?

Motor run for 3 months. why it failed after 3 months?

Moderator's Note: By STAR, he may mean Wye connection.

It would depend on the configuration of your motor. But judging from your use of "Star" vs "Wye", I can assume you are in a place other than North America, and that your motor was a Dual Voltage version, as in 380/220V (or 400/230 or 415/240, basically the same thing). In that type of motor, the Delta connection would be for the LOWER voltage, the Star connection would be for the higher voltage. So if you HAD 230V available, but you used the Star connection instead of the Delta connection pattern, you were essentially starving your motor of voltage. The effective voltage across the coils would be reduced by the square root of 3 (1.732) the inverse of which is 58% of what the motor was expecting to get.

It would appear to the naked eye to run at the normal speed; AC induction motor base speed is only related to the number of poles and the applied frequency, neither of which would be different here. The torque capacity of the motor, however, would be severely curtailed; running torque would be no more than 58% of normal, but starting and re-acceleration (peak) torque would be reduced by the SQUARE of that reduction to just 33% of maximum peak torque capacity. So normally, the peak (Breakdown) torque of the motor. which is uses to re-accelerate after a step change in load, would be 200%, but in this case it would be reduced to just 66% of FLT. The motor would therefore run at a higher slip with any appreciable load, which would cause it to draw higher than normal current, yet perform less work.

when you connect the delta connection motor into a star connection mode, the effective per phase voltage is reduced to (1/1.732) times hence to maintain the torque/speed it will draw 1.732 times higher current.

hence the motor will continue to run for some days/weeks or months depending upon your ambient and other cooling factors. but after a certain period, its winding will definitely burnt out which happened in your case also.

that's it.

re-wind the motor with higher size copper wire i.e. at least double of its existing size, then it will be stable.

By Gaurav Katiyar on 6 December, 2018 - 9:02 pm

Sir,

>we are using 3 hp motor in tray separator. motor is delta
>connected as per name plate but we ran the motor in STAR.
>now motor one coil has burnt. explain me the reason?
>
>Motor run for 3 months. why it failed after 3 months?
>
>Moderator's Note: By STAR, he may mean Wye connection.

pl. note that the motor full load current is specified for the connection type mentioned on the name plate. now considering your case you have delta motor with name plate full load of 3A (say) this is the line current and the actual current in the winding will be 3/1.732 = 1.732A. now if you connect this to star then line current equals winding current and since the winding is rated for only 1.732A you need to change the overload settings from 3A to 1.732A so that the motor winding don't get Overloaded.

As you have said that the motor got burnt, i am sure that this is due to running the winding at higher current than rated.

other things that happen due to changing delta to star is,

reduced starting torque, reduced speed.

it is generally recommended to convert delta motors running at less than 63% load from delta to star. this will conserve energy increasing the efficiency by increasing the loading on the motor.