star delta motors


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if i convert delta connected three phase motor to three phasr star connection, what will be changes in following characterstics of motor: a) current rating b) torque rating c) kw rating d) rpm (any slippage) ?
I have only recollection of this: Standard commercial 3 phases motor are wired delta. If you feed star: power rating goes down 1/3. May be also the amps (does it make sense ?) and torque. RPM is same, but if not strong enough, slippage will occur.

Bruce Durdle

Dipin, By connecting a motor in star instread of delta: Voltage across each phase winding falls, so required back emf falls, and hence magnetic flux falls. The decrease is to 1/sqr(3) = 0.577 of the original value. current through each winding falls, also by the same factor. So overall your torqueas a function of speed will fall to 1/3 the original. The delivered power will also fall to 1/3 the original. Motor rating (in terms of the power the motor can handle) will not change - it may be possible, depending on the motor characteristics, to deliver the rated power. Slippage will increase to about 3 x the original slip. Cheers, Bruce.

Dobrowolski, Jacek

Hi, Assuming that you connect the motor after conversion to the same power lines (voltage), all characteristics go like that: a) rated current is smaller by factor 1/1.73, b) rated torque is smaller by factor 1/3, c) rated power is smaller by factor 1/3, d) rated rpm is the same (slippage doesn't change). But that is true only if a motor is under rated load (by means of torque). Regards Jacek Dobrowolski

Johan Bengtsson

The other answers so far covers what happens when you connect the motor to the same power line, I will therefore not cover that. If you instead connect the motor to power line with a 1.7 times higher voltage (to be exact: sqrt(3)) you will have a motor with the exact same characteristics on all of your points except a. The reason is that you by doing this wire the motor in a different way and still get the same voltage across each winding. The current drawn for any load will be lower by the same factor (sqrt(3)). (Note: the current thru each winding will be the same, but not in the power line, if an explanation of that is needed send me a mail and I explain) /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: ----------------------------------------