star delta motors

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Thread Starter

dipin

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.
 
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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.
 
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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
 
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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: johan.bengt[email protected] Internet: http://www.pol.se/ ----------------------------------------
 
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