wye versus delta motor windings

[HDMANN]

What (if any ) are the advantages of wye versus delta motor windings ?
I understand most two speed motors are equiped with both windings ( wye for start and Delta to run ) but what are the differences ?

 

[jbartos]

Suggestion: The motor has one set of winding. Ends of windings are available in the motor terminal box for delta or wye connection. Normally, wye connection is used for the motor start and delta connection for the motor run. Wye connection develops smaller torque, draws less current, and it is used temporarily only to accelerate motor to high percentage of the motor rated speed.

 

[electricpete]

Wye winding is preferred for large motors since it does not allow any 3rd harmonic currents.

(Reference: Liwschitz-Garak Electric Machinery Volume II Page 138.)

 

[gsimson]

For LT motors, current carrying capacity is important for sizing, hence delta is preferred as phase voltage = Line voltage.
In some applications star is used during starting to reduce the starting current and stress on the power system

For HT motors , voltage withstand capacity is important. Hence star is preferred as Phase voltage = 0.577 * line voltage.

 

[jbartos]

Suggestion: The wye winding is more often applied to medium voltage motors since the electrical field stresses on winding insulations are smaller than at delta windings. However, the wye winding will propagate harmonics into magnetic circuits and into the rotor just like the transformer. Triplen harmonics will not circulate in the stator, as it is the case for the delta winding.

 

[freddycor]

And what about in fault conditions? supossing the same rating in HP but different conection star or delta?

Thanks a lot

Alfredo Corona

 

[jbartos]

Suggestion: With a help of delta star transfiguration, it can be shown that:
1. Short circuit current Iwye,ab,sc between the phase a and phase b on the motor input terminals for wye connected motor is:
Iwye,ab,sc = Vt / 2Z
where
Vt is the motor terminal voltage
Z is impedance of one star winding wing
2. Short circuit current Idelta,ab,sc between the phase a and phase b on the motor input terminals for delta connected motor is:
Idelta,ab,sc = Vt / 2Z

Therefore, Idelta,ab,sc = Iwye,ab,sc

The short circuit current phase to phase on the motor input terminals a and b is the same for delta connected motor and wye connected motor for the same motor terminal voltage Vt.