Why 3 phase induction motors are mostly delta connected?

The reason 3 phase induction motors are mostly delta connected is because using that connection there is no path for ground fault currents. The most common electric faults in any industrial installation are ground faults, using this connection the continuity of the motor operation (availability) is improved.

Lets be specific with the 3-Leads Induction motors. These motors are considered as small motors in industry and can be operated through DOL starter. These motors are mostly Delta connected because:

1. Delta connection winding can be done in a smaller diameter than those of a Wye.

2. This reduces the wire cost and often simplifies manufacturing.

3. Motor provide maximum toruque than the Wye connected motors because Delta connected motors operate on L-L Voltage/phase and Wye Connected Motors operate on L-N Voltageon each phase.

Requirements vary as per the process requirements, manufacturer requirements, motor size requirements etc. Case and details are different for 6 Leads & 9 Leads motors.

Regards,

 Insulation will not be problem as voltage level is less.

1. 1. Starting current will not be problem as starting power in all will be less. So no problem of voltage dips.
   2. Starting torque should be large, as motors are of small capacity.

while designing an ind: motors(IM) starting method has to be considered and most simple method is star-delta if you do not have power system capability for DOL(direct on line starters)starting. for that you have to design it in delta configuration.where i work ,most of the IM's are star connected and are with DOL starters.Wherever delta connected IM is they all are designed for star-delta starting.and of course during its normal operation delta has a marginal(depending on rating)saving in Cu loss and thus heat load.

in delta connected the current in the stater winding is lesser than star connected winding, resulting in less heating for the same power

These motors are mostly Delta connected because to 

1. This reduces the wire cost and often simplifies manufacturing.

2. Delta connection winding can be done in a smaller diameter than  a star.

3. Motor provide maximum toruque than the star  connected motors because Delta connected motors operate on L-L Voltage/phase and star Connected Motors operate on L-N Voltageon each phase.

Requirements vary as per the process requirements, manufacturer requirements, motor size requirements 

For the using full power of motor,but for the big motor,for the reduce starting current using star delta switching

In 3phase induction moter Ruter(armature) mostly still and field are moving .. thats why they whould be mostly in delta connection .. :-)

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The 3 phase induction motors are mostly delta connect for Low voltage as it is cheaper to have this connection for specific KW requirement in comparision to Star cobnneceted motor and Torque is also high.

The 3 Phase induction motor are mostly Star connected for High voltage i.e 3.3 KV, 6.6 KV, 11 KV etc, as the cost of winding is dependent on insulation cost. In delta winding the insulation cost will be high.

So the basic reason for going for Delta or Star is COST optimzation.