Sunday, 4 September 2016

Why LT Motors are Delta connected but HT Motors are Star?

In industries Low Tension i.e. low voltage Motors like Motors supplied by three phase 415 V are normally have Stator connected in Delta while High Tension i.e. Motors supplied by high voltage like 6.6 kV have Stator connected in Star configuration. The reason behind this is technical while making the Motor economical.

Following are the main reasons due to which high voltage Motors stator are connected in Star:

  • As the Stator winding of Motor is to be connected with high voltage, it is better to configure the Stator in STAR as in this configuration, the phase current remains the same as the line current but the phase voltage reduces by to Vph = Vline/1.732 which means that insulation requirement from phase winding will be less.
  • The second most important reason is that, starting current for Motors is 6 to 7 times of full load current. So start-up power will be large if HT motors are delta connected. It may cause instability i.e. voltage dip in case of small Power System. In STAR connected HT Motors starting current will be less compared to delta connected motor as voltage is Vph and current is line current. So starting power and starting torque are reduced.
  • As current is less in STAR configuration, copper (Cu) required for winding will be less.


Following are the main reasons due to which low voltage Motors stator are connected in Delta:

  • In Delta connection, the insulation requirement will not be problem as voltage level is less in LT Motors.
  • Starting current will not be problem as starting power in all will be less. So no problem of voltage dips.
  • Starting torque should be large, as motors are of small capacity and hence Stator should be connected in Delta to have more current and hence more starting torque.

Thank you!


Anonymous said...

How can Iline in star and Iline in delta be the same? If there is no neutral point the Iline will be √3 more than Iline in delta.

Aditya Kumar said...

This is all about phase. You can understand like this, three windings i.e. R, Y and B phase which are 120 degree apart in space are connected either in Star or in Delta. So phase current here means the current flowing in either winding. If you carefully observe Delta connection, you will notice that line current is bifurcated in two different windings i.e. phase, therefore Line Current in Delta = 1.732 Phase Current. But case is different in Star, you observe that line current in Star is going to only one phase winding so phase current = Line Current.

Did you get it friend?