Current Flowing in a star delta motor cables

[johnno]

People,
Can someone explain the mathematics behind the current that flows in one of the legs of a motor in star delta configuration. There are six connections - one from each end of the six coil terminals.

I believe that if the motor full load current is 100%, then the current magnitude in each leg is 58% (=1/sq root of 3).

Much Thanks

 

[llangford]

If you connect star then the phase voltage is 0.577 line voltage. If you connect delta then phase voltage is line voltage.

 

[llangford]

Adding to previous post:
There are three circuits in the motor. Phase A is T1 and T4. Phase B is T2 and T5. Phase C is T3 and T6. The motor you have may have different markings such as U1-U2, V1-V2, W1-W2. Same thing. When the motor is connected star, or wye, the voltage that is applied to the winding is 1/1.732 times phase voltage. When the connection is changed to delta then phase voltage is line voltage.
If you have a nameplate rated 460 volt motor rated star start and delta run then the motor will be connected for 796 volts on start but will only get 460 volts applied; so it starts a little slower. When the contactors connect it to delta then the motor attains full power. Acceleration time is dependent on the load. Hope this helps.

 

[busbar]

Yes, the current in each of six leads is 0.577 per-unit of nameplate. Remember to apply (99)NEC310-15(b)(2)(a) ampacity-adjustment factors if the six leads are in a raceway or cable.