Motor Wye-Delta Starting Circuit: Shifting each phase to the next one over? Shorebrook Exam Problem 51

[akyip]

Hey guys,

On Shorebrook PE Power Exam, question 51 asks to identify which circuit is not a wye-delta starting circuit for motors.

One thing I noticed in each of the wye-delta starting circuits on choices A, B, and C is that each of these circuits shifts each phase over to the next (A --> B, B --> C, and C --> A or A --> C, C --> B, B --> A). I understand the basic purpose and function of a wye-delta starting circuit is to start the motor at Y-phase or line-to-neutral voltage for reduced voltage at start-up, and then later switch over to delta-phase or line-to-line voltage for full rated voltage. This is to reduce starting torque and voltage drop at start-up... but how does the swapping of each phase over to the next help in the wye-delta starting circuit?

Thanks for any input on this!

 

[xnuke]

Sometimes rolling the phases reduces the current spikes that can be caused by a wye/delta open transition so that nuisance breaker trips don't occur, but that doesn't always help. Since the purpose of wye/delta starting is to reduce inrush currents, having large current spikes when open transitioning defeats the purpose of the starter. Closed-transition wye/delta starters don't normally have the same transition current spikes. A good discussion of the factors contributing to nuisance trips of breakers when using the opens transition wye/delta starters is discussed here: Why Wye-Delta Starters Cause Breakers to Trip.

(Also, if the other three answer choice diagrams didn't have the wiring show the phases rolling, there'd be no difference between them, and the correct answer would be obvious. The question isn't really testing knowledge of wye/delta starting, but changing direction by swapping two phases.)