see e.g.:

Q. I am an apprentice and would like your help please. I have wired up a motor in star-delta starter. The motor will run in star but when it switches from star to delta it blows fuses. I have run it in star alone and delta alone, checked the motor wiring, current etc but found nothing any suggestions?

A. It may be that your wye-delta starter is an open-transition type, but your application requires a closed-transition type.

Open-transition means the motor is momentarily disconnected from the supply voltage while the motor windings are switched from wye to delta configuration.

Closed-transition means the motor windings are kept energized until the switchover from wye to delta configuration is completed.

Basic wye-delta starters are open-transition starters. Closed transition starters use an additional contactor and resistor bank to keep the motor windings energized for a few cycles until the transition from wye to delta is completed.

The open-transition starter is recommended for high inertia loads, where the load will not appreciably slow down during the transition from wye to delta. Closed-transition starters are recommended for high friction loads.

On open-transition starters, large voltage transients and high current inrush may occur when the motor is re-connected to the delta winding, depending upon how much the motor slowed down during the transition. This is a function of the load inertia and friction.

Log onto the websites of Square D and Cutler-Hammer, manufacturers of reduced voltage wye-delta starters. Run a search under “wye-delta starters.” Check out wiring diagrams and application notes on wye-delta starters to see the difference between an open-transition type and closed-transition type. Search the Square D website for document number 0140CT9201, Wiring Diagram Book, and for document number 8600PD9201, "Reduced Voltage Starting Of Low-Voltage, Three-Phase Squirrel-Cage Induction Motors." Download and keep them for future reference.

Square D website: www.squared.com

Cutler-Hammer website: www.eatonelectrical.com

http://expert.ecmweb.com/resistance_grounding_experts/page/7...



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Note added at 45 mins (2007-01-03 13:57:12 GMT)
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Or 'star-delta'. IMO 'wye' is the more common term.