Star Delta Starters for AC Motors

[anilkk]

During expansion of our plant we are providing star delta starters for 75kw, 37kw and 15kw pumps. This is to reduce starting current of the pump motors. This would reduce starting torque as well but the reduced torque is adequate for the service. I would like to know the required rating of star delta starters to be provided for these motors such as contact rating (star & delta side), overload relay current rating, cable rating/size and main contactor rating (in Motor Control Centre). Plesae guide me at the earliest. Thanks & regards,
Anil

 

[Marke]

If you are looking to gain a reduction in starting current, I would suggest that you do a fast recording of the starting current when a star delta starter is used on a pump. You may get a surprise!! The peak current is often worse than Direct On Line due to the open transition switching from star to delta.
If you wnat to minimise the start current, use a soft starter or something else.

Best regards, Mark Empson

http://www.lmphotonics.com

 

[d23]

anilkk

The correct way to size cable for your wye delta starter can be found on this thread:
thread237-42197

Manufacture rate their starters by NEMA size. NEMA continuous amp ratings are as follows:

NEMA Size 00 - 9A
NEMA Size 0 - 18A
NEMA Size 1 - 27A
NEMA Size 2 - 45A
NEMA Size 3 - 90A
NEMA Size 4 - 135A
NEMA Size 5 - 270A
NEMA Size 6 - 540A
NEMA Size 7 - 810A

In the post from Marke he describes the current spike that occurs during transition from wye to delta. Maybe more detrimental to your system is the voltage spike produced. If you have some sensitive electronic equipment on this power system the voltage spikes could cause you some problems. If this spike is a concern you may want to consider a "closed transition" starter or as Marke stated a "soft-starter." Closed transition will take care of the spikes for you.

David

 

[JayantDesh]

Soft starter will be more that 3 time expensive than the cost of the star delta starter...
Check with you nearest Siemens office. Siemens has publlished the switchgear selection chart for variouse rated motor starters. Also they have a special kind of motor starter timers with "an inbuilt time delay fearture in it"
How does it help?.........Well.........it opens the star contactor just 40-50 ms after the delta conactor is closed without causing the short circuit, thus it avoides the spike that is caused in in case of normal star delta starters during the star to delta transition.........

 

[jbartos]

Suggestion: The technical data sheets and catalogs are available from star-delta starter manufacturers, e.g. AB, GE, Cutler-Hammer, Square D, Siemens, ABB, etc.
The voltage is needed to select the proper star-delta starter for your motors.