Choosing Contactors For Star-delta Starter

[miki83]

Hi guys, need a hand

 

I'm trying to find an optimal solution for starting a 90 kW three-phase asynchronous motor with rated current of 164 Amps. It would be a star-delta starter and I'm not sure on the following issue:

 

Is it necessary to use contactors rated for 90kW ( 180 Amps if I remember correctly)? This ammount of money is almost enough to get an adequate softstarter from Moeller electric. I seem to remember hearing somewhere that when the power circuit changes from star to delta, the two closed contactors don't draw the full rated current. If i remember correctly one would draw 0.58*164= around 90 Amps. Does this mean I could use contactors rated for, say 110 Amps without the fear of damaging them? If there is an online reference regarding this, I would appreciate it very much for someone to share it with me.

 

Thanx up front

[botshe]

Dear sir,

i hope to be able to provide you with your need and try to give a simple solutions or answers for waht you need .

firrst i would like to tell you that almost you write is nearlly correct , specially the formula of contactor size calculation (multiplyer by 0.58)

second , there is an important thing you must have it , the application , i see that is not correct to convert the starting method of the motor just for save a monay or for extra economical way , i prefer to built you knowlage of the starting method based on the motor application ( for example if you need to start the motor as star/delta sure the application needs to make current limitation and sure the apllication starting current almost high or very high , so it is not logic to use a soft starter which works based on the incraseing of the volatge also related to current increases from zero to the rated voltage )

 

best regards

 

Bothe

 

 

[marke]

Have a look at http://www.lmphotonics.com/star_delta.htm

All explained there

 

Best regards,

Mark.

[miki83]

Thanx guys, You helped me a lot, since this is not my primary area of interest, I now know a lot more than I knew three days ago.

 

To Botshe, it's not my decision to make regarding the money I will spend on the starter, they gave me a task in my company, and gave me as much money as they did, so I'm trying more/less to fit in the figure. I know it's not good to save money this way, since if something goes wrong we have a 1 year guarantee on the product, and most likely it would be a case of a damaged/destroyed motor. By the way, motor is getting fitted on a screw compressor. A very biiig one

 

Thanx once again,both to Botshe and Marke.

 

Best regards.

[Shozza]

Miki83,

 

It seems that you are just considering the initial setup costs here and have chosen a cheaper starting method to suit. In the long term using a star-delta starter would probably prove to be more expensive than the soft starter option if you take in to consideration the maintenance costs and wear and tear on all mechanical components (motors, gearboxes, belts, couplings etc). A soft starter is probably not too much more expensive, provides a better soft start meaning less wear and tear on mechanical components and usually has superior inbuilt overload protection capabilities which enable the motor to be used right up to its thermal limit without nuisance tripping meaning less down time and less lost productivity.

It might be worth the extra cost up front to save money in the future.

Anyway just something to consider.

 

Best regards.

Shozza