AC2 & AC3 DUTY CURRENT DIFERENCE

Dear,

All electrical motors have a higher starting torque / & so, higher starting current.

Normal DOL Starting induction motors have starting current av. 6 - 8 times Ir

Same motor, when operated with star delta / auto tr. start, it have starting current of the order of 3- 4 * Ir

Also, some applications may require 10 start-stop duty per Hr. While, some applications require only one start - stop per day.

Life of the contacts of the contactor depend upon all these parameters, inspite of thermal current rating may be same for all duties.

Manufacturer provide you the data of contact life of contactors versus Start - stop operations , w.r.t. AC3 duty cycle.

You can work out the life of same current rated contactor with AC4 duty - i.e. inching duty say for hoists , cranes etc...

Normal AC3 duty Ir * (3-3.5 ) - current rated contactor required for same contactor life, for inching operations.

check this post

http://cr4.globalspec.com/thread/36017/What-is-AC1-Ac2-AC3-Duty-cycle