Ratings for Coils

Home | Articles | Forum | Glossary | Books


The specification ratings for contactor coils provide information regarding inrush current, sealed current, and operating time. From Ill. 1 notice that the inrush current for a size 00 contactor with a 208 volt coil is 0.29 A. The seal-in current for the same coil is 0.07 A. The inrush current for a size 2 contactor with a 120 volt coil is 1.8 A and the seal-in current for the same coil is 0.25 A.


Above: Ill. 1: Inrush current, sealed currents and operating times for coils of NEMA rated contactors. (Click image to enlarge).



The operating time is provided so that calculations regarding pick-up time and drop-out time can be made. Pick-up time and drop-out time are important in some applications where the contactors are controlled by microprocessor systems or programmable logic controllers where the sequence of operation is crucial. All of the times in this table are listed in milliseconds.

Exercise

Determine the amount of inrush current and sealed (holding) current for the coil of the number 3 starter that one selected in the previous problem for use in controlling the 75 A electric heating coil. Keep in mind that the heating element uses 480 volts, three-phase AC voltage, and the coil for the contactor is 208 volts. Use the table shown in Ill. 1 to determine the inrush and sealed currents. Use the second table shown in this figure to determine the operating time for pick-up and drop-out times for the size 3 contactor.

Solution

Since the contactor in this application is used to control three-phase voltage, the coil has to move three contacts when it opens and closes. The table in Ill. 1 shows that the size 3 starter with three poles and 208 volts coil uses 2.78 A to pull in the contacts. The sealed current for this contactor is 0.21 A. The time it takes to pick up the contacts is 35 msec, and the time for drop-out is 18 msec.

Top of Page previous related article or category Basic Parts of a Contactor Next related page, article or category Motor Starters Home