The voltage rating of the contactor(s) must be equal to or greater than the electricity supply voltage.
The contactor(s) must have type 2 coordination when used in combination with the selected fuses and overloads. The contactor(s) must have a minimum AC-3 (make locked rotor current, break full load current) rating for the motor full load current. The contactor(s) must have a short time withstand (Icw) in excess of the fuse time-current characteristic.
The over riding consideration is the type 2 coordination as this will give the minimum contactor rating, the motor full load current, starting method and the projected frequency of operation should then be taken into account to determine the contactor sizes to be used.
Where direct on line is the starting method then the contactor selection is in accordance with the above, this is also true for main contactor for star delta, Korndorffer and rotor resistance starting.
With star delta starting the star and delta contactors are subjected to a lesser load. The star contactor to ⅓ of the full load current and the delta contactor to √3 of the full load current therefore the star and delta contactors may be selected for a lesser current however they must still fulfil the requirements regarding the type 2 coordination and the short time withstand.
Although not used in this article there is an alternative star delta circuit where the starter is configured such that the main contactor is in the delta circuit. The contactor in this instance is subjected to √3 of the full load current therefore the contactor may be selected for a lesser current however it must still fulfil the requirements regarding the type 2 coordination and the short time withstand.
With Korndorffer starting the star contactor is subjected to a lesser load, that load is 55% of the full load current therefore the star contactor should be rated for 55% of full load current, however the contactors must still fulfil the requirements regarding the type 2 coordination and short time withstand.
Although not used in this article there is an alternative Korndorffer circuit where the starter is configured such that the transformer is fed via a start contactor fed directly from the power supply then the rating of that contactor is dependant on the auto transformer tapping. The start contactor at 80% tap to 80%, at 65% tap to 55% and at 50% tap to 37% of the full load current. For flexibility the worst case that is the 80% tap ratings must be used therefore the start contactor should be rated for 80% of the full load current, however the contactors must still fulfil the requirements regarding the type 2 coordination and short time withstand.
Where rotor resistance starting is used the resistance shorting contactors must have a voltage rating equal to or greater than the rotor voltage, with a current rating equal to or greater than the rotor current at each resistance stage.
It is also necessary to be mindful of the need to keep the heat gain within the starter from the contactor(s) to a minimum.
Contactors differ in this respect from the other power circuit components as their operating coils require a larger operating current as the main contact rating increases. Thus increasing the contactor size to reduce the heat gain from the contact resistance will be offset and may be negated by the increase in heat gain from the operating coil.
The contactors must also be equipped with a number of auxiliary contacts for use in the interlocking, control, indication and alarm circuits. Contactors with ratings up to 100A should be able to be equipped with a minimum three normally open and two normally closed contacts, contactors with ratings above 100A should be capable of being equipped with four normally open and four normally closed contacts.
The contactor coils should be suitable for 110Vac both 50Hz and 60Hz electricity supplies have a minimum pick up voltage of 85% of the nominal coil voltage and a minimum drop out voltage of 60% of the nominal coil voltage.