Technical information

The following terms can be used as an analogy with the motors:

➞ “Locked Rotor Ampacity” (LRA) is the current established at the moment the

contactors close, i.e. the make current;

➞ “Full Load Ampacity” (FLA) is the current that the contactor must open to

interrupt operation, i.e. the break current.

This is the value normally indicated in the catalogues

For the motors it is specified that LRA = 6 x FLA unless they are specifically

marked with a different ratio.

For contactors this is a dummy ratio and by definition LRA = 6 x FLA is imposed.

Contactor functioning currents

The delay set through the special timer for the passage from the wye to the delta

must be sufficient to start up the motor: in this case the contactors switch a

current with a similar to that of full capacity (the FLA of the tables). If instead the

delay set is too short, the contactors switch, with the motor almost at standstill, a

current with a value equal to that with the rotor blocked.

EXAMPLE 1

The extreme case of a start where the switching from wye to delta occurs with a

stopped motor is h

ypothesised:

A 20 hp (15 kW) three-phase motor is assumed at 480 V of which the rated plate

current is unknown and the value of the current with the rotor blocked is

unknown

From the table it is read that the FLA of the motor is 27 A and that LRA is 162 A

(that is 6 x 27 A). In this case the operating currents of the contactors are:

BRANCH CIRCUIT

Contactor code LRA FLA

1M 0.33 x LRA motor 0.577 x LRA motor

2M 0.577 x LRA motor 0.577 x LRA motor

S00.33 x LRA motor

Contactor code LRA FLA

1M 0,33 x 162 A z 53 A0.577 x 27 A z 16 A

2M 0.577 x 162 A z 93 A0.577 x 27 A z 16 A

S00.33 x 162 A z 53 A