Technical information

The same motor as in the previous example is assumed: 20 hp (15 kW) a 480 V.

The line to be used is the second, already highlighted, for the motor values

between 15 hp and 40 hp: il “NEMA size” required is 2 for all three contactors.

To make the example clearer the contactors manufactured according to NEMA

standards, the 500 series is looked for in the catalogue.

It is immediately noted that the choice of the components is made on the basis of

criteria that differs from the European criteria: there are not categories of use; the

contactors are divided first of all on the basis of the use provided for and then on

the basis of the NEMA size.

From the catalogue, we find that the contactors for motors, size 2, suitable for the

start in the example is the 500-COx930. No other type of sizing is required.

EXAMPLE 4

The case is assumed of a start where the switching from wye to delta happens

when the motor has already started at sufficient speed, but the motor is mark ed

with a diff erent r ation between the lock ed r otor curr ent and the full load curr ent.

The following is assumed: LRA = 9 x FLA.

The ration different from 6, between LRA and FLA obliges us to calculate the

sizes of the contactors because table 33.3, cited in example 3, is no longer

applicable.

A three-phase 20 hp (15 kW) at 480 V motor is assumed. From the table it can

be seen that the motor's FLA is 27 A: it follows that the LRA is 243 A (i.e. 9 x 27 A).

The functioning 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