Technical information

The choice of the contactors occurs as in a normal direct starting supposing that

each of them commands a dummy motor with current, at full capacity and with

blocked rotor,equal to those calculated.

In the catalogue, the contactors are specified by power so it is necessary to

convert the calculated current values into power values (hp).The procedure is

opposed to that for obtaining the FLA: starting from the calculated current the

three-phase motor is sought that, at 480 V, has an FLA that is not lower.The

following is obtained

Of course the wye and delta contactors remain the same while the wye is now of

a smaller size.

Scaling in accordance to North American criteria as just shown, while following

precise tables defined in UL508A, brings us to make product choices identical to

the ones of the drawings done in accordance with European criteria.

EXAMPLE 3

The normal case is assumed with a start where the switching from wye to delta

occurs when the motor has already started at sufficient speed: It is furthermore

supposed that the motor is standard: LRA (block ed r otor curr ent\) = 6 x FLA (full

load curr ent).

On the basis of the hypothesis made it is not necessary to size the contactors on

the basis of the currents but reference may be made to table 33.3 of UL508A.This

table on the basis of the power and voltage of the motor defines the NEMA size of

the contactors to use: the NEMA size is a construction size that is shown in the

catalogue or on the component.

BRANCH CIRCUIT

Contactor code

FLA calculated Motor power Contactor

[A] FLA print out Power

1M 16

15 hp 100-C23

21 A15 hp

2M 16

15 hp 100-C23

21 A15 hp

7.5 hp 100-C12

11 A7.5 hp