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 fully 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 output (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

The fault for which the "wye" is larger than the line contactors and delta is

immediately clear: this is a direct consequence of the case for which the

switching occurs when the rotor is blocked.

EXAMPLE 2

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

occurs when the motor has already started at suff icient speed:

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

of which the rated current and the value of the current when the rotor is blocked

are 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 14 A). In this case the operating currents of the contactors are:

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

S53

50 hp 100-C72

65 A50 hp

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 FLA 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 27 A z 9 A