Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 19)

1.6 Control circuits, semiconductor load and electromagnetic load
Regarding the specific aspects of the switching of control circuits, also refer to Section 5.
The utilization categories AC-12 to AC-15 for alternating current and DC-12 to DC-14 for direct
current (see
Tab. 1.1-1 ) make allowance for the specific loading of switchgear for switching of
control circuits with semi-conductors or electromagnetic loads. When electromagnets are
switched, for example contactor coils, particular attention is paid to the increased making load
because of the pull-in current of the magnets and the increased breaking load due to the high
inductance of the closed magnets.
In addition to the switching capacity of the contact in the sense of a maximum permitted load,
very often the key criterion in the switching of control circuits is contact reliability, i.e. the
capability of a contact or a chain of contacts to reliably switch small signals. This is especially
the case for contacts in circuits of electronic controllers and in the signal range of
≤ 24 V / ≤ 20
mA (also see section 5.3.5 ).
1.7 Three-phase asynchronous motors
The three-phase asynchronous motor – also known as the induction motor – is the most
frequently used motor type for industrial drives. Especially in the form of a squirrel-cage
induction motor, it dominates the field of industrial electrical drive technology.
1.7.1 Principle of operation
The key functional elements of the three-phase asynchronous motor (see Fig. 1.7-1 ) are the
fixed stator with a three-phase coil supplied by the three-phase supply network and the revolv-
ing rotor. There is no electrical connection between the stator and rotor. The currents in the
rotor are induced by the stator across the air-gap. The stator and rotor are composed of highly
magnetizable dynamo plates with low eddy current and hysteresis losses.
Fig. 1.7-1
Sectional view of a squirrel-cage three-phase motor with enclosed design
When the stator coil is connected to the power supply, the current initially magnetizes the
laminated metal body. This magnetizing current generates a field that rotates with the synchro-
nous speed n
s
.
n
s
= 60 · f/p
n
s
= synchronous speed in min -1
f = frequency in s
-1
p = pole pair number (pole number/2)
For the smallest pole number of 2p = 2, with a 50 Hz power supply, the synchronous speed is
n
s
= 3000 min -1 . For synchronous speeds with other pole numbers and for 50 and 60-Hz power
LVSAM-WP001A-EN-P - April 2009 1-9