Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 46)

operation, the starting current has already dropped somewhat by the time the motor is switched-
off. This usually compensates for the effect of any disregarded adverse conditions.
For the most common applications of contactors the electrical life span is presented in the
product documentation with various diagrams:
- AC-1 Non-inductive or slightly inductive loads, for example resistance furnaces
(small making current and cos
φ higher than with AC-3, however full recurring voltage
on switching off)
- AC-3 Squirrel-cage induction motors: Starting, switching off motors during running
(high making current, breaking of the motor rated current)
- AC-2 Slip-ring motors: Starting, switching off
- AC-4 Squirrel-cage motors: Starting, plugging, inching (high making and breaking current at
full voltage)
- Mixed service of slip-ring motors, e.g.
AC-3 90 %
AC-4 10 %
With the curves Fig. 2.3-7 for AC-3 and Fig. 2.3-8 for AC-4 the expected electrical life span for
specific applications can be determined. These curves also can be used to determine the
electrical life span for any application (for example jogging motors with very high or especially
low starting current and any mixed service).
Contactor size I
e(AC-3)
[A]
Fig. 2.3-7
Example of a diagram for determining the electrical life span of contactors as a function of the rated
operational current I
e
for utilization category AC-3.
The diagram applies up to 460 V, 50/60 Hz.
Example
Background:
Squirrel-cage induction motor 7.5 kW, 400 V, 15.5 A, AC-3 (switching off only when running),
operating cycle 2 minutes ON / 2 minutes OFF, 3-shift operation, expected service life 8 years.
Objective:
Selection of the contactor
Solution:
2 min ON + 2 min OFF = 15 switching operations/h. This results for 3-shift operation over 8
years in around 1 million switching operations.
From diagram Fig. 2.3-7 yields for a rated operational current of 15.5 A and 1 million required
switching operations the contactor C16 (electrical life span approx. 1.3 million switching
operations).
LVSAM-WP001A-EN-P - April 2009 2-18