Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 47)

Contactor size I
e(AC-3)
[A]
Fig. 2.3-8
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-4.
The diagram applies up to U
e
=690 V, 50/60 Hz.
Example
Background:
Squirrel-cage induction motor 15 kW, 400 V, 29 A, plugging, switching off rotor at standstill at
I
A
= 6·I
e
, expected life span = 0.2 million switching operations.
Objective:
Rating of starting and braking contactors.
Solution:
The starting contactor (circuit making only) is selected according to the maximum permitted
rated power at AC-3 (see
Fig. 2.3-7 ): C30.
The brake contactor is selected according to the maximum permitted rated operational power at
AC-4 and 0.2 million switching operations according to Diagram
Fig. 2.3-8 : C72.
For mixed service, i.e. service of the contactor with AC-3 and AC-4 switching operations, the life
span results from the sum of the loadings. In the catalogs, diagrams for certain %-rates of AC-4
operations, for example 10 %, are provided. The RALVET electronic documentation is available
for determining the life span for other percentage rates, or direct inquires must be made.
If in practice the electrical life span was considerably shorter than desired, there are several
possible causes and explanations:
- More switching operations than expected, e.g. operated by extremely sensitive controller.
- More frequent inching than expected, e.g. unskilled operation.
- Permitted frequency of operation exceeded, e.g. chattering contacts
- Short-circuits, e.g. switching pause too short for reversing or star-delta starters.
- Synchronization with the supply voltage. Semiconductors as controllers could for example
always switch off at the same phase angle and act in the same direction of current-flow
(results in one-sided material migration to the contacts like in direct current control).
Assessment of the contacts
In conjunction with the electrical life span, the question often arises of assessment of contacts
after a certain service period for continued serviceability. At least with large contactors, the
contacts can be inspected.
Already after the first few switching operations, there are clear signs of burn-off on the contact
surface. After a relatively small number of switching operations, the entire contact surface
becomes roughened and blackened. Black deposits and traces of arc extinction can be seen on
LVSAM-WP001A-EN-P - April 2009 2-19