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Revision 8.6
SECTION 2 : SYSTEM DESIGN
26
2.15. SELECTION OF ROTOR CONTACTORS
With Thyromat control rotor contactors are mainly used in Hoist applications.
When a rotor contactor switches ON the current rises due to a shift from a greater slip resistor to a smaller
slip resistor. The current peaks are limited to a value between 1.5 and 2.5 times that of the rotor rated
current under full load conditions.
The rotor contactors will switch off under zero current conditions provided that the switching times of these
contactors are faster or at least equal to the stator contactors switching times. In the event that the
switching times are longer the relevant parameters for phase shifter ON and OFF time delays have to be
adjusted accordingly.
There are four basic configurations used to connect the rotor contactors, the most popular configurations
used are ‘Delta’ and ‘Star’ configurations and although not as popular, ‘V’ and ‘W’ configurations are
sometimes also used. In the interests of promoting reliability, MH Automation has a conservative
approach to the selection of the contactors and recommends that the continuous rating of the contactors
are used rather than intermediate duty which is used for the intermediate rotor contactors.
The formula used to select the correct rotor contactors for the applicable connection configurations are
listed in the following paragraphs:-
2.15.1.
Star Connections
Contactor
thermal current
(Ith)
= Motor Rotor Current
Example: Motor Rotor Current =
100 A
Contactor selected ≥
100 A Ith
Ith
=
AC1
contactor rating
2.15.2.
Delta Connections
Contactor
Ith
= Motor Rotor current
1.4
Example: Motor Rotor Current =
100 A
Contactor selected ≥
100
1.4
=
71.4 A Ith
