Basic application 35
4.2 Main Contactor operation
The control of the main contactor is very important. Incorrect implemention is the main cause of failures.
See also 6.5 CH ANGE PARAMETERS / STOP MODE RA MP and 6.5.1.1 Block diagram of contactor control.
The essential elements of controlling the contactor are as follows.
1) It must be possible to re lease the contactor without relying on electronic s.
2) The contactor must not break current. To obey this rule the foll owing applies:-
a) The PL/X must not attempt to deliver armature current until after the contactor has closed.
b) The armature current must be brought to zero before the contactor has opened.
3) The contactor control circuit must be compatible with all likely application requirements.
The PL/X has been designed to c ontr ol all of the a bove requi rements in the use of the main contactor.
The purpose of the main contactor is to provide mechanical isolation of the motor armature from the po wer
supply. In the event of an emergency it must be possible for the supply to be removed electromechanically
(without the aid of semiconductor electronics). This requirement is usually mandated by safety codes.
Under normal operation the contactor is controlled by the PL/X according to the programmed requirements of
the user. See 6.5 CHAN GE PARA METERS / STOP MODE RA MP. The CSTOP (coast stop) terminal T34 goes
directly to the 24 V coil of the internal contactor control relay. (Relay contact is on T45 and T46). If this
terminal is provided with 24V then the relay (and hence the main contactor) is ready to be controlled by the
PL/X. If the CSTOP terminal is opened then the relay will either not energise, or de-energise and release the
main contactor. There is a capacitor across the relay coil which causes it to have a defined drop out time of
approx. 100mS. This ensures that the PL/X has time to commutate the armature current to zero before the
contacts open.
It may be necessary for installations to have over-riding external independent
systems for de-energi sing the main contactor. In this case it is rec ommended that
the CSTOP terminal be opened 100mS in advance of the main contacts opening.
Failure to achieve this may result in damage to the unit.
Note. If the users main contactor has a closing time delay of greater than 75mS,
then it is essential that steps are taken to delay the release of armature current until
the main contact has closed.
1) Insert an auxiliary normally open contact on the main contactor in series with the RUN input on T31.
2) Alternatively use contactor wiring method shown in 4.3.2.
Contactor coils usually have a high inductance. When the contactor is de-energised it can produce high
energy arcing on the internal PL/X control relay. This may degrade the life of the relay and/or pr oduce
excessive EMC emissions. Ensure that the contactor coil is snubbered.
4.2.1 Contactor control questions and answers
Question. Why is it so important to prevent the contactor 1) Breaking current or 2) Making current?
Answer. 1) Breaking current. The motor armature is an inductive load. This helps to smooth the current by
storing electrical energy during a charging period and releasing it during a discharging period. Ho w ever if the
circuit is suddenly broken then the stored energy has nowhere to go. This results in a rapid rise in voltage as
the inductor (motor armature) seeks to find a discharge path. This rapid transient may cause thyristors in the
armature bridge to avalanche on and become conductive. If this happens to a pair of thyristors then an
effective short circuit may be formed across the armature. Then a second effect occurs. If the motor is
rotating and is suddenly shorted then the mechanical energy stored in the rotation of the motor and load is
then generated into the short circuit. This could be a destructive amount of energy. The thyristors then
become permanently shorted, and the next time that the contactor closes, the supply fuses will blow.
Solution.
Always let the PL/X control the contactor. It has been designed to hold the contactor in while it safely
quenches the armature current. Use the CSTOP for emergency opening of the contactor via the PL/X. This
terminal is electromechanical but also lets the PL/X quench the current in time. If safety codes prevent the
PL/X from being used in the emergency stop sequence, ensure that the CST OP is opened 100mS prior to the
main contactor opening.
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