36 Basic application
Answer. 2) Making current. If the PL/X has been instructed to start making current, but the main contactor
has not yet closed, then the motor will not be able to rotate. This will cause the PL/X to phase further
forw ard in an attempt to produced the desired speed. If the contactor then closes it will present a stationary
motor armature on a fully phased forw ard stack, straight on to the supply, producing destructive current. All
this will occur in a few cycles of current w hich is far too fast for the speed loss alarms to operate.
Solution.
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 show n in 4.3.2.
Question. Plenty of systems do not appear to suffer from failures due to opening the contactor incorrectly so
why is it so important?
Answer. If the armature current is discontinuous, which is very common, then there is much less stored
inductive energy and the current also goes to zero every current cycle. This makes it highly unlikely that a
destructive situation occurs. The high risk situations are regenerative applications and continuous current
modes. Even in these cases it does not al ways result in a destructive sequence.
Question. Even if the contactor operates according to the recommendations ho w is protection afforded if the
contactor coil supply is lost.
Answer. This is a difficult problem to solve using electronics. The only reliable insurance is to insert a DC
semiconductur fuse in the armature circuit. This fuse should open before the thyristor junction fails.
Question. What if the grid system fails totally?
Answer. This is not as bad as losing the contactor coil supply. Most installations naturally have other loads
that provide a safe discharge path before the contactor opens.
Question. What if the grid system fails for a few cycles? (Brown outs)
Answer. The PL/X is designed to ride through these kinds of supply dips. As soon as it loses synchronisation
the armature current is quenched. The armature voltage is then monitored so that when the supply returns
the PL/X picks up into the rotating load at the correct speed.
Question. What other sorts of problems occur?
Answer. Most problems occur when users are retro-fitting the PL/X into an existing system. Sometimes these
systems have previously controlled the contactor via a PLC or Drive healthy relay. These control systems
may not be interfaced correctly with PL/X and situations occur that drop out the contactor too quickly, or
bring it in too late.
Another common problem is that the contactor is controlled correctly for normal running but incorrectly
during jogging or emergency stopping.
Another instance is the installation is designed correctly but the commissioning engineer uses a local op
station to get each PL/X going, that has an in built control problem.
Summary. Use the PL/X to control the main contactor for STOP, START, jogging and eme rgency stop. All
sequencing occurs automatically. Fit semiconductor fuses in the AC supply and armature circuits.
The cost of a fuse is marginal compared to the cost of repairing a damaged dri ve and suffering machine
downtime and engineer call out costs.
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