OK. But it will be a lengthy one...
The controlled rectifier switches from one phase to another. The switching (delay angle) determines what voltage is output to the motor. 30 - 90 degrees produce a positive (hoisting) voltage and 90 - 150 degrees lowers the load (negative voltage).
A conducting thyristor cannot be switched off in any other way than taking current down below holding current. There are several ways of doing it. In a single-phase system, you just wait for voltage to reverse or (if load is inductive) current to become zero. In a three-phase system, things get more complicated. If you fire the next thyristor while the actual thyristor is conducting (it always is when you have continuous current in the armature) then you get a short across the phases.
This short carries as much current as you have in the armature (inductive, constant current for the duration of the short) and the current in the conducting thyristor is reduced while the current in the recently fired thyristor is building up. The time it takes to switch from one thyristor to the next is called "commutation time" and during this time, the two phases are short-circuited. The resulting voltage is the mean voltage of the two voltages involved. That is why you get the deep notches shown in the picture.
The commutation time, the width of the notches, is determined by the impedance of the sourcing grid. The impedamnce is mainly inductive (generator X'') and a high value makes the notch wider than a small value. The width is usually controlled by putting commutating reactors before the rectifier. In many cranes, the generator X'' serves as reactors. It is so high that additional reactors are not needed or possible.
The supervising circuitry in the controlled rectifier has a limit as to what it can accept before saying "Enough!" and shutting the system down. Since the depth of the notch is independent of current and reactance, it is the width (which determines both higher component harmonics contents and RMS of the resulting voltage) that is critical.
By using a generator with higher rated output, the X'' is reduced, the notch width reduced and the rectifier is happier. That's why a higher rated generator helps. Or one with a lower X'' - but usually not possible to reduce X'' for a given generator size.
There are also other things to consider - a crane usually needs to handle dynamic overloads and if the generator cannot handle that as good as motor and rectifier can, then the crane is not going to be popular with the harbour people. It may ultimately result in the harbour losing customers. It happened in the harbour where the picture in the earlier posting was taken.
Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
