What is a PLC and how it works?

READ:

https://en.wikipedia.org/wiki/Programmable_logic_c...

https://en.wikipedia.org/wiki/Variable-frequency_d...

The first slot on the left is a digital computer on a PLC and it turns on and shuts down the VFD. High tech stuff that pays more than a living wage. Allen-Bradley made my high tech stuff. Allen-Bradley has schools to teach you with a catered lunch.

It's a little computer with inputs and outputs designed to interconnect with (usually) 24V switching signals, so the inputs can work from switches and relay contacts etc., and the outputs can drive lights, relays and such.

They are usually made to be programmed by the installers or users, often using a simple "language" known as ladder programming, that graphically emulates a circuit diagram using switches and relays.

There are hundreds of different types of PLC, some all-in-one, some modular where you have a main controller then stack extra input and output units with it - possibly hundreds or even thousands of connections.

There are also units to read or output analog voltages, counters, position encoder interfaces, communicate with other devices or PLCs to expand the system, give web browser access to monitor and control things & numerous other interfaces and add-ons.

If a machine or plant that's controlled by PLCs also uses motors, it's not at all unusual for those to be controlled by the PLC.

That can be simple on/off switching for fixed speed motors, an analog speed and on/off/direction control to a servo drive or VFD, or they are also often connected by a standard network system such as PROFIBUS, so just a single data cable can link PLCs, displays, drives and remote input/output units or other PLCs together.

See the photo below - that's the PLC section of a machine we built some years ago.

The main PLC is at the left of the top row, then the rest of that row is additional input and output units connected to that.

The middle row has a couple of 24V power supplies and power distribution terminals & fuses, then the machine safety controller (red item), another small power supply and a second PLC.

The third row has fuses and terminals for power in to the PSUs, then a line of 24V relays that switch higher voltage or power circuits (eg. 110V), then more I/O expansion units connected to the second PLC.

The light gray pieces are "cable trunking" to keep the wiring neat; the one between the first two PLC rows has the cover off, so you can see all the wires inside it.

The thick purple wire is a network cable that connects those two PLCs; it also goes to another PLC in the machine control station, rather than having a massive bundle of individual wires and more PLC inputs etc. in the main electrical box.

(The PLCs & expansion modules in that machine are all Siemens S7-200 series; they are each approximately 100mm tall, to put the overall picture in proportion.).

That system controls seven motor drives of various types, some BLDC servos and some thyristor speed controllers.

Plus many other motors and monitoring dozens of sensors for positions, pressures, oil levels etc. to ensure everything is working as it should.