Building the PLC panel
It is uncommon for engineers to build their own PLC panel designs (but not impossible of course). For example, once the electrical designs are complete, they must be built by an electrician. Therefore, it is your responsibility to effectively communicate your design intentions to the electricians through drawings. In some factories, the electricians also enter the ladder logic and do debugging.
This article discusses the design issues in implementation that must be considered by the designer.
Electrical wiring diagrams of a PLC panel
In an industrial setting a PLC is not simply “plugged into a wall socket”. The electrical design for each machine must include at least the following components.
- Transformers – to step down AC supply voltages to lower levels
- Power contacts – to manually enable/disable power to the machine with e-stop buttons
- Terminals – to connect devices
- Fuses or circuit breakers – will cause power to fail if too much current is drawn
- Grounding – to provide a path for current to flow when there is an electrical fault
- Enclosure – to protect the equipment, and users from accidental contact
This power must be dropped down to a lower voltage level for the controls and DC power supplies. 110Vac is common in North America, and 220 V AC Is common in Europe and the Commonwealth countries. It is also common for a control cabinet to supply a higher voltage to other equipment, such as motors.
Motor controller example
An example of a wiring diagram for a motor controller is shown in Figure 1. Note that symbols are discussed in detail later).
Dashed lines indicate a single purchased component. This system uses 3 phase AC power (L1, L2 and L3) connected to the terminals. The three phases are then connected to a power interrupter. Next, all three phases are supplied to a motor starter that contains three contacts, M, and three thermal overload relays (breakers).
The contacts, M, will be controlled by the coil, M. The output of the motor starter goes to a three phase AC motor. Power is supplied by connecting a step down transformer to the control electronics by connecting to phases L2 and L3. The lower voltage is then used to supply power to the left and right rails of the ladder below. The neutral rail is also grounded.
The logic consists of two push buttons:
- Start push button is normally open, so that if something fails the motor cannot be started.
- Stop push button is normally closed, so that if a wire or connection fails the system halts safely.
The system controls the motor starter coil M, and uses a spare contact on the starter, M, to seal in the motor starter.
Aside: The voltage for the step down transformer is connected between phases L2 and L3. This will increase the effective voltage by 50% of the magnitude of the voltage on a single phase.
The diagram also shows numbering for the wires in the device. This is essential for industrial control systems that may contain hundreds or thousands of wires. These numbering schemes are often particular to each facility, but there are tools to help make wire labels that will appear in the final controls cabinet.
In the cabinet the AC power would enter at the terminal block, and be connected to the main breaker.
It would then be connected to the contactors and overload relays that constitute the motor starter. Two of the phases are also connected to the transformer to power the logic. The start and stop buttons are at the left of the box (note: normally these are mounted elsewhere, and a separate layout drawing would be needed).
The final layout in the cabinet might look like the one shown in Figure 1.
When being built the system will follow certain standards that may be company policy, or legal requirements. This often includes items such as;
- Hold downs – the will secure the wire so they don’t move
- Labels – wire labels help troubleshooting
- Strain reliefs – these will hold the wire so that it will not be pulled out of screw terminals
- Grounding – grounding wires may be needed on each metal piece for safety
A photograph of an industrial controls cabinet is shown in Figure 4:
When including a PLC in the ladder diagram still remains. But, it does tend to become more complex. Figure 5 below shows a schematic diagram for a PLC based motor control system, similar to the previous motor control example.
This figure shows the E-stop wired to cutoff power to all of the devices in the circuit, including the PLC. All critical safety functions should be hardwired this way.
Electrical control panels including PLCs and HMIs
Reference // Automating manufacturing systems with PLCs by Hugh Jack