1. PLC Power Supply Design
Generally speaking, the basic power supply of PLCs is usually of two types: ACI00V/240V and DC24V.
When the PLC is powered by AC 100V/240V, the allowable fluctuation range of the input power supply voltage is typically -15% to +10%. For example, when the rated input voltage is selected as AC 100V, the allowable input voltage variation range is typically AC 85 to 110V; when the rated input voltage is selected as AC 240V, the allowable variation range is AC 200 to AC 264V. The PLC has relatively low requirements for the frequency of the external AC power supply, and the allowable frequency variation range is typically ±3Hz. That is, when the rated input frequency is selected as 50Hz, the allowable input frequency variation range is 47 to 53Hz; when the rated input frequency is selected as 60Hz, the allowable variation range is 57 to 63Hz.
When a PLC uses a DC 24V power supply, the allowable input voltage variation range is generally -15% to +20% (i.e. , DC 20.4 to 28.8V , such as SIEMEN SLC). Some PLCs can reach -35% to +30% (i.e., DC 15.6 to 31.2V , such as Mitsubishi Q series PLCs). Compared with other computer control systems, its input power requirements are relatively low and are usually easy to meet.
However, to ensure the normal operation of the PLC and suppress line interference, for PLCs powered by AC 100V/240V, an isolation transformer, surge absorber, or voltage stabilization measures should, in principle, be added to the power input circuit. The PLC input power supply should be wired separately from the equipment power supply, AC control circuit power supply, and AC output power supply, and should have an independent protection circuit and an independent isolation transformer.
For PLCs powered by DC 24V, a regulated power supply should be used in principle: at least a three-phase bridge rectifier and filter should be used before power supply; generally, a DC power supply that only uses a single-phase bridge rectifier should not be used to directly power the PLC. The PLC input power supply should be wired separately from the equipment's DC power supply, DC control circuit power supply, and DC output power supply, and should have an independent protection circuit. When the winding configuration is complex, an independent regulated power supply should be used to power the PLC separately.
When the system adopts a modular structure, the capacity of the power supply module should meet the power supply capacity requirements of the PLC system. The rated output capacity of the power supply module should be greater than the total power consumed by all the components in the system, with a margin of 20% to 30%.
2. I/O device external power supply
External power supply for I/O refers to the power supply used for PLC input modules, PLC output modules, input sensors (such as proximity switches), and output actuators.
The external power supply used for PLC input signals is generally DC 24V. Since voltage fluctuations in the input signals can directly affect changes in the PLC input state, the requirements for them are high. In principle, a regulated power supply should be used; at least a three-phase bridge rectifier and filter should be used for power supply; a DC power supply that only uses a single-phase bridge rectifier should not be used to prevent errors in input signal sampling.
The external power supply used for PLC output signals depends on the PLC's output type and load requirements; it can be AC or DC. Especially when relay contact outputs are used, the power supply requirements are entirely dependent on the load.
Generally, PLCs have lower requirements for output power than input power. For example, for DC 24V intermediate relays and solenoid valves...
For typical loads, a single-phase bridge rectifier DC power supply can be used. However, when the PLC output needs to be used as an input for other control devices in the system (such as CNC), the output power supply must be selected according to the requirements of the latter.
3. PLC Main Power Supply System
The design of the main power supply system must be based on a specific analysis of the nature of the controlled object, technical requirements, system composition, and operating environment conditions. The following section of this chapter provides examples of PLC control system hardware design for readers' reference.
As a basic design principle for the PLC main power supply system, the following aspects should be noted:
① In the system, all power supplies related to the PLC can be disconnected through the main power switch of the equipment to achieve isolation from the power grid.
②As the main control device of the system, the PLC should, in principle, be able to start working immediately after the main power supply of the equipment is turned on without any other startup operations, so that the control system can effectively monitor the controlled object.
③ For control systems that use both basic units and expansion units, the power supply to the expansion units should be turned on before or simultaneously with the basic units so that the basic units can effectively monitor the expansion units.
④ The external power supply used for PLC input signals can be shared with the PLC's basic power supply, but an independent protection device (such as a circuit breaker) must be installed in the circuit.
⑤ When the external power supply for PLC input signals is set up independently, this power supply should be put into operation immediately after the main power supply of the equipment is turned on, so that the PLC can effectively monitor the current status of the equipment through input signals.
⑥ The external power supply for PLC output signals can be shared with the input power supply or set independently. For control systems with complex components and many actuators, multiple power supplies can be set as needed.
⑦ When the PLC output uses a common external power supply, multiple protection devices (such as circuit breakers) should be set according to the different output objects, and the power supply connection sequence for each type of output should be different. The design should ensure that the switching on and off of the power supply for each type of PLC output is constrained and controlled by the "interlocking" condition of the high-voltage control circuit.
⑧ Power supplies used for other control circuits in the system can share the same input or output power supply as the PLC when the voltage is the same (such as DC24V control circuit), but must be equipped with independent protection components (such as circuit breakers).
