I. Reduced-voltage starting control of three-phase asynchronous motors
1. Y-Δ reduced voltage starting control of three-phase asynchronous motor
The relay contactor control for the Y-Δ reduced voltage starting of a three-phase asynchronous motor was modified into a PLC control system.
(1) Determine the I/O signals and draw the external wiring diagram of the PLC.
(a) Main circuit
(b) PLC I/O wiring diagram
Wiring diagram for Y-Δ reduced voltage starting of an electric motor
(2) Design the ladder diagram for Y-Δ reduced voltage starting of a three-phase asynchronous motor.
Ladder diagram for Y-Δ reduced voltage starting control of electric motor
2. Series autotransformer reduced-voltage starting control for three-phase asynchronous motors
The relay contactor control for series autotransformer step-down starting is transformed into a PLC control system:
(1) Determine the I/O signals and draw the external wiring diagram of the PLC.
PLC input signals: Start button SB1, Stop button SB2, Thermal relay normally open contact FR.
PLC output signals: running contactor KM2, series autotransformer contactor KM1.
(a) Main circuit
(b) PLC I/O wiring diagram
Wiring diagram for autotransformer step-down starting of electric motor
(2) Design the ladder diagram for the series autotransformer step-down starting of a three-phase asynchronous motor.
Ladder diagram for series autotransformer step-down starting control of a three-phase asynchronous motor
II. Control of Three-Phase Wound-Rotor Asynchronous Motors
1. Three-phase wound-rotor asynchronous motor series resistance starting control
The relay contactor control circuit for starting a wound-rotor asynchronous motor with a series resistor is modified into a PLC control system:
(1) Determine the I/O signals and draw the external wiring diagram of the PLC.
PLC input signals: Start button SB1, Stop button SB2, Thermal relay normally open contact FR.
PLC output signals: power contactor KM, shorting contactor KM1 (R1 shorted), shorting contactor KM2 (R2 shorted)
(a) Main circuit
(b) PLC I/O wiring diagram
Wiring diagram for starting a three-phase wound-rotor asynchronous motor with series resistance.
2. Starting circuit for a three-phase wound-rotor asynchronous motor with a series frequency-sensitive resistor
The relay contactor control circuit for starting a wound-rotor asynchronous motor with a series frequency-sensitive resistor is modified into a PLC control system:
(1) Determine the I/O signals and draw the external wiring diagram of the PLC.
PLC input signals: Start button SB1, Stop button SB2, Thermal relay normally open contact FR.
PLC output signals: running contactor KM1, shorting frequency-sensitive resistor contactor KM2, and connecting intermediate relay KA to thermal relay.
(a) Main circuit
(b) PLC I/O wiring diagram
(2) Design the ladder diagram for starting a three-phase wound-rotor asynchronous motor with a series frequency-sensitive resistor.
Three-phase wound-rotor asynchronous motor with series frequency-sensitive resistor starting ladder diagram
III. Automatic Reciprocating Control
The automatic reciprocating control relay contactor control circuit will be converted into a PLC control system:
(1) Determine the I/O signals and draw the external wiring diagram of the PLC.
PLC input signals: forward start button SB1, reverse start button SB2, stop button SB3, normally open contact of thermal relay FR, forward limit switch SQ1, reverse limit switch SQ2, forward limit switch SQ3, and reverse limit switch SQ4.
PLC output signals: forward running contactor KM1, reverse running contactor KM2.
Schematic diagram of automatic round-trip control
(a) Main circuit
(b) PLC I/O wiring diagram
Wiring diagram for automatic reciprocating control
(2) Design the trapezoidal diagram
Ladder diagram for automatic reciprocating control
IV. Trapezoid Diagram Empirical Design Method
(I) Characteristics of ladder diagrams in PLC control systems
(1) Input signals and output loads of the PLC control system: The actuators such as AC contactors and solenoid valves in the relay circuit diagram are controlled by the output relays of the PLC, and their coils are connected to the output terminals of the PLC. Buttons, control switches, limit switches, proximity switches, etc. are used to provide control commands and feedback signals to the PLC, and their contacts are connected to the input terminals of the PLC.
(2) The functions of intermediate relays and time relays in the relay circuit diagram are accomplished by auxiliary relays and timers inside the PLC, which are independent of the input relays and output relays of the PLC.
(3) Setting up intermediate units: In a ladder diagram, if multiple coils are controlled by a series-parallel circuit of a certain contact, in order to simplify the circuit, an auxiliary relay controlled by the circuit can be set up in the ladder diagram. The auxiliary relay is similar to the intermediate relay in the relay circuit.
(4) Handling of instantaneous contacts of time relays: In addition to contacts that operate with a time delay, time relays also have instantaneous contacts that operate immediately when the coil is energized or de-energized. For time relays with instantaneous contacts, an auxiliary relay can be connected in parallel across the coil of the corresponding timer in the ladder diagram. The contacts of the latter are equivalent to the instantaneous contacts of the time relay.
(5) Handling of power-off delay time relay. The FX series PLC does not have a timer with the same function, but a timer that delays after the coil is energized can be used to implement the power-off delay function.
(6) Establishment of external interlocking circuit. In order to prevent the two contactors controlling forward and reverse rotation from operating simultaneously and causing a short circuit in the three-phase power supply, in addition to setting up a soft interlocking circuit in the ladder diagram consisting of normally closed contacts connected in series with the coils of their corresponding output relays, a hard interlocking circuit should also be set up outside the PLC.
(7) Handling of thermal relay overload signals: If the thermal relay is an automatic reset type, the overload signal must be provided to the PLC through the input circuit, and overload protection is implemented using a ladder diagram. If it is a manual reset type thermal relay, its normally closed contact can be connected in series with the coil of the AC contactor controlling the motor in the PLC's output circuit.
(8) Rated voltage of external load: The relay output module and bidirectional thyristor output module of PLC can generally only drive loads with a rated voltage of AC220V. If the original AC contactor coil voltage of the system is 380V, the coil should be replaced with 220V, or an intermediate relay should be set outside the PLC.
(ii) Experience-based design method
The programming method used in the above examples is the "experience-based design method." As the name suggests, the "experience-based method" is a design method that relies on the designer's experience.
1. Key points of the experience-based design method
(1) PLC programming, from the perspective of ladder diagram, is fundamentally about finding the working conditions of each output of the system that meet the control requirements. These conditions are always implemented by combining various internal devices according to certain logical relationships.
(2) The basic pattern of ladder diagram is start-protect-stop circuit. Each start-protect-stop circuit generally only targets one output, which can be the actual output of the system or an intermediate variable.
(3) There are some conventional basic steps in ladder diagram programming. They all have certain functions and can be applied in many places like building blocks.
2. Programming Steps Using the "Empirical Method"
(1) After accurately understanding the control requirements, reasonably allocate input and output ports to the events in the control system. Select necessary internal components, such as timers, counters, and auxiliary relays.
(2) For some outputs with relatively simple control requirements, their operating conditions can be written directly, and the relevant ladder diagram branches can be completed according to the start-protect-stop circuit mode. For outputs with slightly more complex operating conditions, auxiliary relays can be used.
(3) For more complex control requirements, in order to draw the ladder diagram of each output port using the start-stop circuit mode, it is necessary to correctly analyze the control requirements and determine the key points that make up the overall control requirements.
(4) Express the key points using ladder diagrams. Key points are always expressed using internal components, which need to be arranged reasonably. When drawing ladder diagrams for key points, common basic components can be used, such as timer timing components and oscillation components.
(5) Based on the completed key point ladder diagram, draw the ladder diagram for the final output of the system. Use the key points to synthesize the control requirements of the final output.
(6) Review the above sketches, and on this basis, add any missing functions, correct any errors, and make final improvements.
