The relay is replaced by programmable logic controller (PLC). The contactor controls the step-down starting process of the autotransformer of the motor, the conversion process of the electric control circuit and the ladder diagram control circuit, and the equivalent transformation method of the non-programmable problems after the conversion.
In order to change the traditional Y-r step-down starting control circuit controlled by relays and contactors, it is obvious that the control system reformed by PLC has the characteristics of simple wiring, convenient operation, good man-machine performance and high reliability by comparing with each other. Y-r step-down starting refers to connecting the stator windings into Y-shape when the motor starts, in order to reduce the starting voltage and limit the starting current. After the motor starts, the stator winding is changed into R-shape to make the motor run at full voltage. This step-down starting method can be applied to all asynchronous motors with R-shaped stator windings in normal operation. When the motor starts, it is connected into Y-shape.
The starting voltage added to each phase stator winding is only 1/3 of the R-type connection, the starting current is 1/3 of the R-type connection, and the starting torque is only 1/3 of the R-type connection. Therefore, this step-down starting method is only applicable to start under light or no-load conditions. Taking the Y-r step-down starting circuit of time relay automatic control as an example, this paper introduces the installation and maintenance method of Y-r step-down starting control circuit. The time relay automatically controls the installation of Y-r step-down starting circuit. The Y-r step-down start circuit diagram of time relay automatic control is shown in Fig. 1-5-17. The circuit consists of three contactors, a thermal relay, a time relay and two buttons. The time relay KT is used to control Y type buck start time and complete Y-r automatic switch. The working principle of the circuit is as follows: first close the power switch QS. In this circuit, after the KMY contactor is powered up, the KMY contactor can be operated electronically through the usual auxiliary contacts of KMY, so that the main contacts of KMY are closed without load, so the service life of the main contacts of KMY contactor can be prolonged. A star-triangle step-down start control program for three-phase asynchronous motor is designed. The power switch is closed. After pressing the start button SB1, the motor starts with a star connection. After turning 5S, KMY is cut off and the star start ends. In order to prevent arc short circuit effectively, after 300 ms delay, the KM Delta contactor coil gets electricity, and the motor rotates according to the triangle connection. No overload protection is considered. The allocation of input and output points is shown in Table 1-1. Complete the connection of PLC according to Figure 8-11. The 24V power supply at the input end of the figure can be supplied either by DC power supplied by PLC or by power alone. If the input of PLC is relay input, 220V AC power supply can also be used. In Figure 8-11, the main contactor KM and the auxiliary contact of triangular full-voltage running contactor of the circuit are connected to the input of PLC as input signals, which facilitates the monitoring of the actual action of the two contactors in the program and ensures the safety of the actual operation of the motor through the program. The hard interlocking links of star and triangle contactor coils are reserved at the output end of the PLC.
Soft interlocking is also needed in the program. Figure 8-12 is a ladder diagram of startup control of motor star-triangle buck. In wiring figure 8-11, the main contactor KM1 and the triangular contactor KM2 auxiliary contacts are connected to the input terminals X2 and X3 of the PLC, and the starting and closing contacts X1 and X3 of the start button are connected in series as the starting conditions of the motor.
The purpose is to prevent the motor from starting with triangular direct full voltage.
Because, if the contactor KM2 fails, if the main contact burns or the armature can not be opened, the KM2 dynamic contact of the input terminal of the PLC closes, which makes the input relay X3 in the on-off state, and its dynamic breaking contact is disconnected. At this time, even if the start button SB2 (X1 closes), the output Y0 will not turn on, and the KM1 as the load will not be able to turn on. Under normal circumstances, Y0 turns on and KM1 main contact acts after pressing the start button. If KM1 is faultless, then its dynamic contact closes, and X2’s dynamic contact closes. It is connected in series with Y0’s dynamic contact and forms self-locking to Y0.
At the same time, the timer T0 starts to time for 5 seconds. Under normal circumstances, Y0 turns on and KM1 main contact acts after pressing the start button. If KM1 is faultless, then its dynamic contact closes, and X2’s dynamic contact closes. It is connected in series with Y0’s dynamic contact and forms self-locking to Y0. At the same time, the timer T0 starts to time for 5 seconds. Y0 is turned on, thermostatic element its dynamic contact is closed. In line 2 of the program, the two latter dynamic contact is closed, so that Y2 is turned on, the main contact of contactor KM3 is closed, and the motor starts in star shape.
When T0 is timed for 5 seconds, Y2 is disconnected, that is, the starlike start-up ends. The Y1 breaking contact in the line acts as interlock to ensure that the contactor KM3 is disconnected if it has entered the triangular full-pressure start-up. When T0 arrives at the same time, that is to say, after starlike start-up, to prevent arc short circuit, it is necessary to delay the connection of KM2.
Therefore, the timer T1 in line 3 of the program plays the role of 0.3s delay.
After T1 is turned on, Y1 is turned on in program 4, KM2 main contact is operated, and the motor starts at full voltage triangle. The Y2 breaking contact also acts as a soft interlock. Because Y1 leads to power loss of T0 and T1 also causes power loss of T0, Y2’s dynamic breaking contact is used to self-lock Y1 in the program. Press the stop button, Y0 power loss, so that Y1 or Y2 power loss, that is, at any time, as long as the stop button, the motor will stop. A. Input the ladder diagram program into the computer.
B. Download the program to PLC and debug the program. Observe whether the motor can realize automatic star-triangle step-down starting under program control. C. Debugging and running and recording debugging results.