PLC sequence control based on MM420

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PLC sequence control based on MM420

Abstract: the principle and disadvantages of relay sequence control are analyzed, and the transformation design scheme of MM420 frequency converter is discussed

in the process of industrial production, many production machines are driven by more than two motors, and there are certain requirements for the operation sequence of each motor. For example, metal cutting machine tools often require that the oil pump motor be started first and then the spindle motor, while some machines require that the feed motor be started only after the spindle motor is started. With the needs of production, people have more strict and new requirements for the source power of production machinery. The traditional relay sequence control can not meet the needs of production

1 relay sequence control

the principle of relay sequence starting is shown in Figure 1. AC contactors KM1, km2 and KM3 control three squirrel cage motors M1, M2 and M3 respectively. Its working process is as follows: press the start button SB1, the two coils of AC contactor KM1 and time relay kt1 are powered on, the main contact KM1 of the main circuit is closed, the motor M1 is started and delayed. 5. The electro-hydraulic servo universal material testing machine, after a certain time, the normally open kt1 is closed, the coils km2 and kt2 are powered on, the motor M2 is started, and after a certain time delay, the normally open kt2 is closed, the coil KM3 is powered on, and M3 is started; When the motor needs to stop, press the stop button SB2 to stop m3 first. After KT3 delay, normally closed KT3 is disconnected, M2 stops, and after kt4 delay, M1 stops. Thus, the sequential start and reverse stop of three motors are realized. This traditional relay control method uses large electrical components, many contacts, high failure rate, troublesome function change, and the starting performance of the motor is difficult to meet. With the development of PLC technology and power electronics technology, it has become an inevitable trend to use PLC and frequency converter to control the sequential operation of motors

2 transformation scheme

2.1 hardware schematic diagram

the transformed system adopts s PLC and Siemens MM420 frequency converter. The hardware schematic diagram of the transformation scheme is shown in Figure 2

use PLC to control and inspect the damage and deformation of low carbon cast iron and terminals 5, 6 and 7 of MM420, and then control the start, stop and speed regulation of the motor

2.2 functional features

(1) all relay contactor controls are connected with hardware devices, hard contacts and "hard" wires, which are all hardware control; Most of the internal PLC uses "soft" electrical appliances, "soft" contacts and "soft" wires, which are controlled by software

(2) the relay contactor control system uses many electrical appliances with large volume and high failure rate; PLC control system has compact structure, less electrical appliances and small volume

(3) the control of relay contactor is all mechanical contact, with slow action and strict arc discharge. The formula is heavy through adjustment; PLC is full of "soft contacts" which act quickly

(4) the control function of relay contactor is changed, and it is troublesome to remove wires, wiring and even replace components; The PLC control function can be changed only by modifying the program, which is extremely convenient

(5) it is easy to realize the start and stop of the motor by the relay contactor sequence control, but it is very cumbersome and troublesome to meet the start and stop performance indicators and speed regulation requirements, and even some performance indicators can not meet the requirements at all; PLC sequence control is easy to meet the performance requirements of motor start, stop and speed regulation

2.3 parameter setting

the fifth feature of PLC sequence control is the advantage of PLC sequence control. This feature is realized through the setting of MM420

(1) start and stop time of motor. Adjust the set value of p1120 (ramp rise time) to realize the time required for the motor to accelerate from static to the maximum motor frequency; Adjust the set value of p1121 (ramp down time) to realize the time required for the motor to decelerate from its maximum frequency to stationary stop

(2) forward and reverse rotation of motor. There are two ways to achieve this: by adjusting the set values of p0701, p0702 and p0703, when 1 is set as "on on-line forward rotation and off stop", and when 2 is set as "on-line reverse rotation and off stop"; It is realized by setting the positive and negative frequency values. When the frequency is set to positive, the motor rotates forward; When the frequency is set to a negative value, the motor reverses

(3) motor speed regulation. The terminals 5, 6 and 7 of the MM420 are set to multi-stage speed, and a total of 7 speeds can be converted. Parameter setting: p0004=7; P0701=17; P0702=17; P0703=17; P1001~ p1007 set the fixed frequency (selected by the user as required)

2.4 after setting the parameters of the control program

mm420, use the q0.0~q1.0 output port of PLC to control the terminals 5, 6 and 7 of the three frequency converters respectively (if only one or two terminals are used, the unused terminals can be disconnected). The main program of PLC sequence control is shown in Figure 3. M0.1 controls the operation of the first motor, and m0.2 controls the operation of the second motor. When m0.1 has output, start the first frequency converter, and motor 1 starts working. "Qiaojinliang thinks it works; When m0.2 has output, start the second frequency converter and motor 2 starts to work; When m0.3 has output, start the third frequency converter and motor 3 starts to work. On the contrary, when m0.1 has no output, turn off the first frequency converter, and motor 1 stops working

attention should be paid during programming: when pressing the start button, m0.4 and m0.5 must be set to "0", otherwise m0.2 and m0.3 will be set to "0" and "1" at the same time, so that the motor cannot work

3 conclusion

it is a wise move to use PLC and frequency converter to realize motor control, especially for industrial and mining enterprises with poor working environment. The PLC sequence control based on MM420 has been proved to meet the control requirements

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