发布时间 : 星期四 文章电气工程及自动化毕业论文更新完毕开始阅读8df5f730eefdc8d376ee32d0
中国矿业大学2008届本科生毕业设计 第50页
ladder program, converting the source program to binary object code which will run on the PLC’s microprocessor and downloading the object code from the PC to the PLC system via the serial communication port. The PLC system is online when it is in active control of the machine and monitors any data to check for correct operation.
A. PLC Speed Control Software
In Fig. 4, the flowchart of the speed control software is illustrated. The software regulates the speed and monitors the constant speed control regardless of torque variation. The inverter being the power supply for the motor executes this while, at the same time, it is controlled by PLC’s software. The inverter alone cannot keep the speed constant without the control loop with feedback and PLC.
From the control panel, the operator selects the speed setpoint and the forward/backward direction of rotation. Then, by pushing the manual start pushbutton, then the rotation stops. The corresponding input signals are interfaced to the DIM and the output signals to the DOM as shown in Table IV.
The AIM receives the trip signal IS from the stator current sensor, the speed feedback signal from the tachogenerator, and the signal from the control panel. In this
中国矿业大学2008届本科生毕业设计 第51页
way, the PLC reads the requested speed and the actual speed of the motor. The difference between the requested speed by the operator and the actual speed of the motor gives the error signal. If the error signal is not zero, but positive or negative, then the PLC according to the computations carried out by the CPU decreases or increases the V/F of the inverter and, as a result, the speed of the motor is corrected.
The implemented control is of proportional and integral (PI) type (i.e., the error signal is multiplied by gain KP, integrated, and added to the requested speed). As a result, the control signal is sent to the DOM and connected to the digital input of the inverter to control V/F variations. At the beginning, the operator selects the gain KP by using a rotary resistor mounted on the control panel (gain adjust) and the AIM receives its voltage drop as controller gain signal (0–10 V).
The requested speed is selected using a rotary resistor and the AIM reads this signal. Its value is sent to the AOM and displayed at the control panel (speed set point display). Another display of the control panel shows the actual speed computed from the speed feedback signal. A third display shows the load torque computed from the load current signal in Newton-meters (N.m). Their corresponding signals are output to the AOM (Table IV).
B. Monitor and Protection Software
During motor operation, it is not possible to reverse its direction of rotation by changing the switch position. Before direction reversal, the stop button must be pushed. For motor protection against overloading currents during starting and loading, the following commands were programmed into the software.
i) Forward/backward signal is input to DIM.
ii) Speed set point signal nsp , the load current IL , the stator current IS, and the speed feedback signal are input to AIM.
iii) At no load IS<1.0A , if the speed set point is lower than 20% or nsp<300 r/min, the motor will not start.
iv) At an increased load over 0, 4 N m (40% of rated torque),IS ≥1.3A, and a speed setpoint lower than 40% or nsp≤600r/min, the motor will not start.
v) If the load is increased more than 1, 0N m(rated torque) IS≥1.5A and if the speed set point exceeds 100% or nsp ≥1500r/min, the motor enters the cutoff procedure.
vi) In all other situations, the motor enters in the speed control mode and the speed control software is executed as described in Subsection A.
In Fig. 5, the flowchart of this software is shown.
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中国矿业大学2008届本科生毕业设计 第53页
C. Cutoff and Restart Motor Software
In Fig. 6, the flowchart of this software is shown.
? In overloading situations, the motor is cut off and the trip lamp (yellow) is lit. The operator must release the thermal relays and then must turn off the trip lamp by pushing trip or stop button. The thermal relays are set to the motor rated current 1, 5 A. Following this, the motor can be started again.
? The motor can be cut off by the operator pushing the stop button: the display of the actual speed is set to zero, the start lamp (green) turns off, and the stop lamp (red) turns on and remains lit for 3 s.
? The load must be disconnected immediately after the motor cuts off and before the drive system is restarted. The motor will not start before 3 s after cutoff even if the start button is pushed.