Traditional squirrel-cage asynchronous motor starting and braking control methods generally include four types: stator circuit series resistance starting, Y/△ starting, autotransformer starting, and extended delta starting; there are three braking methods: reverse braking, dynamic braking, and capacitor braking. The implementation of any of these starting and braking control methods is usually completed by relay-contactor control systems. The following uses stator circuit series resistance reduced-voltage starting and reverse braking as examples to analyze the starting and braking control of squirrel-cage asynchronous motors implemented by relays and contactors.
This control circuit contains three contactors and one intermediate relay coil, with 12 contacts. During starting, both KM2 and KM3 coils are in the off state. Press the start button SB1, the KM1 coil is energized and self-locked, and the motor starts with series resistance for reduced-voltage starting. When the motor speed rises to a certain value, the normally open contact of speed relay KS1 closes, the intermediate relay KA is energized and self-locked, and the normally open contact of KA connects the contactor coil KM3. The main contacts of KM3 short-circuit the stator resistance R in the main circuit, and the motor speed rises to the given value for stable operation.
During braking, press the stop button SB2, the KM1 coil is de-energized, and its main contacts disconnect the three-phase power supply; in the control circuit, the normally open contact opens, KM3 loses power, and the current-limiting resistor is connected in series; the normally closed contact closes, connecting the reverse braking contactor KM2, swapping the phase sequence of the two-phase power supply, and the motor is in a reverse braking state. When the speed drops to a certain value, the normally open contact of KS1 disconnects KA, then disconnects KM2, the motor loses power, and stops quickly. The capacitor tester uses a new type of excitation coil for non-disassembly measurement of vacuum degree, featuring convenient use, non-disassembly measurement, and high testing accuracy. It is a practical detection instrument suitable for electric power, steel, petrochemical, textile, coal, railway, and other departments. This traditional relay-contactor control method has clear control logic and adopts a combination of mechanical and electrical components, which is convenient for ordinary mechanical or electrical technicians to maintain. However, due to the large size of electrical components, many contacts, and high failure rate, the reliability of operation is relatively low. With the development of PLC technology, using PLC for motor operation control has become an inevitable trend.
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Comparison between PLC and relay-contactor control systems
By comparing the traditional control method and PLC control method for starting and braking of squirrel-cage asynchronous motors, in a sense, PLC control is developed from relay-contactor control. Both have similarities and many differences.
1. Differences between the two schemes
(1) PLC internally mostly uses "soft" logic
Relay-contactor control uses all hardware devices, hard contacts, and "hard" wiring, making it fully hardware control; PLC internally mostly uses "soft" electrical components, "soft" contacts, and "soft" wiring, making it software control;
(2) PLC control system has a compact structure
Relay-contactor control systems use many electrical components, are large in size, and have a high failure rate; PLC control systems have a compact structure, use fewer electrical components, and are small in size;
(3) PLC internally uses all "soft contacts" for fast action
Electrical contactor control uses all mechanical contacts, which are slow in action and have severe arc discharge; PLC internally uses all "soft contacts" for fast action;
(4) PLC control function changes are extremely convenient
Changing the control function of relay-contactor control requires rewiring, reconnecting, and even replacing components, which is troublesome; changing PLC control functions generally only requires modifying the program, which is extremely convenient;
(5) PLC control system has a short manufacturing cycle
Due to its simple and compact structure, and being basically software control, the design, construction, and debugging of PLC control systems have a shorter cycle than relay-contactor control systems.
In addition, since PLC technology is developed on the basis of computer control, its software and hardware settings have advantages that traditional relay-contactor control cannot match, and its working reliability is extremely high.
2. Design points of the PLC scheme
(1) Set up filtering
In PLC, π-type filters are generally set at the input and output interfaces, which not only filter out high-frequency interference from the outside but also reduce mutual interference between internal module signals;
(2) Provide isolation
In PLC systems, almost all CPUs and I/O circuits (mainly digital ports) are equipped with optocouplers for isolation to prevent interference or possible damage to the CPU, etc.;
(3) Set up shielding
There are two types of shielding: one is dual shielding of the transformer with magnetic and electric fields, using materials that are both magnetically conductive and electrically conductive as the shielding layer; the other is shielding of modules such as CPU and programmer only against electromagnetic fields, using conductive metal materials as the shielding layer;
(4) Adopt modular structure
PLC usually adopts a building-block structure, which is convenient for users to repair and replace templates. At the same time, each template is equipped with fault detection circuits and corresponding indicators to mark its status, allowing users to quickly determine the fault location;
(5) Provide interlock function
Interlock functions are provided between the output channels in PLC to prevent accidents that may be caused by misoperation between controlled objects;
(6) Set up environment detection and diagnostic circuits
This part of the circuit is responsible for detecting the operating environment of the PLC (such as grid voltage, working temperature, environmental humidity, etc.), and also monitors the working status of each module in the PLC. This part of the circuit often works in coordination with software to achieve automatic fault diagnosis and prediction;
(7) Set up Watchdog circuit
This circuit in PLC is specifically used to monitor whether the PLC operation process proceeds according to the predetermined sequence. If a fault occurs in the PLC or the user program area is damaged, it will alarm because the CPU cannot work according to the predetermined sequence (predetermined time interval);
(8) PLC input and output control is simple
PLC works in a scanning mode, that is, the input of signals, processing of data, and output of control signals are carried out in batch processing at different time intervals within one scan cycle. This not only makes user programming simple and less error-prone, but also makes the PLC work less susceptible to external interference; at the same time, the data processed by PLC is relatively stable, reducing errors in processing; in addition, the input and output control of PLC is relatively simple, and it is not easy to cause problems due to inappropriate timing.
