As a favorable tool, automobile has been widely used in daily life. DC motor has good starting and braking performance, so it is mostly used in some controllable electric traction places.
Among DC motors, the DC speed regulation system with voltage cut-off negative feedback is also the most widely used, which is widely used in automatic control of rolling mills, metallurgy, printing, metal cutting machine tools and many other fields. He usually uses three-phase full-bridge rectifier circuit to supply power to the motor, so as to control the speed of the motor. Traditional control systems use analog components, such as thyristors and various linear operation circuits. Although it meets the production requirements to a certain extent, the components are prone to aging in use, affected by external interference, complicated circuits and poor versatility. The control effect is affected by device performance, temperature and other factors, which leads to the change of system operation characteristics, the reliability and accuracy of the system can not be guaranteed, and even accidents occur. The DC speed control system consists of power thyristor, phase-shifting control circuit, rotating speed circuit, double closed-loop speed control system circuit, integrating circuit, current feedback circuit, phase failure and overcurrent protection circuit. Usually refers to manually or automatically changing the motor speed to meet the requirements of working machinery. In the aspect of mechanical characteristics, the mechanical characteristics of the motor are changed by changing the parameters of the motor or the applied voltage, so as to change the intersection of the mechanical characteristics of the motor and the mechanical characteristics of the working characteristics and change the stable running speed of the motor.
Due to my limited ability, mistakes or shortcomings are inevitable. Please criticize and correct me.
Chapter 1 Selection of main circuit and composition of closed-loop system
Brief introduction of 1. 1 V-M system
Thyristor-motor speed control system (V-M system for short) has a simple schematic diagram as shown in figure 1. In the figure, VT is a thyristor controlled rectifier, which can be single-phase, three-phase or more, half-wave, full-wave, semi-controlled and fully controlled.
Advantages: By adjusting the control voltage of trigger device GT to move the phase of trigger pulse, the rectified voltage can be changed and smooth speed regulation can be realized.
Disadvantages:
1. Because of the unidirectional conduction of thyristor, it does not allow the current to reverse, which makes the reversible operation of the system very difficult.
2. Components are very sensitive to overvoltage, overcurrent and excessively high du/dt and di/dt, and any index exceeding the allowable value may damage components in a short time.
Therefore, there must be a reliable protection device and meet the requirements of heat dissipation conditions, and there should be enough margin when selecting components.
Figure 1. 1 V-M system
1.2 determination of main circuit
Although the number of thyristors used in three-phase half-wave controllable rectifier circuit is only half that of three-phase fully controlled bridge rectifier circuit, its performance is not as good as that of three-phase fully controlled bridge rectifier circuit. Three-phase full-controlled bridge rectifier circuit is the most widely used rectifier circuit at present, and its output voltage fluctuation is small, which is suitable for the load of DC motor. The speed regulating device composed of this circuit has a wide adjustment range (nearly 50). The application of this circuit in this design can meet the technical requirements of continuous smooth speed regulation and irreversible operation of the motor.
The three-phase fully controlled bridge rectifier circuit is actually a cathode group in the three-phase half-wave thyristor rectifier circuit.
And a series circuit of * * * anode group, as shown in Figure 6. Three-phase full-control bridge rectifier circuit can control cathode group and anode group at the same time, and the control angles are all? . All six thyristors should be triggered once in one cycle, and the triggering sequence is: VT 1? VT2？ VT3？ VT4？ VT5？ VT6, the phases of the six trigger pulses are.
The difference is 60o. In order to form a complete current loop, two thyristors are required to be turned on at the same time, one of which is in the anode group and the other is in the cathode group. Therefore, thyristors must be turned on in strict accordance with the number. Thyristors VT 1 and VT4 are in phase a, thyristors VT3 and VT6 are in phase b, thyristors VT5 and VT2 are in phase c, thyristors VT 1, VT3 and VT5 are connected to the anode group, and thyristors VT4, VT6 and VT2 are connected to the cathode group. exist
Under the control of the circuit, only when the thyristor with the highest potential in the circuit cathode group, the thyristor with the lowest potential in the circuit anode group and the trigger pulse are turned on at the same time can a complete rectifier circuit be formed.
Because the grid voltage is often inconsistent with the working voltage (U2), it is necessary to configure a rectifier transformer at the front end of the main circuit to obtain a voltage matching the load, and isolate the thyristor device from the grid, so as to reduce or reduce the interference of the thyristor converter device to the grid and other electrical equipment.
Considering that the increase of control angle α will make the load current discontinuous, and when the load is a DC motor, the thyristor conduction angle θ decreases, the equivalent internal resistance of the rectifier increases, the mechanical characteristics of the motor soften, the commutation conditions deteriorate and the motor loss increases. Therefore, the smoothing reactor is connected in series on the DC side to limit the fluctuation component of the current and keep the current continuous.
In order to prevent the component from exceeding the voltage and current it bears in an emergency, overvoltage and overcurrent protection devices are added to the circuit.
Chapter II Composition of Closed-loop Speed Regulation System
The open-loop DC speed regulation system can change the speed of the motor by adjusting the control voltage Uc. If the static error rate of load is not high in the production process, such an open-loop speed regulation system can realize stepless speed regulation in a certain range, but when the static error rate is high, the open-loop speed regulation system often cannot meet the requirements. At this time, a closed-loop speed regulation system will be adopted.
PI-regulated single-speed closed-loop DC speed regulation system can realize the speed without static error on the premise of ensuring the stability of the system.
Chapter 3: Determination of main circuit components of speed regulation system and its parameter calculation.
3. 1 capacity calculation of rectifier transformer
3. 1. 1 secondary voltage U2:
In order to ensure the normal operation of the load, the voltage U2 on the AC side of the thyristor can only change in a small range when the wiring form of the main circuit and the rated voltage required by the load are determined, so it is necessary to accurately calculate the secondary voltage U2 of the rectifier transformer. The factors influencing U2 value are:
The value of (1)U2 must first meet the maximum DC value Ud required by the load.
(2) Thyristors are not ideal controllable switching elements, and there is a certain tube voltage drop when conducting, which is expressed by UT.
(3) The existence of transformer leakage reactance will produce commutation voltage drop.
(4) The smoothing reactor has a certain DC resistance, and when the current flows through this resistance, it will produce a certain voltage drop.
(5) Voltage drop of armature resistance
Based on the above factors, the accurate expression of U2 is:
A = ud0/U2, which indicates the average rectified voltage and the effective value of the transformer secondary phase voltage when the control angle α = 0.
The proportion of.
B=Udα/Ud0, which indicates the ratio of average rectified voltage when the control angle is α and α = 00. Uk%-percentage of transformer short-circuit voltage; British KVA% = 5 for transformers below 100, and 100 ~ 1000 British KVA% = 5 ~ 8.