DC speed control system

  Overview Speed ​​control methods are usually mechanical, electrical, hydraulic, pneumatic, and mechanical and electrical speed control methods can be used only for mechanical and electrical speed control methods. Improve transmission efficiency, easy to operate, easy to obtain stepless speed regulation, easy to achieve long-distance control and automatic control, therefore, widely used in production machinery due to DC motor has excellent motion performance and control characteristics, although it is not as structure as AC motor Simple, inexpensive, easy to manufacture, and easy to maintain, but in recent years, with the development of computer technology, power electronics technology and control technology, the AC speed control system has developed rapidly, and in many occasions it is gradually replacing the DC speed control system. But the main form. In many industrial sectors in China, such as rolling steel, mining, marine drilling, metal processing, textile, papermaking and high-rise buildings, high-performance controllable electric drag speed control systems are required in theory and practice, from control technology From the perspective, it is the basis of the AC speed control system. Therefore, we first focus on the DC speed regulation 8.1.1 DC motor speed control method According to the basic principle of the third chapter DC motor, from the induced potential, electromagnetic torque and mechanical characteristics equation, there are three speed control methods for DC motors: (1) Adjust the armature supply voltage U.

Changing the armature voltage is mainly to lower the armature voltage from the rated voltage and shift the speed from the rated motor speed. This is the best method for a constant torque system. The change encounters a small time constant and can respond quickly, but requires a large-capacity adjustable DC power supply. (2) Change the main magnetic flux of the motor. Changing the magnetic flux can realize stepless smooth speed regulation, but only weaken the magnetic flux for speed regulation (referred to as weak magnetic speed regulation). The time constant encountered from the motor amount is much larger than that encountered by the change, and the response speed is higher. Slower, but the required power capacity is small. (3) Change the armature loop resistance. The method of speed regulation of the string resistor outside the motor armature circuit is simple and convenient to operate. However, it can only be used for step-regulated speed regulation; it also consumes a lot of power on the speed-regulating resistor.

There are many shortcomings in changing the resistance speed regulation. At present, it is rarely used. In some cranes, hoists and electric trains, the speed control performance is not high or the low speed running time is not long. The speed is increased in a small range above the rated speed. Therefore, the automatic control of the DC speed control system is often based on the voltage regulation and speed regulation. If necessary, the current in the armature winding of the voltage regulation and the weak magnetic DC motor interacts with the main magnetic flux of the stator to generate electromagnetic force and electromagnetic rotation. The moment, the armature thus rotates. The electromagnetic rotation of the DC motor is very conveniently adjusted separately. This mechanism makes the DC motor have good torque control characteristics and thus has excellent speed regulation performance. Adjusting the main magnetic flux is generally still or through the magnetic regulation, both need adjustable DC power. 8.1.3 Speed ​​Control System Performance Indicators Any equipment that requires speed control must have certain requirements for its control performance. For example, precision machine tools require machining accuracy of tens of microns to several speeds, with a maximum and minimum difference of nearly 300 times; a rolling mill motor with a capacity of several thousand kW has to complete from positive to reverse in less than one second. Process; all of these requirements for high-speed paper machines can be translated into steady-state and dynamic indicators of motion control systems as a basis for designing the system. Speed ​​control requirements Various production machines have different speed control requirements for the speed control system. The following three aspects are summarized: (1) Speed ​​regulation.

The speed is adjusted stepwise (stepped) or smooth (stepless) over a range of maximum and minimum speeds. (2) Steady speed. Stable operation at the required speed with a certain degree of accuracy, without due to various possible external disturbances (such as load changes, grid voltage fluctuations, etc.) (3) acceleration and deceleration control. For equipment that frequently starts and brakes, it is required to increase and decelerate as soon as possible, shortening the starting and braking time to increase productivity; sometimes it is necessary to have three or more aspects that are not subject to severe, sometimes only one or two of them are required, Some aspects may still be contradictory. In order to quantitatively analyze the performance of the problem. Steady-state indicators The performance indicators of the motion control system when it is running stably are called steady-state indicators, also known as static indicators. For example, the speed range and the static rate of the speed control system during steady-state operation, the steady-state tension error of the position system, and so on. Below we specifically analyze the steady state index of the speed control system. (1) Speed ​​regulation range D The ratio of the maximum speed nmax and the minimum speed nmin that the motor can meet is called the speed regulation range, which is indicated by the letter D, that is, where nmax and nmin generally refer to the speed at the rated load, for a few loads Very light machinery, such as precision grinding machines, can also use the actual load speed. Set nnom. (2) Static error rate S When the system is running at a certain speed, the ratio of the speed drop corresponding to the ideal no-load speed no when the load changes from the ideal no-load to the rated load is called static, and the static difference is expressed.

The stability of the speed regulation system under the load change, it is related to the hardness of the mechanical characteristics, the harder the characteristics, the smaller the static error rate, the steady diagram of the speed 8.3 the static rate at different speeds (3) the pressure regulation system The relationship between D, S and D in the DC motor voltage regulation speed regulation system is the rated speed of the motor nnom. If the speed drop at the rated load is, then the system's static rate and the minimum speed at the rated load are considered. To equation (8.4), equation (8.5) can be written as the speed range is to substitute equation (8.6) into equation (8.7), and equation (8.8) expresses between speed range D, static rate S and rated speed drop. The relationship that should be satisfied. For the same speed control system, the smaller the characteristic hardness, the smaller the speed range D allowed by the system. For example, the rated speed of a certain speed control motor is nnom=1430r/min, and the rated speed drop is such that if the static error rate is S≤10%, the speed regulation range is only the performance index of the dynamic index motion control system during the transition process. Dynamic indicators, including dynamic performance indicators and anti-interference performance indicators. (1) Following performance index Under the action of a given signal (or reference input signal) R(t), the change in system output C(t) is described by following performance indicators. For different performance indicators, the initial response is zero, and the system responds to the output response of the unit step input signal (called unit step response). Figure 8.4 shows the following performance index. The unit step response curve 1 rise time tr The time required for the unit step response curve to rise from zero for the first time to the steady state value is called the rise time, which indicates the rapidity of the dynamic response. 2 overshoot