In mechanical engineering, due to production needs, the motor is frequently in the process of starting, braking and reversing, so the starting and braking time of the motor determines its production efficiency to a large extent. In order to shorten the time, it is difficult for the single closed-loop speed control system using negative speed feedback to meet the requirements, while the double closed-loop DC speed control system adds current negative feedback on the basis of single closed-loop control, and obtains good performance.
1 Speed regulation method of separately excited DC motor
The mechanical characteristics of the separately excited DC motor refer to the relationship between the rotational speed and the electromagnetic torque under the condition of constant excitation current, armature voltage and total resistance of the armature circuit, that is, n=f(7). The mechanical characteristic expression of the separately excited DC motor is:
In the formula, n0 is the ideal no-load speed: UN is the rated voltage of the motor: CE is the electromotive force constant of the DC motor: CT is the electromagnetic torque constant: o is the excitation flux: Ra is the armature resistance: 7em is the electromagnetic torque: 8 is the slope of the mechanical properties, the smaller the 8, the harder the mechanical properties.
The inherent mechanical characteristics of the separately excited DC motor are shown in Figure 1. In the figure, nN is the rated speed of the motor, and 7N is the rated electromagnetic torque.
There are three kinds of speed regulation methods for separately excited DC motors: speed regulation by reducing armature voltage, speed regulation by reducing armature circuit series resistance, and speed regulation by weakening magnetic flux.
2 Design of double closed-loop DC speed control system
The motor speed regulation system (VM system) realizes speed regulation by adjusting the control angle of the thyristor to change the motor armature voltage. The speed is greatly affected by the load, and it is difficult to maintain the stability of the speed and cannot meet the production process requirements. Therefore, the negative feedback of the speed is introduced in this paper, and the output voltage of the rectifier circuit is automatically adjusted according to the deviation of the speed, so as to keep the speed stable. In order to shorten the start-up time, a current negative feedback circuit is added to form a double closed-loop control circuit, as shown in Figure 2.
The current loop ACR in Figure 2 is mainly based on the following performance, using a P1-type current regulator, and its transfer function is as follows:
In the formula, Ki is the proportional coefficient of the current regulator: TI is the leading time constant of the current regulator, and s is the Laplace transform parameter.
The speed loop AsR also uses a P1-type speed regulator, and its transfer function is as follows:
In the formula, Kn is the proportional coefficient of the speed regulator: Tn is the leading time constant of the speed regulator , and s is the Laplace transform parameter.
According to the above principles and analysis, combined with the three-phase full-bridge rectifier circuit composed of thyristors, the circuit diagram of the double closed-loop DC motor speed control system can be obtained, as shown in Figure 3.
Among them, regulator 1 is a speed regulator, and regulator II is a current regulator. When the system is working, changing the size of the given voltage Ug can change the speed of the motor , and the output of the speed regulator is used as the output of the current regulator. The purpose of limiting the starting current is achieved by using the output limiter of the speed regulator. The output of the current regulator is used as the control voltage Uct  of the trigger circuit. In the experiment, the oscilloscope was used to observe the waveform, as shown in Figure 4. It can be seen from the waveform that the full-bridge rectifier waveform is stable, and the double closed-loop speed control system designed in this paper is used, and the overshoot of the motor startup is small.
In this paper, based on the single closed-loop speed control system of speed control, the current negative feedback is added to form a double closed-loop speed control system. And it ensures that the static speed difference rate is less than or equal to 1%, and the speed overshoot is less than or equal to 10%. At the same time, it has the function of weak magnetic speed regulation, which improves the speed regulation performance, speed regulation accuracy and system stability of the DC motor speed regulation system.