Speed control accuracy is one of the core performance indicators of a DC motor, especially in industrial scenarios requiring precise speed matching, such as cold rolling mills, precision machine tools, or automated production lines. Fluctuations in the excitation current directly disrupt speed stability, triggering a series of chain reactions. The impact mechanism of this fluctuation can be analyzed from four aspects: changes in magnetic field strength, armature current response, mechanical characteristic deviation, and system anti-interference capability.
The core function of the excitation current is to establish the motor's main magnetic field, and its magnitude directly determines the magnetic field strength. When the excitation current fluctuates, the magnetic field strength changes synchronously, leading to an imbalance in the electromagnetic relationship within the motor. For example, when the excitation current increases, the magnetic field strength strengthens. Although the rate at which the armature winding cuts the magnetic field lines remains unchanged, the torque generated per unit magnetic flux increases, and the motor will attempt to accelerate under constant load. Conversely, when the excitation current decreases, the magnetic field weakens, the torque output capability decreases, and the motor speed may decrease due to insufficient power. This speed fluctuation caused by changes in magnetic field strength is the direct cause of the excitation current's impact on control accuracy.
The automatic adjustment mechanism of the armature current further amplifies the impact of excitation current fluctuations. In a DC motor, the armature current and load torque must maintain a dynamic balance: when the load torque increases, the armature current increases to provide more torque; when the load torque decreases, the armature current decreases to avoid energy waste. However, this regulation process relies on a stable magnetic field strength. If fluctuations in the excitation current cause changes in the magnetic field strength, the armature current regulation can fall into a "misjudgment"—for example, when the excitation current suddenly decreases, the motor's actual torque output capability decreases, but the armature current may remain unchanged due to the load, causing the speed to drop sharply due to insufficient power; when the armature current reacts and increases to compensate for the torque, the speed may overshoot due to overcompensation. This "hysteresis-overshoot" cycle significantly reduces speed stability.
A deviation in mechanical characteristics is another important consequence of excitation current fluctuations. The mechanical characteristic curve of a DC motor describes the relationship between speed and torque, and its slope reflects the "stiffness" of the motor: the flatter the curve, the smaller the fluctuation in speed with torque; the steeper the curve, the greater the speed fluctuation. Excitation current fluctuations directly alter the slope of the mechanical characteristic curve. When the excitation current increases, the curve flattens, reducing the motor's sensitivity to torque changes and decreasing speed fluctuations. Conversely, when the excitation current decreases, the curve steepens, increasing the motor's sensitivity to torque changes and increasing speed fluctuations. In scenarios requiring high-precision control, this shift in mechanical characteristics can prevent the motor from operating stably at the target speed, or even induce oscillations.
A decrease in the system's anti-interference capability further weakens speed control accuracy. Excitation current fluctuations can be caused by power quality issues, sudden load changes, or control algorithm defects. These fluctuations themselves become new sources of interference, creating a vicious cycle of "fluctuation-interference-greater fluctuation." For example, when the grid voltage fluctuates, the output of the excitation power supply may change accordingly, leading to excitation current fluctuations. These fluctuations can cause speed fluctuations, which may further affect the regulation of the excitation current through the feedback control loop, ultimately causing the system to fall into an unstable state. Furthermore, if the control algorithm fails to compensate for the impact of excitation current fluctuations in a timely manner, the speed recovery time will be prolonged, and control accuracy will significantly decrease. In practical applications, the hazards of excitation current fluctuations are particularly prominent. Taking a cold rolling mill as an example, its main motor needs to frequently switch between shutdown, startup, and stable operation cycles, and a stable excitation magnetic field needs to be quickly established each time it starts. If the excitation current fluctuates too much during the startup phase, the motor may fail to reach the target speed due to insufficient torque, or it may overspeed due to excessive torque, leading to a decrease in the quality of the billet processing or even equipment damage. Similarly, in precision machine tools, excitation current fluctuations may cause relative positional deviations between the tool and the workpiece, affecting machining accuracy; in automated production lines, they may lead to unstable material transport speeds, affecting overall production efficiency.
To suppress the impact of excitation current fluctuations on speed control accuracy, measures need to be taken from both hardware and software aspects. On the hardware side, high-precision excitation power supplies, low-ripple filters, and high-stability excitation windings can be used to reduce fluctuation sources. On the software side, advanced control algorithms, such as fuzzy PID control, adaptive control, or model predictive control, can be introduced. By monitoring the feedback signals of excitation current and speed in real time, control parameters can be dynamically adjusted to achieve fast and accurate compensation. In addition, regular maintenance of the motor and excitation system, and timely replacement of worn parts, are also important means to ensure the accuracy of speed control.
