In the operation of a DC motor, adjusting the excitation current is one of the core means of speed control. This principle is based on the interaction between the law of electromagnetic induction and the mechanical characteristics of the motor. The magnetic field of a DC motor is generated by the excitation winding. The magnitude of the excitation current directly determines the magnetic field strength, which in turn affects the back EMF generated by the armature winding cutting the magnetic flux lines. When the armature voltage is constant, a weakened magnetic field reduces the back EMF, increases the armature current, and subsequently increases the electromagnetic torque, ultimately increasing the motor speed. Conversely, a stronger magnetic field reduces the speed. This process demonstrates the inverse proportional relationship between excitation current and speed: lower excitation current increases speed, while higher excitation current reduces speed.
In practical applications, excitation speed control is often combined with armature voltage speed control to expand the speed control range. Below base speed, constant torque speed control is achieved by adjusting the armature voltage. During this period, the excitation current is maintained at its maximum value to provide a stable magnetic field. When operation above base speed is required, the armature voltage is fixed and the excitation current is gradually reduced, entering the field-weakening speed control stage. During this stage, the motor output power remains constant, but the torque decreases with increasing speed. This segmented control strategy balances torque requirements at low speeds with the speed regulation range at high speeds. It is commonly used in applications requiring a wide speed regulation range, such as electric locomotives and rolling mills.
Excitation speed regulation can be implemented in a variety of ways. Early methods employed manual adjustment of the excitation resistor, controlling the current by varying fixed or variable resistors in the excitation circuit. Technological advancements have enabled more precise excitation control with the application of power electronics. For example, adjusting the output voltage of the excitation power supply using a thyristor rectifier or PWM controller enables continuous and smooth adjustment of the excitation current. Modern systems also often incorporate closed-loop control, using speed sensors to provide feedback on the actual speed and automatically adjusting the excitation current after comparing it with a setpoint, resulting in a highly accurate speed control system.
The advantage of excitation speed regulation lies in its high energy efficiency. Compared to speed regulation using series resistance in the armature circuit, excitation speed regulation does not incur additional resistance losses and only requires a relatively small amount of power in the excitation circuit to control the motor speed. Furthermore, excitation speed regulation offers robust mechanical properties, resulting in excellent speed stability under load fluctuations, making it suitable for applications requiring high speed accuracy. However, this method also has limitations. For example, the speed regulation range is limited by the saturation characteristics of the motor's magnetic circuit, and torque capacity decreases during high-speed field weakening.
In the design of DC motors, the choice of excitation method directly affects speed regulation performance. Separately excited motors, because their excitation current is independent of the armature voltage, offer high speed regulation accuracy and are often used in precision control scenarios. Shunt motors offer a simpler structure, but their excitation current is affected by the armature voltage, resulting in less stable speed regulation. Compound motors combine the advantages of both and are suitable for applications requiring high starting torque. In practical applications, the excitation method should be selected based on a comprehensive consideration of factors such as load characteristics, speed regulation range, and cost.
The reliability of excitation speed regulation depends on a comprehensive protection mechanism. For example, during field weakening, it is important to avoid speed runaway caused by insufficient excitation current. This can be achieved by installing a current surge protection unit or overcurrent circuit breaker for real-time monitoring. Furthermore, the contact between the carbon brushes and the commutator must be regularly checked to prevent sparking or overheating due to poor contact. Furthermore, the continuous fluctuation of the magnetic field intensity can lead to hysteresis, which can cause heating of the stator poles. Forced air cooling or heat sink fins are required to improve heat dissipation.
With advancements in control technology and materials science, excitation speed regulation technology continues to evolve. The application of new amorphous alloys has increased motor speed regulation accuracy by approximately fourfold, while also reducing metal fatigue and extending the life of core components. In the future, as the performance of power electronics improves and digital control technology becomes more widespread, excitation speed regulation will demonstrate its value in even more areas, providing more reliable power support for the efficient operation of DC motors.
