Speed control of low-voltage DC servo motors often refers to speed control of separately excited brushed DC motors. According to the speed equation of a DC motor, the speed n = (armature voltage U - voltage current Ia * internal resistance Ra) ÷ (constant Ce * air gap flux Φ). Since the internal resistance Ra of the armature is very small, the voltage current Ia * internal resistance Ra ≈ 0. Thus, the speed n = (armature voltage U) ÷ (constant Ce * air gap flux Φ). By adjusting the armature voltage U while keeping the air gap flux Φ constant, the speed n of the DC motor can be adjusted; or by adjusting the air gap flux Φ while keeping the armature voltage U constant, the speed n of the motor can also be adjusted. The former is called constant torque speed control, and the latter is called constant power speed control.
Constant torque speed regulation method for low-voltage DC servo motors
In constant torque mode, the air gap flux Φ must first be kept constant. The stator and rotor magnetic fields of a DC motor are orthogonal and do not affect each other. To keep Φ constant, it is sufficient to ensure that the current in the excitation coil is stable at a certain value. Theoretically, using a constant current source to control the current in the excitation coil would be ideal, but since current sources are difficult to find, applying a stable voltage to the excitation coil can generally approximate the stabilization of the excitation current, thereby keeping the air gap flux Φ constant. If it is a permanent magnet DC servo motor, a permanent magnet replaces the excitation coil, and the flux is permanently constant, so this is not a concern.
Simple voltage adjustments are insufficient for applications with significant load fluctuations. Therefore, a cascade speed control system was introduced. By detecting the motor's current and speed, separate current loops (inner loop) and speed loops (outer loop) are created. Using a PID algorithm, this effectively addresses speed regulation under fluctuating load conditions, making the DC motor's speed control characteristics very "rigid"—meaning the maximum torque remains unchanged regardless of speed fluctuations, achieving true constant torque output. This speed control method has long been imitated by AC speed control systems; for example, vector control in frequency converters is modeled after this approach. Using only the inner current loop, a specific torque output from the motor can be directly controlled to meet various stretching and coiling control requirements.
Armature voltage control was not an easy task before the invention of thyristors and IGBTs, given their high power requirements. In the early days, it was controlled by a generator that produced DC power. By adjusting the generator's magnetic flux, the generator's output voltage could be controlled, which in turn adjusted the armature voltage.
After the invention of the thyristor (SCR), by applying an AC input voltage to the thyristor and using phase-shift triggering technology to control the thyristor's conduction angle, AC power could be rectified into a pulsating DC current. Because DC motors are highly inductive loads, the pulsating DC current is buffered and stabilized by a large inductor. This DC voltage is adjustable and proportional to the thyristor's conduction angle. This speed control technology is very mature and reliable, and it was widely used in industry in the mid-to-late 20th century.
Furthermore, with the advent of devices such as MOSFETs and IGBTs, the speed control of low-voltage DC servo motors can be made even more precise. PWM chopping technology can be used to make the output DC voltage very stable, resulting in very small fluctuations in the DC motor speed. If the motor rotor is made longer, the moment of inertia is reduced, and a position loop is added, precise positioning control can be achieved. This is what is known as a DC servo system.
Constant power speed regulation method for low-voltage DC servo motors
This is what is known as field weakening speed regulation. Essentially, this speed regulation method is a supplement to constant torque speed regulation. It is mainly used in some situations where a wider speed regulation range is required. For example, some gantry milling machines require very slow feed and high torque during machining, while the torque is very light and the speed is very fast during retraction. In this case, constant torque speed regulation is used during feed, and field weakening speed regulation is used during retraction. In this case, the maximum power of the motor remains unchanged.
Some electric vehicles need to travel very slowly uphill, requiring a lot of torque, while wanting to travel very fast on flat roads where resistance is low. In this case, constant power speed regulation is also needed, similar to mechanical gear shifting or adjusting the reduction ratio. Generally, field weakening speed regulation is not suitable for permanent magnet motors, so the magnetic flux Φ cannot be controlled independently.
To weaken the magnetic field, the size of the air gap magnetic flux Φ is directly reduced. This can reduce the current of the excitation coil. Generally, a thyristor or field-effect transistor is used in the excitation coil to make a PI adjustment and output a current source to achieve this.
When using field weakening speed regulation, the higher the motor speed, the lower the maximum torque output of the motor will be. This needs to be noted, and it generally cannot be reduced indefinitely. It can be controlled at about 90% of the rated excitation current.
