Before frequency converters existed, DC motor speed control systems dominated the market. Even the Apollo moon landing required precise speed control systems and position control, which were accomplished using DC servos. Before the 1960s, when thyristors were not yet invented, DC motors were driven by generators for speed control. Adjusting the generator's excitation current controlled the generator's output voltage, thereby controlling the DC motor's speed. This type of speed control system can be seen in early textbooks on electric motors and drives, although it was a bit cumbersome. However, it had a very wide speed range, high torque, and was stable and reliable. Furthermore, the theory of DC speed control was already very mature, and early electric vehicles used this type of speed control system.
DC motor speed control system
The DC motor mentioned here refers to a brushed DC motor, because its magnetic field and armature coils are controlled independently and are orthogonal at 90°, eliminating coupling issues. When the excitation current remains constant, the stator excitation winding flux linkage remains unchanged.
Motor torque = excitation winding flux linkage * armature current
Therefore, by simply adjusting the armature current, precise torque control of the motor can be achieved directly, easily meeting the constant torque control requirements. This is the fundamental reason why DC motor speed control systems have excellent low-speed torque.
For speed regulation of a DC motor, the speed n = (armature voltage U - armature current I * armature internal resistance R) / constant Kφ.
Because the internal resistance R of a DC motor is very small, the rotational speed n ≈ armature voltage U / constant.
The armature voltage U is almost directly proportional to the rotational speed n, which is an important reason why DC motor speed regulation can be achieved by adjusting the voltage through a generator.
Later, devices such as silicon controlled rectifiers (SCRs) were invented. Through a fully controlled bridge or a half-controlled bridge, alternating current could be directly converted into controllable direct current, and the voltage could be adjusted arbitrarily and quickly. This was used to control the armature voltage of a DC motor, thereby changing the motor speed.
After the development of control theory, cascade systems were also used to regulate the speed of DC motors. That is, the speed loop is on the outside, the speed deviation is used as the reference for the current loop, and the current loop is the inner loop. Both loops use PID controllers to complete the control, which has fast response, high accuracy, large torque, and wide speed range.
In addition to constant torque speed regulation, the DC motor can also be operated in the constant power region by reducing the excitation flux by reducing the excitation current. In this way, the torque decreases as the speed increases, the power remains unchanged, but the speed regulation range can be widened.
In fact, today's vector control mode for frequency converter speed regulation mimics the speed regulation method of DC motors, and the effect is not as ideal as that of DC speed regulation systems. It's only because brushed DC motors suffer from severe carbon brush wear, are difficult to maintain, and have high manufacturing costs that brushed DC motor speed regulation systems have gradually been phased out of the market. Even so, many low-power motors still use DC speed regulation systems, given their price advantage and relatively good performance.
| Variable Pole Speed Regulation
Besides frequency conversion speed regulation, asynchronous motors can also be adjusted by changing the number of pole pairs. For example, a four-pole motor can reach 1500 RPM, while an eight-pole motor can only reach 750 RPM. This speed regulation method has significant limitations and is generally called a dual-speed motor. It often only has two speed ranges, but the torque is relatively high and stable. It is ideal for applications that only require two speed ranges, such as some mixing systems. These systems allow for low-speed operation for a period of time before switching to high-speed mode. This control system is very simple, somewhat similar to star-delta switching, so it is inexpensive. Even today, despite the widespread use of frequency conversion, this speed regulation method is still widely used.
The speed of an asynchronous motor is n=60f/p. In addition to changing the frequency, the speed n can also be changed by adjusting the number of pole pairs p.
| Slip speed regulation
This type of speed regulation, as the name suggests, means speed regulation by "slipping". The actual motor speed remains unchanged after startup. It is achieved by slipping through the "slipper" between the motor and the load, which reduces the speed on the load side. This slipper can also be understood as an electromagnetic clutch. There are many types of clutches, but they all utilize electromagnetic effects to create resistance. The following is a simple explanation using the principle of a magnetic powder clutch.
For example, inside this clutch, there are coils and a lot of magnetic powder. When the current is applied, the magnetic powder will stick together due to the magnetic field of the coil. The greater the current, the stronger the magnetic field, and the tighter the magnetic powder is bonded. When it reaches a certain level, it can become a rigid object, which can directly connect the output and the output shaft to maintain a consistent output speed. This allows the load and the motor to rotate at the same speed.
When there is no current at all, the magnetic field disappears, and the magnetic powder will become a pile of loose sand. There will be no magnetic powder connection between the output and input shafts. Although the motor is still turning, the load speed can become zero.
If the magnetic field current is at a certain value, the magnetic powder will have some adhesion, but if the stiffness is insufficient, it will slip inside. This will create a certain speed difference between the input and output shafts. By controlling the value of the magnetic field current, the speed difference can be controlled, thus changing the speed of the load.
Because it involves slippage, friction and heat will inevitably occur, resulting in wasted electrical energy and low speed control efficiency. However, it also has its advantages: it allows for closed-loop speed control, and at low speeds, the torque is even more ideal than that of a frequency converter.
