DC motors are characterized by low speed and high torque, which cannot be replaced by AC motors. Therefore, DC motor speed control equipment has a wide range of applications. DC motor is divided into two kinds: with commutator and without commutator. Then how to adjust its speed?
The potentiometer speed control method is one of the most common DC motor speed control methods. It uses a potentiometer to change the current to the motor and thus the speed of the motor. The motor speed is adjusted by turning the potentiometer knob. The potentiometer speed control method can be used for single-speed DC motor speed control and multi-speed DC motor speed control. The pulse width modulation speed regulation method uses the pulse width modulation technique to regulate the motor speed by changing the duty cycle of motor operation. The controller can generate periodic pulse signals and regulate the motor speed by changing the pulse width. This is a high-precision, high-reliability, low-noise, and low-energy method. The code plate feedback speed regulation method utilizes a device such as a code plate to feed back the motor's rotational speed, thus achieving precise speed control. The motor usually comes with a rotary code for detecting the position and speed of the motor.
The information from the code disk is sent back to the controller, which uses this information to regulate the output of the motor driver, thereby controlling the speed of the motor. The magnetic field excitation speed control method uses the magnetic field of the motor to control the speed of the motor. By changing the magnitude of the motor excitation current, the torque and speed of the motor can be changed. This is a very simple and easy-to-realize speed control method, but its control accuracy is relatively low. in the late 1930s, the development of motor systems made DC motors with excellent speed control functions widely used. This control method can obtain a wider speed range, smaller speed ratio, and smooth speed regulation function. However, the main disadvantages of this method are large system weight, large footprint, low power consumption, and maintenance difficulties.
In recent years, with the rapid development of power electronics technology, the thyristor converter DC motor speed control system has replaced the FA motor speed control system, and its speed control function has far exceeded the FA motor speed control system. Especially with the rapid development of large-scale integrated circuit technology and computer technology, the precision, dynamic function, and reliability of DC motor speed control systems have been greatly improved. The development of high-power devices such as IGBT in power electronics technology is replacing thyristors, presenting a DC speed control system with better functions.
The formula for calculating the speed of a DC motor is: n=(U-IR)/ K φ, where U is the armature terminal voltage, I is the armature current, r is the total resistance of the armature circuit, φ is the magnetic flux of each pole, and k is the structural parameter of the motor. There are three methods of speed regulation for DC motors: lowering the armature voltage and adjusting the speed below the base speed; using series resistance in the armature circuit to adjust the speed; and weakening the magnetic field and adjusting the speed above the base speed.
When the armature voltage is reduced for speed regulation, the armature circuit must have an adjustable DC supply. The resistance of the armature and excitation circuits should be as low as possible. When the voltage is reduced, the rotational speed is also reduced. The artificial characteristic hardness is constant, the running speed is stable, and step-less speed regulation can be realized.
The armature circuit is controlled by the series resistance. The larger the series resistance, the weaker the mechanical characteristics and the more unstable the speed. At low speeds, the series resistance is very large, the more power is lost, the lower the power.The speed regulation range is affected by the load, with large load and small light load.
Weak magnetic speed regulation, general DC motor, to prevent the magnetic circuit oversaturation can only be used weak magnetic and not strong magnetic. The armature voltage is kept at the rated value, the series resistance of the armature circuit is reduced to the minimum, and the excitation current and magnetic flux are reduced by increasing the resistance of the excitation circuit Rf so that the motor speed increases and the mechanical characteristics become soft. When the rotational speed rises, if the load torque remains at the rated torque, the motor power will exceed the rated power and the motor will run overloaded, which is not allowed. Therefore, when adjusting the weak magnetic speed, the load torque will decrease accordingly with the increase of motor speed, which is constant power speed control. In order to prevent the motor rotor windings from falling off and being damaged due to excessive centrifugal force, the motor speed should not exceed the permissible limit when using a weak magnetic field.
In a DC motor speed control system, a stable DC voltage is first selected to supply power to the motor, and then speed control is accomplished by changing the resistance in the armature circuit. This method is simple, easy to manufacture and inexpensive. However, the disadvantage is that the power is small, the mechanical characteristics are soft, and it is not possible to obtain a wide and smooth speed regulation function. This method is only suitable for small power, low speed regulation range.