With advancements in high-performance permanent magnet materials, microelectronics, automatic control, and power electronics, permanent magnet brushless DC motors have experienced rapid development. Overcoming the inherent drawbacks of mechanical commutation devices, brushless DC motors offer numerous advantages, including long lifespan, superior speed control performance, small size, light weight, high efficiency, low moment of inertia, and good electromagnetic compatibility. The application and research of brushless DC motors have received widespread attention, and their technological advantages have led to their replacement of other types of electric motors in many applications. Especially in the field of micromotors, brushless DC motors occupy a dominant position in the application of low-power, high-speed speed control. Next, let's learn about three speed control methods for DC motors with Songwen Electromechanical:
1. Adjusting the armature supply voltage U. Changing the armature voltage mainly involves reducing the armature voltage from the rated voltage, thus decreasing the motor speed from the rated speed; this is a constant torque speed regulation method. This method is best for systems requiring stepless smooth speed regulation within a certain range. The time constant encountered by armature current changes is small, allowing for a fast response, but a large-capacity adjustable DC power supply is required.
2. Change the main magnetic flux φ of the motor. Changing the magnetic flux can achieve stepless smooth speed regulation, but it can only reduce the magnetic flux. Speed regulation from the motor's rated speed upwards is a constant power speed regulation method. The time constant encountered when the armature current changes is much larger, resulting in a slower response speed, but the required power supply capacity is smaller.
3. Changing the armature circuit resistance R. This method of speed regulation by adding a resistor in series with the motor's armature circuit is simple and easy to operate. However, it only allows for stepped speed regulation, resulting in poor speed smoothness and a relatively soft mechanical characteristic; it also consumes a large amount of electrical energy in the speed-regulating resistor. Due to these drawbacks, speed regulation by changing resistance is rarely used nowadays.
Automatic DC speed control systems often primarily rely on voltage regulation, sometimes combining it with field weakening. Voltage regulation requires a dedicated controllable DC power supply. Since the 1970s, power electronic devices have rapidly developed, leading to the development and production of various high-performance, fully controllable devices capable of controlling both conduction and shutdown. Pulse Width Modulation (PWM) DC speed control systems, constructed from these devices, have seen rapid growth in low- and medium-power DC drives in recent years. Compared to older controllable DC power supply speed control systems, PWM speed control systems offer the following advantages:
1. The PWM speed control system using fully controllable devices has a high switching frequency of its pulse width modulation circuit, resulting in a wide bandwidth, fast response speed, and strong dynamic anti-interference capability.
2. Due to the high switching frequency, a DC current with very small pulsation can be obtained solely through the filtering effect of the motor armature inductance. The armature current is easy to be continuous, the system has good low-speed performance, high speed stability, and a wide speed range. At the same time, the motor loss and heat generation are relatively small.
3. In a PWM system, the power electronic devices in the main circuit operate in a switching state, resulting in low losses, high device efficiency, and minimal impact on the AC power grid. There is no "pollution" to the power grid caused by thyristor rectifiers, and the power factor and efficiency are high.
4. The main circuit requires fewer power components, has a simple circuit, and is easy to control.
Currently, due to limitations in device capacity, PWM DC speed control systems are only used in medium and low power systems. The speed setting of a brushless DC motor depends on the speed command Vc. Once Vc is set, regardless of load changes, power supply voltage changes, or ambient temperature changes, when the speed is lower than the commanded speed, the feedback voltage decreases, the duty cycle of the modulation wave increases, and the armature current increases, causing the electromagnetic torque generated by the motor to increase and produce acceleration until the actual speed of the motor equals the commanded speed. Conversely, if the actual speed of the motor is higher than the commanded speed, the duty cycle decreases, the armature current decreases, and deceleration occurs until the actual speed equals the commanded speed. It can be said that within the allowable range of power grid fluctuations and below the allowable overload capacity, the steady-state speed of a brushless DC motor differs from the commanded speed by about 1%, and it can achieve constant torque operation within the speed control range, with the speed unaffected by voltage and load changes.
Disclaimer: This article is a reprint. If there are any copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
