01 Characteristics of DC Servo Motors
DC servo motors, with their excellent speed control performance, high starting torque, smooth operation, and high speed, have held an important position in the field of motor speed control for a considerable period of time. With the development of power electronics technology, especially the advent of high-power electronic devices, DC servo motors have gradually been replaced by AC servo motors. However, DC servo motors still have a place in low-power applications. The following mainly introduces the working principles of brushed DC motors, brushless DC motors, and the control principle of DC servo motors.
02 Working principle of brushed DC motor
The working principle of a brushed DC motor is shown in the left figure. N and S in the figure are a pair of fixed permanent magnets. The rotor of the motor is installed between the two magnetic poles, and coils abcd are fixed on it. The coil segment has two commutator segments (also called commutators) and two brushes.
When current flows from the positive terminal of the power supply, through brush A, commutator 1, coil, commutator 2, and brush B, back to the negative terminal, the current flow in the coil is a→b→c→d. According to the left-hand rule, this generates a counter-clockwise electromagnetic torque in the coil. When this electromagnetic torque exceeds the motor's load torque, the rotor rotates counter-clockwise.
After the rotor rotates 180°, the coil ab side rotates from the N pole of the magnet to near the S pole, and the cd side rotates to near the N pole. Because the relative positions of the brushes and commutator segments have changed, the direction of the current in the coil becomes d → c → b → a. According to the left-hand rule, the coil still generates a counterclockwise electromagnetic torque, and the rotor continues to rotate counterclockwise, as shown in the diagram below.
During the rotation of an electric motor, due to the action of brushes and commutators, direct current flows alternately in the coil in both forward and reverse directions, always generating electromagnetic torque in the same direction, causing the motor to rotate continuously. Similarly, when the external power supply is reversed, the motor will rotate clockwise.
03 Working principle of brushless DC motor
The structure of a brushless DC motor is shown in the left figure. To achieve brushless commutation, the armature winding of a brushless DC motor is mounted on the stator, while the permanent magnets are mounted on the rotor, a structure opposite to that of a conventional DC motor. By eliminating the sliding contact commutation mechanism of the brushes and commutator, the main source of DC motor failures is eliminated.
A common brushless DC motor is a three-phase permanent magnet synchronous motor. Its principle is shown in the diagram below. The commutation principle of a brushless motor is as follows: three Hall effect sensors are used as rotor position sensors, installed at 120° intervals on the circumference. The magnets on the rotor trigger the Hall effect sensors to generate corresponding control signals. These signals control transistors VT1, VT2, and VT3 to sequentially switch on and off, causing the stator windings U, V, and W on the motor to be sequentially energized and commutated as the rotor position changes, forming a rotating magnetic field that drives the rotor to move continuously. The control technology used in brushless DC servo motors is the same as that used in AC servo motors.
04 Control Principle of DC Servo Motor
Speed control of DC servo motors typically employs pulse width modulation (PWM), as shown in the left figure. The square wave control signal Vb controls the switching on and off of transistor VT, which in turn controls the power supply voltage. When Vb is high, transistor VT is on, the power supply voltage is applied to the motor, generating a current im. Since the motor windings are an inductive load, the current im experiences a rising process. When Vb is low, transistor VT is off, the power supply voltage is cut off, but the electrical energy stored in the motor windings is released, generating a current im, which then experiences a falling process.
Duty cycle is the ratio of the time a pulse (high level) occupies to the total time within a continuous operating period. DC motors regulate speed by controlling the duty cycle of the pulse signal. When the duty cycle of the control pulse signal is 60%, meaning the high level occupies 60% of the total time, the average voltage applied to the motor stator windings is 0.6µF. When the system is running stably, the average current in the motor windings is also 0.6µF of the peak value. Clearly, the duty cycle of the control signal determines the average current and average voltage applied to the motor, thus controlling the motor's speed. The current curve of a brushless DC motor is shown in the figure below.
