6 brushless DC
Brushless DC motors use semiconductor switching devices to achieve electronic commutation, that is, electronic switching devices replace traditional contact commutators and brushes. It has the advantages of high reliability, no commutation spark, low mechanical noise, etc. It is widely used in high-end recording stands, video recorders, electronic instruments and automated office equipment.
The brushless DC motor is composed of a permanent magnet rotor, a multi-pole winding stator, a position sensor, and the like. The position sensing commutates the current of the stator winding in a certain order according to the change of the rotor position (ie, detecting the position of the rotor pole relative to the stator winding, and generating a position sensing signal at the determined position, after being processed by the signal conversion circuit To control the power switch circuit, the winding current is switched according to a certain logic relationship). The operating voltage of the stator windings is provided by an electronic switching circuit controlled by the position sensor output.
Position sensors are available in magnetic, photoelectric and electromagnetic types. A brushless DC motor using a magnetically sensitive position sensor, the magnetic sensing element (such as a Hall element, a magnetic sensitive diode, a magnetically sensitive diode, a magnetoresistor or an ASIC) is mounted on the stator assembly. To detect the change of the magnetic field generated when the permanent magnet and the rotor rotate.
A brushless DC motor using a photoelectric position sensor is provided with a photoelectric sensor device at a certain position on the stator assembly, and a light shielding plate is mounted on the rotor, and the light source is a light emitting diode or a small light bulb. When the rotor rotates, the photosensitive components on the stator will intermittently generate pulse signals at a certain frequency due to the action of the visor.
A brushless DC motor using an electromagnetic position sensor is provided with an electromagnetic sensor component (for example, a coupling transformer, a proximity switch, an LC resonance circuit, etc.) on the stator assembly. When the position of the permanent magnet rotor changes, the electromagnetic effect will cause the electromagnetic sensor. A high frequency modulated signal is generated (the amplitude of which varies with rotor position).
Superiority
DC motors have fast response, large starting torque,
From zero speed to rated speed, it has the performance of providing rated torque, but the advantage of DC motor is also its shortcoming. Because DC motor must produce constant torque under rated load, the armature magnetic field and rotor magnetic field must be constant. Maintain 90°, which is done by carbon brushes and commutators. Carbon brushes and commutators generate sparks and toner when the motor rotates. In addition to damage to the components, the use is also limited. AC motors do not have carbon brushes and commutators. They are maintenance-free, rugged, and widely used. However, to achieve the equivalent performance of DC motors, complex control technology can be used. Today's semiconductors are developing rapidly and the power component switching frequency is much faster, improving the performance of the drive motor. The speed of the microprocessor is also faster and faster, and the AC motor control can be placed in a rotating two-axis orthogonal coordinate system, and the AC motor's current component in two axes can be appropriately controlled to achieve DC motor control and is equivalent to the DC motor. performance.
In addition, many microprocessors have the necessary functions to control the motor in the chip, and the volume is getting smaller and smaller; like analog-to-digital converter (ADC), pulse width modulation (Pulsewide Modulator, PWM)... Wait. The DC brushless motor is an electronically controlled AC motor commutation, which has an application similar to the DC motor characteristics and lacks the DC motor mechanism.
Control structure
The brushless DC motor is a kind of synchronous motor, that is to say, the speed of the rotor of the motor is affected by the speed of the rotating magnetic field of the motor stator and the number of poles of the rotor (p):
n=120. f/p. In the case of a fixed number of poles of the rotor, changing the frequency of the rotating magnetic field of the stator can change the rotational speed of the rotor. The DC brushless motor is a method in which the synchronous motor is electronically controlled (driver), the frequency of the rotating magnetic field of the stator is controlled, and the rotational speed of the motor rotor is fed back to the control center for repeated correction, in order to achieve a near-DC motor characteristic. That is to say, the brushless DC motor can control the rotor of the motor to maintain a certain speed when the load changes within the rated load range.
The DC brushless driver includes a power supply unit and a control unit, and a power supply unit that supplies three-phase power to the motor, and the control unit converts the input power frequency according to requirements.
The power supply unit can directly input DC (usually 24v) or AC input (110v/220v). If the input is AC, it must be converted to DC by the converter. Whether the DC input or the AC input is to be transferred to the motor coil, the DC voltage must be converted from the inverter to the 3-phase voltage to drive the motor. The inverter is generally divided into six upper power transistors (q1, q3, q5)/lower arms (q2, q4, q6) by six power transistors (q1 to q6) to connect the motor as a switch for controlling the flow through the motor coil. The control unit provides pwm (pulse width modulation) to determine the switching frequency of the power transistor and the timing of commutation of the inverter (Inverter). DC brushless motors generally use a speed sensor that can stabilize the set value without changing too much when the load changes, so the Hall is equipped with a Hall-sensor that can sense the magnetic field. Closed loop control is also used as the basis for phase sequence control. But this is only used as speed control and can not be used as positioning control.
Control principle
In order for the motor to rotate, the control unit must determine the position of the motor rotor according to the Hall-sensor, and then according to the stator winding, the order of the power transistors in the inverter (inverter) is turned on (or off), so that the current flows sequentially. A forward (or reverse) rotating magnetic field is generated by the motor coil and interacts with the magnet of the rotor, so that the motor can be rotated clockwise/reversely. When the rotor of the motor rotates to the position where the Hall-sensor induces another set of signals, the control unit turns on the next set of power transistors, so that the circulating motor can continue to rotate in the same direction until the control unit decides to stop the motor when the rotor is stopped. Transistor (or only the lower arm power transistor); to reverse the motor rotor, the power transistors are turned on in reverse order.
