Brushless DC motor is the use of semiconductor switching devices to achieve electronic commutation, that is, electronic switching devices instead of traditional contact commutator and brush. It has the advantages of high reliability, no reversing spark, low mechanical noise, and is widely used in high-grade recording booth, video recorder, electronic instrument and automatic office equipment.
Brushless DC motor is composed of permanent magnet rotor, multipole winding stator, position sensor and so on. Position sensing according to the change of rotor position, along a certain sequence of stator winding current converter (i.e. detecting the rotor magnetic pole relative to the position of the stator winding, and in determining the location of position sensor signal, the signal conversion circuit to control the power switch circuit, after processing according to certain logic relation between winding current switch). The operating voltage of the stator windings is provided by an electronic switching circuit controlled by the output of the position sensor.
There are three types of position sensors: magneto-sensitive, photoelectric and electromagnetic. Brushless DC motor with magnetic-sensitive position sensor, its magnetic-sensitive sensor (such as Hall element, magnetic-sensitive diode, magnetic-sensitive polar tube, magnetic-sensitive resistor or special-purpose integrated circuit, etc.) is mounted on the stator assembly, which is used to detect the magnetic field changes caused by permanent magnet and rotor rotation.
The brushless DC motor with photoelectric position sensor is equipped with photoelectric sensor in a certain position on the stator component, and the light source is light-emitting diode or small bulb. When the rotor rotates, the light-sensitive components on the stator will generate pulse signals intermittently at a certain frequency due to the effect of the mask.
Using electromagnetic position sensor brushless dc motor, is in the electromagnetic sensors are installed on the stator component parts (such as the coupling transformer, close to switch, LC resonance circuit, etc.), when the permanent magnet rotor position changes, electromagnetic effect will make the electromagnetic sensor produces high frequency modulation signal (the amplitude changes with the rotor position).
1. Superiority
Dc motor has a rapid response, large starting torque, rotating speed of from zero to rated speed can provide the rated torque performance, but the advantages of dc motor is also its drawback, because the direct current machine to produce a constant torque under the rated load performance, the armature magnetic field and rotor magnetic field must be constant to maintain 90 °, this is about to borrow from the carbon brush and commutator. Carbon brush and commutator will produce spark and toner when the motor is rotating, so in addition to causing component damage, the use of the occasion is also limited. Ac motor without carbon brush and commutator, maintenance-free, strong, widely used, but in terms of characteristics to achieve the same performance as DC motor must be achieved by complex control technology. Nowadays, the semiconductor develops rapidly and the switching frequency of power components is much faster, which improves the performance of the driving motor. Microprocessor speed is also getting faster and faster, can realize the ac motor control in a rotating two axis coordinate system, appropriate control of ac motor current components in the two axis, to achieve similar dc motor control and has the same performance as dc motor.
In addition, many microprocessors have done the necessary functions to control the motor in the chip, and the size is becoming smaller and smaller; Like analog-to-digital Converter (ADC), Pulse Wide Modulator (PWM)... And so on. Brushless DC motor is an application that electronically controls the commutation of ac motor to obtain similar characteristics of DC motor without the loss of DC motor mechanism. [1]
Second, control structure
Brushless DC motor is a type of synchronous motor, that is to say, the rotor speed 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. When the number of poles of the rotor is fixed, the rotor speed can be changed by changing the frequency of stator rotating magnetic field. Brushless DC motor is the synchronous motor plus electronic control (driver), control the frequency of stator rotating magnetic field and the rotor speed of the motor back to the control center for repeated correction, in order to achieve close to the characteristics of dc motor. That is to say, the brushless DC motor can maintain a certain rotor speed when the load changes within the rated load range.
The brushless DC driver includes a power supply and control unit. The power supply unit provides three-phase power to the motor, and the control unit converts the input power frequency according to the demand.
The power supply can be directly input by DC (usually 24V) or ac (110V / 220V). If the input is AC, it must be converted to DC through a Converter. Whether dc input or AC input to the motor coil before the DC voltage converter (Inverter) into the 3-phase voltage to drive the motor. Inverter (Inverter) generally consists of six power transistors (Q1 ~ Q6) divided into upper arm (Q1, Q3, Q5)/ lower arm (Q2, Q4, Q6) connected to the motor as a switch to control the flow through the motor coil. The control section provides PWM (pulse width modulation) to determine the switching frequency of the power transistor and the timing of the commutator (Inverter). Brushless DC motor generally hopes to use the speed control that the speed can be stable at the set value without too much change when the load changes, so the motor is equipped with a Hall sensor that can sense the magnetic field, which serves as the closed-loop control of speed and the basis of phase sequence control. But this is only for speed control and not for positioning control.
Three, control principle
To make the motor rotate, the control department must according to the position of the motor rotor induced by hhall sensor, and then according to the stator winding decision to open (or close) the order of the power transistor in the converter (Inverter), so that the current flows through the motor coil in order to generate a forward (or reverse) rotating magnetic field, and interact with the magnet of the rotor. This allows the motor to turn in/out of time. When the motor rotor rotates to the position where the hhall sensor senses another set of signals, the control part turns on the next set of power transistors again, so the cycle motor can continue to rotate in the same direction until the control part decides to stop the motor rotor and turn off the power transistor (or only turn on the lower arm power transistor); To reverse the rotor of the motor, the power transistor turns on in reverse order.
Four, fixed magnetic field brushless motor
General brushless DC motor is a servo motor in nature, composed of synchronous motor and driver, is a frequency conversion speed regulating motor. Brushless DC motor with variable voltage speed regulation is a brushless DC motor in the real sense. It is composed of stator and rotor. The stator is composed of iron core. The rotor consists of a cylindrical magnet (with a shaft in the middle), or of an electromagnet plus a collector ring. The brushless DC motor can generate torque, but not control the direction. In any case, the motor is a very meaningful invention. When used as a DC generator, the invention can generate dc current of continuous amplitude, thus avoiding the use of filter capacitors, and the rotor can be permanent magnet, brush-excited or brushless excited. When used as a large motor, the generator will generate self-induction and need to use protection devices.