Introduction to the principle and characteristics of brushless DC motor (BLDC)

A brushless DC motor (BLDC) can be thought of as the polar opposite of a brushed DC motor, where the permanent magnets are on the rotor and the wires are wound on the stator. Therefore, the motor is brushless and commutator free, eliminating the disadvantages associated with brushed DC motors producing sparks. The following will introduce the principle and characteristics of brushless DC motors to help you.

Unlike an open-loop stepper application where the driven stator coils determine the rotor position, in a BLDC motor the rotor position determines which stator coils are driven. The stator flux vector position must be synchronized with the rotor flux vector position (not opposite) for smooth motor operation. To do this, knowledge of the rotor position is required to determine which stator coils to drive. There are several techniques to accomplish this, but the most common technique is to monitor the rotor position using Hall effect sensors. Unfortunately, these sensors and their associated connectors and wiring harnesses add to system costs and reduce reliability.

To reduce these problems, various techniques have been developed to eliminate these sensors, thereby enabling sensorless operation. Most techniques rely on extracting position information from the back EMF waveform of the stator windings as the motor rotates. However, techniques that rely on back-EMF sensing are useless when the motor is spinning slowly or stationary, where the back-EMF waveform is weak or non-existent. Therefore, new technologies need to be continuously developed to obtain rotor position information from other signals at low or zero speed.

BLDC motors have an absolute advantage in terms of efficiency ratings, generally around 95%. Current research into new amorphous alloy materials is pushing that number to new highs. Efficiencies of 96% in the 100W range have been reported. BLDC motors are still competing for the title of "world's fastest motor", with some motor speeds reaching hundreds of thousands of RPM (400KRPM has been reported in one application).

The most commonly used BLDC motors use a 3-phase stator structure. Therefore, a standard 6-transistor inverter is the most commonly used power stage. Depending on the operational requirements (sensored vs. sensorless, commutated vs. sinusoidal, PWM vs. SVM, etc.), there are many ways to drive a transistor to achieve the desired goal, the list goes on and on. This places high demands on the flexibility of the PWM generator, which is typically located in a microprocessor.

Brushless DC Motor Features

●Comprehensively replace DC motor speed control, fully replace frequency converter + variable frequency motor speed control, fully replace asynchronous motor + reducer speed control;

●It has all the advantages of traditional DC motors, and at the same time cancels the structure of carbon brushes and slip rings;

●It can run at low speed and high power, and can directly drive large loads without the reducer;

●Small size, light weight, large output;

●Excellent torque characteristics, good torque performance at medium and low speeds, large starting torque, and small starting current;

●Stepless speed regulation, wide speed regulation range, strong overload capacity;

●Soft start and soft stop, good braking characteristics, can save the original mechanical brake or electromagnetic brake device;

●High efficiency, the motor itself has no excitation loss and carbon brush loss, eliminating multi-stage deceleration consumption, the comprehensive power saving rate can reach 20%~60%, and the purchase cost can be recovered in one year only for power saving;

●High reliability, good stability, strong adaptability, simple repair, and maintenance;

●Bump and vibration resistance, low noise, small vibration, smooth operation, and long service life;

●No radio interference, no sparks, especially suitable for explosive places, with explosion-proof type;

●Trapezoidal wave magnetic field motor and sine wave magnetic field motor can be selected as required.