The Basics of a BLDC Motor
Unlike brushed motors, BLDC motors do not have carbon brushes and copper commutator. Instead, current to the fixed coils in the rotor is controlled electronically.
BLDC motors are popular in applications like cordless power tools, RC vehicles, and HVAC systems. They offer a high power-to-weight ratio and quiet operation. For demanding applications requiring position control, they can also be equipped with optical encoders.
Stator
The fixed part of a BLDC motor is called the stator. It is surrounded by a permanent magnet. A rotor, which contains the poles, is fixed within the stator. The rotor magnets generate the force that causes the motor to rotate. The rotor can have from two to eight pole-pairs.
To make a BLDC motor operate, its coils are sequentially energized in a specific pattern with varying electrical — and thus magnetic — directions by a specialized driver. This enables the motor to achieve a much wider range of speeds and torques than is possible with brushed DC motors of similar size.
A BLDC motor has fewer moving parts, enabling it to produce significantly less noise than a brushed motor. This BLDC motor makes it ideal for applications where the motor must operate quietly. It also produces less heat, and is not prone to the sparking that occurs when brushes contact the commutator in a brushed motor.
Unlike stepper motors, which use pulses to control positioning, a BLDC motor can hold its position using a small current proportional to the external force. This makes it suitable for service robots that need to remain stationary while delivering services. A BLDC motor is an excellent choice for this type of application because it can deliver the required amount of torque with precision.
Rotor
A BLDC motor’s rotor is composed of permanent magnets and coils. Unlike brushed DC motors, the current in the coils is not supplied by brushes. Instead, a method of commutation must be employed. This process involves the use of a sensor (often a Hall element) to determine the rotor position. This information can be fed into an MCU or controller that can then generate the appropriate analog voltage to control the coil current.
The most straightforward approach to this task is called trapezoidal control, in which each phase of the motor is powered individually by energizing the coils in a sequence. In the example shown in Figure 4, the RED winding labeled as “001” will be energized as the NORTH pole and the BLUE winding will be energized as the SOUTH pole. This sequence, repeated over and over again, produces the desired rotational speed.
Another commutation method, known as field oriented control, works by controlling the current vector directly in a biaxial coordinate system of the rotor. This allows the torque to be generated from a combination of repulsion and attraction forces. It also eliminates the need for converting current flows into current values, which can cause distortion of the rotor’s magnetic field and excessive power losses.
Windings
The rotor in a BLDC motor is made up of permanent magnets. They can be arranged with two to eight pole-pairs and are typically manufactured from a rare earth alloy of Neodymium, Ferrite and Boron (NdFeB). The magnetic material used for the rotor needs to have high flux density to achieve appropriate motor torque and propeller efficiency.
Brushed DC motors use brushes and a commutator to switch current to Waterproof BLDC motor the stationary magnet coils. In a brushed motor, this switching process generates significant electrical noise that can get coupled into sensitive circuits. With a brushed DC motor, arcing between the brushes and rotor are also common and this can cause damage to the brushes and commutator.
A BLDC motor uses an electronic switch to commutate the windings of the motor, eliminating the need for brushes and a commutator. This improves the reliability of the motor and increases speed stability/control. It also reduces the thermal losses of the windings.
Changing the winding connection schemes dynamically on demand during the operation of the BLDC motor allows you to extend the operating range of the motor and provides higher performance in terms of rotation synchronization, power efficiency and torque. This approach confirms the original expectations that the use of optimal schemes for specific operational conditions increases the speed of a motor and reduces heat losses in the windings.
Control
BLDC motors eliminate brushes and their commutator. This eliminates the contact resistance that causes arcing during starting and switching between open and closed switches. It also avoids the high current flow and associated electromagnetic noise generated by commutator contacts and their inductive switching.
Instead, a sensor is used to sense the position of the rotor. The sensor consists of a Hall element or Hall effect IC that detects the magnetic flux density in the rotor shaft. The sensor outputs a low impedance digital signal representing the sensor’s output polarity. The sensor signals are fed to a motor controller or MCU. These are typically designed exclusively for BLDC control and have optimized comparator circuits.
The MCU or controller compares the rotor position signal with an opcode to determine the phase of the rotor windings to be energized. Depending on the application, the motor may be operated with simple trapezoidal control, which energises each coil in either a high or low state, or with more advanced sinusoidal control techniques that can achieve much smoother acceleration and deceleration responses.
Most BLDC motors use IGBT or MOSFET driver chips. Several major semiconductor vendors offer integrated BLDC motor drivers that include an 8-bit microcontroller and a three-phase inverter. This simplifies the design process and helps keep costs down. In addition, 8-bit microcontroller development kits, such as those available from Atmel, are an inexpensive way to explore different control regimes before designing a full-size motor.
