Permanent Magnet Brushless DC Motor
Brushed motors use fixed brushes to supply electric energy to the commutator, which reverses current to the coil polarities to generate rotation. This method is fine for low cost applications like motorized toys.
To control speed and torque, an “H-bridge” drive circuit of electronic switches (transistors, IGBTs, or MOSFETs) is used. This method also requires a sensor to detect the physical position of the rotor.
No Drive Electronics
A brushed motor requires brushes that rub on a portion of the rotor called a commutator to transfer current from the power source to the rotor windings. As they rub, arcing occurs that generates significant amounts of electrical noise and stray magnetic fields that can cause interference with sensitive circuitry. To avoid this, the commutator segments are insulated with carbon-copper composite graphite, and significant effort is made to avoid contact between the commutator and other components.
In contrast, brushless motors have no brushes. Their armature windings are connected in a star or delta configuration, with the three motor terminals wired to a DC supply that has polarity-reversible outputs controlled by a microcontroller. The microcontroller can manage acceleration, motor speed and fine-tune efficiency.
The rotor is a cylindrical structure with a pair of magnetic poles (North and South) that face the armature. Its stator houses the magnets and also provides a low reluctance return path for magnetic flux from the rotor to the stator core. This helps the rotor maintain a constant speed, even at very low speeds.
The armature windings in the rotor and stator are energized from the DC supply in a sequence to produce a rotating electromagnetic field that generates torque. Unlike brushed motors, which Permanent magnet brushless DC motor are “hard switched” and thus generate torque ripple, brushless motors gradually transition current between windings to reduce the mechanical pulsation of energy onto the rotor that causes vibration and noise.
No Position Sensors
Unlike the traditional brushed DC motor, Permanent magnet brushless motors do not require position sensors to commutate current in their windings. Instead, their motion is managed by controller electronics. This eliminates mechanical wear and tear, as well as the generation of heat which weakens the magnetic field and destroys the insulation of the motor windings.
With the right control algorithms, positioning can be estimated from back-EMF and current sensing alone. In this way, position sensors are completely eliminated from the motor assembly, further reducing its cost and size. For this, a simple method uses the virtual neutral point of each phase based on the rotor speed-dependent zero crossing of the back-EMF signal VSUM (which is obtained from the summation of stator phase voltages, rotor flux third harmonic components lr3, and motor phase currents) to estimate the required commutation points.
Alternatively, more sophisticated methods use the online adaptation of rotor speed and stator resistance. An example is the AFFO observer (Assumption-Focused Observer) which estimates both the rotor speed and the stator resistance based on Lyapunov stability criteria, and converges to their real values at steady state.
No Reversal Switch
Brushed DC motors use a mechanical switch (commutator) to reverse current from the brushes as they contact them. This creates significant arcing between the brushes and generates electrical noise, even with the use of commutation circuits (RC snubbers).
Brushless motors have an electronic control system that replaces the mechanical commutator contacts. This is called “electronic commutation”.
The electronic commutation circuit contains semiconductor switches (like transistors) that alternately connect or disconnect the rotor windings to the stator coils. The electronic controller can control these switches to make the magnetic field rotate in either direction, based on how the rotor is oriented.
In the case of the permanent magnet BLDC motor, the magnetic field can be rotated in either of two physical configurations: inrunner or outrunner. In an inrunner configuration, the rotor magnets are part of the stator core, while in a outrunner design, the rotor magnets are overhanging the core.
Once the rotor has been properly aligned, the motor can be started by applying a DC current to the rotor windings for a few seconds. This forces the rotor magnets to rotate in the correct position to be compatible with the armature coils. Once the magnetic field has settled, the motor can begin operating normally. This approach also minimizes startup current draw and reduces thermal management challenges. Ultimately, it enables the motor to operate more efficiently and with lower pollutant emissions and nonrenewable energy consumption than conventional DC brushed motors.
High Efficiency
The brushes of a brushed DC motor act as a kind of electrical switch that continuously opens and closes, allowing significant current to flow through the rotor windings. This is not ideal because it generates significant heat, which reduces efficiency and Permanent magnet brushless DC motor company can cause arcing that could damage the rotor and brushes. These arcs also create high-frequency electrical noise that can interfere with sensitive electronic circuitry.
Brushless DC motors replace the brushes and commutator with an “H-bridge” of electronic switches – transistors, IGBTs or MOSFETs – that allow the armature to be energized with opposite polarity voltages simultaneously, allowing the magnets to attract and repel each other across the air gap and keep the rotor spinning. This type of commutation also allows the speed and torque to be controlled by pulse-width modulating one or more of these switches.
Once the rotor is turned, the Hall sensors (either on the rotor or stator) generate a high or low signal based on which way the rotor is rotating and which of the two identical windings are energized. Using this information, the microcontroller can calculate which of the stator coils to supply with dc power to rotate the rotor at its required speed.
ANSYS Maxwell simulations of no-load operations show that the resulting cogging torque is very low and the radial air gap flux density waveform has a relatively perfect shape. This is conducive to smooth operation and reduced vibration and noise, as well as minimizing the potential for demagnetization.
