Brushless DC electric motor


A brushless DC electric motor, also known as an electronically commutated motor, is a synchronous motor using a direct current electric power supply. It uses an electronic controller to switch DC currents to the motor windings, producing magnetic fields that effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the current pulses that control the speed and torque of the motor. It is an improvement on the mechanical commutator used in many conventional electric motors.
The construction of a brushless motor system is typically similar to a permanent magnet synchronous motor, but can also be a switched reluctance motor, or an induction motor. They may also use neodymium magnets and be outrunners, inrunners, or axial.
The advantages of a brushless motor over brushed motors are high power-to-weight ratio, high speed, nearly instantaneous control of speed and torque, high efficiency, and low maintenance. Brushless motors find applications in such places as computer peripherals, hand-held power tools, and vehicles ranging from model aircraft to automobiles. In modern washing machines, brushless DC motors have allowed replacement of rubber belts and gearboxes by a direct-drive design.

Background

s were invented in the 20th century and are still common. Brushless DC motors were made possible by the development of solid-state electronics in the 1960s.
An electric motor develops torque by keeping the magnetic fields of the rotor and the stator misaligned. One or both sets of magnets are electromagnets, made of a coil of wire wound around an iron core. DC running through the wire winding creates the magnetic field, providing the power that runs the motor. The misalignment generates a torque that tries to realign the fields. As the rotor moves and the fields come into alignment, it is necessary to move either the rotor's or stator's field to maintain the misalignment and continue to generate torque and movement. The device that moves the fields based on the position of the rotor is called a commutator.

Brush commutator

In brushed motors this is done with a rotary switch on the motor's shaft called a commutator. It consists of a rotating cylinder or disc divided into multiple metal contact segments on the rotor. The segments are connected to conductor windings on the rotor. Two or more stationary contacts called brushes, made of a soft conductor such as graphite, press against the commutator, making sliding electrical contact with successive segments as the rotor turns. The brushes selectively provide electric current to the windings. As the rotor rotates, the commutator selects different windings and the directional current is applied to a given winding such that the rotor's magnetic field remains misaligned with the stator and creates a torque in one direction.
The brush commutator has disadvantages that has led to a decline in use of brushed motors. These disadvantages are:
  • The friction of the brushes sliding along the rotating commutator segments causes power losses that can be significant in a low-power motor.
  • The soft brush material wears down due to friction, creating dust, and eventually the brushes must be replaced. This makes commutated motors unsuitable for low particulate or sealed applications like hard disk motors and for applications that require maintenance-free operation.
  • The electrical resistance of the sliding brush contact causes a voltage drop in the motor circuit called, which consumes energy.
  • The repeated abrupt switching of the current through the inductance of the windings causes sparks at the commutator contacts, which is a fire hazard in explosive atmospheres and a source of electronic noise, which can cause electromagnetic interference in nearby microelectronic circuits.
During the last hundred years, high-power DC brushed motors, once the mainstay of industry, were replaced by alternating current synchronous motors. Today, brushed motors are used only in low-power applications or where only DC is available, but the above drawbacks limit their use even in these applications.

Brushless solution

In brushless DC motors, an electronic controller replaces the brush commutator contacts. An electronic sensor detects the angle of the rotor and controls semiconductor switches such as transistors that switch current through the windings, either reversing the direction of the current or, in some motors turning it off, at the correct angle so the electromagnets create torque in one direction. The elimination of the sliding contact allows brushless motors to have less friction and longer life; their working life is limited only by the lifetime of their bearings.
Brushed DC motors develop a maximum torque when stationary, linearly decreasing as velocity increases. Some limitations of brushed motors can be overcome by brushless motors; they include higher efficiency and lower susceptibility to mechanical wear. These benefits come at the cost of potentially less rugged, more complex, and more expensive control electronics.
A typical brushless motor has permanent magnets that rotate around a fixed armature, eliminating problems associated with connecting current to the moving armature. An electronic controller replaces the commutator assembly of the brushed DC motor, which continually switches the phase to the windings to keep the motor turning. The controller performs similar timed power distribution by using a solid-state circuit rather than the commutator system.
Brushless motors offer several advantages over brushed DC motors, including high torque to weight ratio, increased efficiency producing more torque per watt, increased reliability, reduced noise, longer lifetime by eliminating brush and commutator erosion, elimination of ionizing sparks from the commutator, and an overall reduction of electromagnetic interference. With no windings on the rotor, they are not subjected to centrifugal forces, and because the windings are supported by the housing, they can be cooled by conduction, requiring no airflow inside the motor for cooling. This in turn means that the motor's internals can be entirely enclosed and protected from dirt or other foreign matter.
Brushless motor commutation can be implemented in software using a microcontroller, or may alternatively be implemented using analog or digital circuits. Commutation with electronics instead of brushes allows for greater flexibility and capabilities not available with brushed DC motors, including speed limiting, microstepping operation for slow and fine motion control, and a holding torque when stationary. Controller software can be customized to the specific motor being used in the application, resulting in greater commutation efficiency.
The maximum power that can be applied to a brushless motor is limited almost exclusively by heat; too much heat weakens the magnets and damages the windings' insulation.
When converting electricity into mechanical power, brushless motors are more efficient than brushed motors primarily due to the absence of brushes, which reduces mechanical energy loss due to friction. The enhanced efficiency is greatest in the no-load and low-load regions of the motor's performance curve.
Environments and requirements in which manufacturers use brushless-type DC motors include maintenance-free operation, high speeds, and operation where sparking is hazardous or could affect electronically sensitive equipment.
The construction of a brushless motor resembles a stepper motor, but the motors have important differences in implementation and operation. While stepper motors are frequently stopped with the rotor in a defined angular position, a brushless motor is usually intended to produce continuous rotation. Both motor types may have a rotor position sensor for internal feedback. Both a stepper motor and a well-designed brushless motor can hold finite torque at zero RPM.

Controller implementations

Because the controller implements the traditional brushes' functionality, it needs to know the rotor's orientation relative to the stator coils. This is automatic in a brushed motor due to the fixed geometry of the rotor shaft and brushes. Some designs use Hall effect sensors or a rotary encoder to directly measure the rotor's position. Others measure the back-EMF in the undriven coils to infer the rotor position, eliminating the need for separate Hall effect sensors. These are therefore often called sensorless controllers.
Controllers that sense rotor position based on back-EMF have extra challenges in initiating motion because no back-EMF is produced when the rotor is stationary. This is usually accomplished by beginning rotation from an arbitrary phase, and then skipping to the correct phase if it is found to be wrong. This can cause the motor to run backwards briefly, adding even more complexity to the startup sequence. Other sensorless controllers are capable of measuring winding saturation caused by the position of the magnets to infer the rotor position.
A typical controller contains three polarity-reversible outputs controlled by a logic circuit. Simple controllers employ comparators working from the orientation sensors to determine when the output phase should be advanced. More advanced controllers employ a microcontroller to manage acceleration, control motor speed and fine-tune efficiency.
Two key performance parameters of brushless DC motors are the motor constants and .

Variations in construction

Brushless motors can be constructed in several different physical configurations. In the conventional inrunner configuration, the permanent magnets are part of the rotor. Three stator windings surround the rotor. In the external-rotor outrunner configuration, the radial relationship between the coils and magnets is reversed; the stator coils form the center of the motor, while the permanent magnets spin within an overhanging rotor that surrounds the core. Outrunners typically have more poles and have a higher torque at low RPM. In the flat axial flux type, used where there are space or shape constraints, stator and rotor plates are mounted face to face. In all brushless motors, the coils are stationary.
There are two common electrical winding configurations: the delta configuration connects three windings to each other in a triangle-like circuit, and power is applied at each of the connections. The wye configuration, sometimes called a star winding, connects all of the windings to a central point, and power is applied to the remaining end of each winding. A motor with windings in delta configuration gives low torque at low speed but can give higher top speed. Wye configuration gives high torque at low speed but not as high top speed. The wye winding is normally more efficient. Delta-connected windings can allow high-frequency parasitic electrical currents to circulate entirely within the motor. A Wye-connected winding does not contain a closed loop in which parasitic currents can flow, preventing such losses. Aside from the higher impedance of the wye configuration, from a controller standpoint, the two winding configurations can be treated exactly the same.