Brushless DC electric motor
From Wikipedia, the free encyclopedia
A brushless DC motor (BLDC) is an AC synchronous electric motor that from a modeling perspective looks very similar to a DC motor. Sometimes the difference is explained as an electronically-controlled commutation system, instead of a mechanical commutation system, although this is misleading, as physically the two motors are completely different. (The rest of this article assumes the reader is familiar with the principles of electrical motors.)
Three subtypes exist:
- The three-phase AC synchronous motor type has three electrical connections
- The stepper motor type may have more poles on the stator.
- The reluctance motor has all its poles on the stator, and a magnetic core on the rotor.
In a conventional (brushed) DC-motor, the brushes make mechanical contact with a set of electrical contacts on the rotor (called the commutator), forming an electrical circuit between the DC electrical source and the armature coil-windings. As the armature rotates on axis, the stationary brushes come into contact with different sections of the rotating commutator. The commutator and brush-system form a set of electrical switches, each firing in sequence, such that electrical-power always flows through the armature-coil closest to the stationary stator (permanent magnet.)
In a BLDC motor, the brush-system/commutator assembly is replaced by an intelligent electronic controller. The controller performs the same power-distribution found in a brushed DC-motor, only without using a commutator/brush system. The controller contains a bank of MOSFET devices to drive high-current DC power, and a microcontroller to precisely orchestrate the rapid-changing current-timings. Because the controller must follow the rotor, the controller needs some means of determining the rotor's orientation/position (relative to the stator coils.) Some designs use Hall effect sensors 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, and therefore are often called "sensorless" controllers. (The BLDC motor has a trapezoidal backEMF, while a brushless AC motor has a sinousoidal backEMF.)
BLDC motors can be constructed in two different physical configurations: In the 'conventional' (also known as 'inrunner') configuration, the permanent magnets are mounted on the spinning armature (rotor.) The stator coils surround the rotor. In the 'outrunner' configuration, the radial-relationship between the coils and magnets are reversed; the stator coils form the center (core) of the motor, while the permanent magnets spin on an overhanging rotor which surrounds the core. In all BLDC motors, the stator-coils are stationary.
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[edit] Comparison with brushed-DC motors
BLDC motors offer several advantages over brushed DC-motors, including higher efficiency and reliability, reduced noise, longer lifetime (no brush erosion), elimination of ionizing sparks from the commutator, and overall reduction of electromagnetic interference (EMI.) BLDC's main disadvantage is higher cost, which arises from two issues. First, BLDC motors require complex electronic speed control to run. Brushed DC-motors can be regulated by a comparatively trivial variable-resistor (potentiometer or rheostat), which is inefficient but also satisfactory for cost-sensitive applications. Second, when comparing manufacturing techniques between BLDC and brushed motors, some BLDC designs, especially in RC modelling area, require manual-labor to hand-wind the stator coils. On the other hand, brushed motors use armature coils which can be inexpensively machine-wound.
BLDC motors are considered more efficient than brushed DC-motors. This means for the same input power, a BLDC motor will convert more electrical power into mechanical power than a brushed motor, mostly due to absence of friction of brushes. The enhanced efficiency is greatest in the no-load and low-load region of the motor's performance curve. Under high mechanical loads, BLDC motors and high-quality brushed motors are comparable in efficiency.
[edit] Applications
BLDC motors can potentially be deployed in any field-application currently fulfilled by brushed DC motors. Cost and control complexity prevents BLDC motors from replacing brushed motors in most common areas of use. Nevertheless, BLDC motors have come to dominate many applications: Consumer devices such as computer hard drives, CD/DVD players, and PC cooling fans use BLDC motors almost exclusively. Low speed, low power brushless DC motors are used in direct-drive turntables. High power BLDC motors are found in electric vehicles and some industrial machinery. These motors are essentially AC synchronous motors with permanent magnet rotors.
The Honda Civic hybrid car uses a BLDC motor to supplement the output of the internal combustion engine when the extra power is needed. It is also used to start the engine vice a conventional starter and solenoid method.
[edit] Model aircraft scene
Recently, an increase in the popularity of electric-powered model aircraft has spurred demand for high-performance BLDC motors. Many hobbyists have begun salvaging BLDC motors from scrap CD/DVD-ROM drives, refurbishing them for use in radio controlled planes. This has led to increased direct consumer-availability of DIY (do-it-yourself) motor kits, for use in radio-controlled vehicles. BLDC motors sold as parts kits allow the buyer to save money through additional assembly work.
