Brushless motor
From Combat Robot
A brushless motor is, as its name implies, an electric motor without brushes.
Contents |
Advantages
Traditional motors have carbon brushes that ride along a commutator, a series of metal pads. These brushes conduct power to the pads, which are wired to the rotating armature. The pads are arranged in such a fashion that armature current is properly switched in a sequence every rotation. For more information on brushed motors, see electric motor
There are several disadvantages to a brushed system:
- High current switching results in electrical arcing between adjacent contacts. This wears both brush and commutator and can be damaging if sustained for long periods of time.
- Excess friction from the brushes which have to be spring-loaded into the commutator to maintain contact at high speeds. This results in heat generation, which can be hazardous to the motor if it is continuously loaded.
- Sliding contacts wear out. Through a combination of mechanical abrasion and vaporization by arcing, both surfaces eventually become worn to the point of failure. Although the lifespan of a motor in a combat robot is often not long enough to completely wear out brushes, it is common in industrial and commercial fields.
Theory
The layout of a brushless motor is the opposite of a brushed motor. Instead of the coils and armature rotating, they are fixed and called the stator. The magnets then spin, becoming the rotor. Specialized controllers approximate magnet position and switch coils accordingly. Brushless motors of interest to robot builders(thus excluding most alternating current motors) accomplish this in one of two methods:
- Sensored - These motors require special electronic controllers and have a series of Hall-effect switches that are sensitive to magnetic fields. When the controller spins the motor, it will read the position of the magnets using these sensors and switch current accordingly. Having positive feedback on rotor position, the controllers can power the motor in either direction and start and stop accurately.
- Sensorless - These motors eliminate the Hall-effect sensors and instead rely on the back-EMF generated by coils and magnets. This type reduces motor cost, and leaves sensing duty to the controller. The downside is that since magnet positions are not known, startup can be unreliable, often leaving the motor cogging, or rapidly switching back and forth in an attempt to start a heavy load, such as a weapon or drivetrain.
It is important to remember that a brushless motor will not operate without a motor controller. Unlike a brushed motor, which can simply be hooked up to an appropriate power suply.
Layout
Hobby brushless motors come in two distinct types by component layout:
- The inrunner. In this type of motor, the stator coils form a ring shape, and the magnets are mounted on the central shaft and spin with it. For a given power, these motors tend to spin faster with less torque.
- The outrunner. The layout in this motor is reversed: instead of having the magnets rotating with the shaft, they are mounted on the inside of the motor can, and the entire can spins, coupled on one end to the shaft, which is often supported by bearings inside the stator mount. Mounting these motors is only possible by the back of the motor, which the coils are attached to and does not rotate. These motors tend to have higher torque outputs due to the concentration of generated force around the edges, and have higher inertia due to the higher spinning mass. This is advantageous in robots that use these for kinetic energy weapons.
Most hobby brushless motors are intended for use in model airplanes due to their high power density. This often means sacrificed structural integrity for lighter weight. Thus, use in a robot warrants protection from impulses and shocks, although some motors, such as the AXI, are sturdy enough to be used directly on a weapon.
Common items with brushless motors
- DC fans, such as computer case fans, and larger ventilation type fans.
- Computer drives. Hard drives and compact-disc drives often have a high speed spindle motor supported by ball or fluid bearings. It is common for an airplane hobbyist to hack motors out of these, modify them for more power, and use them on model planes. Several robots with these types of motors also exist.
External Links
Aveox - a manufacturer of high performance DC brushless motors.
Hobby Lobby - Has an immense selection of motors.
GoBrushless.com - Motor kits based on the hard drive and CD-type motor.
