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Unlike inner rotor motors that require more length to house the magnets, an outer rotor design can be much shorter. This means a lower weight, smaller footprint and more efficient power output.
An outer rotor motor also reduces iron losses by strongly limiting leakage flux in the iron ribs that contain the magnets against centrifugal force. This is especially important for high-speed applications.
Brushed motors use brushes and a mechanical commutator to switch power to the rotor coils. This causes the rotor coils to be constantly attracted and repelled by the fixed magnets in the stator, which causes the rotor to spin.
The armature of a brushless DC motor contains wire coils around a laminated soft iron ferromagnetic core. When current flows through the armature, it makes the coils act as temporary magnets and interacts with permanent magnets in the stator to produce the force that drives the shaft.
With the brushless rotor, a motor can achieve higher efficiency and more precise control. This is why it’s found in cordless tools like Makita’s line of string trimmers and chainsaws, as well as electric vehicles such as golf carts and mobility carts. It’s also used in applications that require low noise and high efficiency, such as servo motors. BLDC motors are typically 85%+ efficient, while simple brushed motors are only 75-80% efficient.
Unlike inner rotor motors, which can wobble in and out of the stator and lose efficiency, outer rotor designs allow for higher magnetic poles and coil counts to maximize power. Depending on the application, this may mean you can shrink the motor length (axial dimension) and reduce weight or maintain the size but boost power output.
The external rotor design also allows the motor to be mounted directly to the fan impeller, eliminating the need for an output shaft. This saves on space and cost and improves performance.
The larger rotor enables higher motor inertia, ideal for applications with load fluctuations. It’s also a great choice for applications where vibration is an issue such as premium tattoo machines, E-bikes, and battery-powered weedeaters. In these types of devices, a larger rotor also helps with torque smoothing. This makes it easier to meet the required speed and power requirements.
Unlike brushed DC motors that use fixed brushes to supply electric current into coils arranged within a fixed magnetic field, external rotor brushless motors achieve commutation electronically. As the rotor rotates, the sensed position of the permanent magnets in the rotor and the alternating electrical fields generated by the copper windings in the stator continually flip coil polarities to maintain rotation.
This topology allows for a higher number of magnetic poles in the rotor and less coils in the stator. This enables the motor to produce more power for the same size.
In addition, the frameless design of EC flat motors makes them ideal for applications where space is at a premium. For example, they can be used to replace large hydraulic actuation systems in bomb disposal robots, industrial and marine winch systems, propulsion of commercial electric vehicles and trucks, and hybridisation of vehicle drivetrains.
Brushless DC motors use magnets on the rotor to attract and repel each other to create torque, rather than mechanical brushes. This allows a smaller motor diameter to be used for a given power output and weight.
The copper shorting bars on the iron laminations of the rotor’s armature create an alternating magnetic field that reacts with the stationary magnets on the stator to spin the motor. To control the motor’s speed and direction, an electronic switch called an H-bridge (or a transistor or IGBT) is connected between the stator windings and the rotor windings to pass current in a sequence that rotates the field.
Outer rotor motors are used in applications where high power and low weight are key such as battery-powered weedeaters, fans, and pumps. They are also widely used in other electric vehicle applications such as replacement of large hydraulic actuation systems, industrial and marine winch systems, bomb disposal robots, and traction for commercial and military hybrid vehicles.