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The external rotor motor is the motor of choice for many applications, including robotics, AGVs, and handheld power tools. It provides high torque in a small footprint without the need for V-belts.
The individual winding ends of an external rotor motor must be connected to one another in such a way that the motor can be switched depending on the electrical operating mode required, e.g. 3-phase operation in star or delta connection, or 1-phase operation for clockwise or counterclockwise rotation.
With the rotor on the outside, outer rotation motors can use larger magnetic poles than inner rotor motors. This can give the motor more power while still fitting within a device’s physical limitations.
The fixed part of the motor, called the stator, uses permanent magnets to provide a stationary magnetic field around the rotor. The rotor then uses these magnets to create torque to rotate the shaft. By varying the applied DC voltage, the speed of the rotor can be varied.
In order to operate the BLDC motor correctly, the drive electronics needs to know the position of the magnetic rotor relative to the stationary magnetic field. This information is obtained using Hall sensors or similar devices that are mounted on the rotor.
From there, the control circuit can apply the right sequence of switching commands to the motor – such as switching transistors TR1 and TR2 on and off at the appropriate times. Choosing the wrong combination of switches can short out the motor terminals, making it stop running instantly.
While they require a little more energy to get started than inner rotor motors, an outer rotation motor has a much higher moment of inertia because the weight of the rotor is distributed across a larger diameter. This makes them more robust and less likely to damage if they are subjected to excessive acceleration.
Because the magnets are against the outside of the motor, outer rotor brushless motors have more magnetic surface area than inner rotor designs. This allows them to achieve higher power density. For the same axial length, you can shrink the motor’s size and weight, or increase its power with an efficiency bump.
The outer rotor design also allows you to attach a fan impeller directly to the motor. This eliminates the need for an output shaft and significantly reduces your motor’s footprint. This is ideal for E-bikes, drones, pumps, premium tattoo machines, and surgical robots. You can also use an external rotor brushless motor in high-torque applications like weed eaters.
Brushed motors use a rotary switch called a commutator to convert direct current into alternating current. This switch is a series of metal contacts on the rotor that are connected to conductor windings. Two or more stationary brushes made of soft, electrically-conductive material press against the commutator, sliding across each segment to selectively provide electric current to different windings. This causes the magnetic field to rotate the rotor in a particular direction, creating torque.
Outer rotor motors eliminate this step, eliminating the need for an output shaft and reducing the overall footprint of the motor. They also allow a fan impeller to be attached directly to the outer rotor, creating a motorized fan.
The flat multi-pole design of the BLDC motors allows for an extremely compact outer rotor. Hall sensors mounted on the stator pick up the magnetic position of the rotor and relay this information to the drive electronics, which then passes current through the stators in a sequence that causes the rotor to spin. This is known as trapezoidal commutation and requires no sophisticated control circuitry.
An external rotor motor has the stationary component (stator) fixed to the motor housing and the rotating component (rotor) located outside the stator. This allows the fan impeller to be attached directly to the rotor which significantly reduces the size and footprint of the motor and the overall system. This makes it easier to install and maintain.
It is also possible to simplify the inverter circuit design by using a unipolar PWM scheme for terminal voltage sensing instead of analogue LPFs. This means that only two inverter switches conduct at any one time, and the true phase back-EMF can be measured from the difference between these switching events.
In order to achieve this, the Hall sensor magnets must be placed on the rotor in addition to the main rotor magnets, but this can lead to misalignment between the rotor and the sensors and therefore errors in shaft angular position detection. This can be solved by the use of a special type of Hall sensor that has separate magnets from the other inverter switch magnets, allowing the sensors to be positioned without having to align them with the rotor.