The primary goal for our motor designers is to maximize the motor output ratings while minimizing the mass. Fisher & Paykel Technologies uses the external rotor design for its direct drive motors in order to achieve this. Let us explain why.
Conventional brushless DC (BLDC) motors are constructed with a permanent-magnet rotor located inside a multiphase wound stator—this is also known as an internal (inner) rotor motor. External rotor motors have the opposite construction. The rotor is placed around the stator and has permanent magnets mounted on the inner diameter of the rotor housing. It then rotates around the internal stator.
The following are some significant advantages of an external rotor motor.
Bigger inertia and air gap radius
An external rotor is naturally bigger in diameter than the rotor of an internal rotor motor. The larger diameter of the rotor creates higher inertia, which helps to reduce the torque ripple and provide smooth, stable operation even at low speeds. In addition, the rotor’s larger inertia helps ride through torque variations.
High torque and magnetic flux
Torque is the product of the magnetic force times the radius of the air gap, and is also known as the length of magnetic flux. In a set diameter, external rotor motors have a larger air gap area than inner rotor designs, and the larger air gap allows a higher force to build. So, the longer the distance between the magnetic flux and the axle, the greater the torque that is generated. External rotor motors have a larger air gap radius, which increases the lever for torque production. The larger size of the rotor means that the rotor can accommodate more poles, which increases the magnetic flux and reduces the cogging torque. Result? More torque with less material.
Compact and lightweight form
External rotor designs are axially shorter than internal rotor motors but retain similar performance characteristics. The thinner magnets use a smaller amount of magnetic material, making external rotor motors lighter. The compact nature of external rotor motors can be a significant space-saving solution that works well for many applications.
Smooth and quiet operation
Lower audible noise due to greater inertia and reduced cogging makes external rotor motors ideal for use in “quiet” applications. With the higher inertia combined with an optimized magnetic design of outer rotor technology, we have minimized the cogging of the motor.
In general, the external rotor principle leads to efficient and powerful motors. These motors are ideal for saving space and reducing noise within equipment such as fans, blowers, pumps, home appliances, automatic doors, and more.