The primary goal for our motor designers is to maximise the motor output ratings while minimising the mass. Fisher & Paykel Technologies uses the external rotor design for its direct drive motors in order to achieve this. Let us explain why.
Brushless direct current (BLDC) motors are constructed with a rotor and when this rotor has magnets, it is called a Permanent Magnet Synchronous Motor (PMSM). The rotor can be configured in different ways, the most common configuration is an internal rotor motor (rotor located inside a multiphase wound stator). 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.
Our direct drive motors are external rotor motors and have a thin design with big diameter. The large 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.
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. Generally, external rotor PMSM motors can be designed with a large air gap. This means that the greater the distance from the air gap to the drive shaft, the greater the torque that can be produced. The larger size of the rotor means that the rotor can accommodate more poles, which increases the magnetic flux and reduces the cogging torque. As a result it can produce more torque with less material.
External rotor motors allow for a more axially compact design while offering the same performance characteristics as internal rotor motors. The thinner magnets use a smaller amount of magnetic material, making external rotor motors lighter in weight. The compact nature of external rotor motors can be a significant space-saving solution that works well for many applications.
The greater inertia from the large diameter rotor smooths out cogging which BLDC motors are often plagued with. This results in lower audible noise, and makes this design of motor ideal for low noise applications.
In general, the external rotor design leads to efficient and powerful motors. These motors are ideal for saving space and reducing noise in applications such as fans, blowers, pumps, home appliances, automatic doors, and many more.
The following are some significant advantages of an external rotor motor:
Higher inertia and air gap radius.
High torque and magnetic flux.
Compact and lightweight form.
Smooth and quiet operation.