At its simplest, a rotary motor consists of a rotor and a stator. One of these (stator) remains stationary while the other (rotor) moves to perform the work. In an internal permanent magnet motor, such as our HVAC and high-speed motors, the rotor with the permanent magnets rotates on a shaft mounted inside a stationary stator.
The stator incorporates a collection of copper coils that are energized in succession to generate magnetic flux. This flux interacts with the rotor, which is mounted in such a way that it can turn to align itself with the fields generated by the coils as they are energized one after another. This alignment force produces torque. Thus, the rotor starts rotating inside the stator, turning the shaft and with that, the load.
Internal rotor motors have great ability to dissipate heat. This explains why the overwhelming majority of BLDC (brushless DC) motors use an inner rotor design—it’s a very commonly-used motor topology. Heat directly impacts the motor’s ability to generate torque, so better heat dissipation improves efficiency.
Another benefit of the internal rotor motor lies in the small moment of inertia (tendency to resist angular acceleration) of the rotating shaft. This quality gives this type of electric motor an incredible ability to run at high speeds.
Creating a large number of coils inside the stator is a more difficult task than winding coils outside the rotor. For this reason, inner rotor motors are currently used for specialised applications that require a compact and high-output motor with superior dynamic characteristics.
It’s a good idea to consider both internal and external rotor motors when looking for an electric drive solution. Picking one is tricky, and sometimes it is necessary to try both before drawing conclusions. At Fisher & Paykel Technologies, we believe that an application-specific motor and control design is most beneficial and can really add value to a product.
Summarised, some major advantages of internal rotor motors are:
Small moment of intertia.
Easier to wind.