Direct drive torque motor consists of two main components: stator and rotor. The stator is a stationary iron core containing coils and electrical connections. The rotor is the corresponding component of dynamic rotation and is equipped with a permanent magnet. The stator of the motor is mounted on the frame, and the rotor is fixed on the shaft or rotating machine components, located inside the stator. In contrast, the stator of the external rotor motor is still stationary, located in the center of the motor, and the rotor (designed to be annular) is outside the stator and rotates around the stator.
property
Inside the motor, torque is generated by the interaction between the permanent magnet on the rotor and the magnetic field generated by the stator coil. In the external rotor layout, the diameter of the air gap is larger.
Since the force generated by the air gap acts on a larger radius, the larger the diameter, the greater the torque. Therefore, in the same motor volume, the outer rotor can produce greater torque. However, we must recognize that the motor torque is not only limited by the maximum magnetic force of the air gap, it is also indirectly limited by cooling.
Thermal resistance
The loss comes from the motor coil wound around the stator core. The heat mainly reaches the mounting surface through the stator core. The internal and external rotor motors have different thermal paths from the coil to the mounting surface. In the inner rotor device, the stator is located on the outer diameter. As a result, the area of heat conduction and cooling is greater than that of the outer rotor. Similarly, as shown in Figure 2, the stator mounting surface area of the inner rotor motor is also much larger. Better heat transfer makes the motor cooler.
Install
Depending on the type of motor, either the outer ring or the inner ring can be rotated. Which installation is most appropriate depends on the application requirements. For example, an external rotor motor can drive a belt or roller directly, such as for high inertia applications. When the spindle is running, such as in high-speed indexing applications, the inner rotor is easier to integrate.
It is worth noting that the moving parts of the external rotor motor are far from the center, so the inertia is large. This effect is further amplified when the applied shaft is connected to the outer diameter. When accelerating, greater inertia requires greater torque.
Sum up
In short, in the same motor volume, the larger the diameter of the air gap of the external rotor motor, the larger the torque, but the lower the speed. Sustained torque depends on the ability to cool the motor. Because the heat from the coil must pass through a smaller area, the thermal resistance of an external rotor motor of the same volume is usually higher. This negates the torque advantage of the larger diameter air gap. The inner rotor motor responds faster and cools faster, but produces less peak torque.
The internal rotor motor and the external rotor motor have different mechanical fit in different applications. Each motor has its own advantages and considerations. Which motor is the best choice depends on the specific machine layout, available space and the components to be driven.
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