Comparison of structure motor with internal rotor and external rotor

In general, the stator of the motor is on the outside and the rotor is on the inside.In a conventional brush dc motor, the stator field is on the outside and the rotor armature is on the inside.Brushless dc motor appeared, so that the rotating armature from the inside to the outside, from the rotor into a stator;And the original stationary stator permanent magnet went inside, and from the stator become the rotor.This kind of structure is called an "internal rotor structure".

In practical use, sometimes in order to meet the special technical requirements of some electromechanical, the motor stator armature is made inside, and the rotor with permanent magnet is made outside. We call this structure "external rotor structure", or "internal stator structure".

In the design of brushless dc motor, the external rotor structure and the internal rotor structure have their own advantages and disadvantages.The differences in motor characteristics between the two designs can be summarized as follows: the rotor inertia, output torque/output power ratio of the motor with external rotor structure are higher, the number of parts is relatively large, and has the approximate  hall reflects position;However, the rotor inertia, output torque/output power ratio of the motor with internal rotor structure are low, and the number of parts is relatively small. The hall reflects position is precise.

Consideration of inertia

The ratio of load inertia to rotor inertia is one of the key factors for selecting a suitable motor.In the rigid mechanical system with gear reducer or worm gear reducer, the maximum recommended ratio of the load inertia on the motor shaft to the rotor inertia(relative ratio) is 10;For systems with belt and pulley deceleration, the maximum recommended ratio is 4.

In the design of the system, whenever possible, the maximum relative ratio shall not be exceeded.In a system with a high inertia of load, the external rotor motor will improve the stability of the system.However, since the acceleration torque is equal to the product of the inertia and acceleration, the load of the internal rotor motor with small inertia can obtain higher acceleration rate, higher system sensitivity, faster response and wider frequency band.

Another advantage of the internal rotor structure is that it is relatively easy to achieve dynamic equilibrium.Therefore, from the point of good rotor dynamic balance, the internal rotor structure is suitable for running at a high speed of close to 9000 (r/min) to 12000 (r/min).

However, when the motor is running, there is a limit of high speed in the machinery: when the inner rotor structure has no special mechanical casing on its outer surface, the limit of high speed is about 15000 RPM;The high speed limit of the outer rotor structure is about 30000 (r/min).This has obvious advantages for very high speed operation.

Consideration of output torque and output power

For a given volume, the external rotor motor has a higher output torque and output power.At high speeds, a large portion of the supply voltage will be eaten up by the electrical back motive force of the motor's windings, leaving only a small portion of the supply voltage to be used to pull the current.In this case, the external rotor motor of this advantage is particularly important.However, because the stator armature of the inner rotor motor is on the outside, it can dissipate heat directly through the outer surface of the motor, so the inner rotor motor can be cooled more effectively than the outer rotor motor.If forced ventilation or cooling device is installed on the outer surface of the inner rotor motor, the performance of the motor will be improved remarkably.

Consideration of the number of motor parts

In this field, compared with the external rotor motor, the internal rotor motor provides two advantages: first, in the design of the internal rotor structure, fewer parts means it has greater inherent reliability;Secondly, because of the simple design, the manufacturing cost of the motor is reduced, and the user saves money at the same time.Therefore, from the economic point of view, in the running speed and the loading equipment allowance, we should choose the inner rotor motor.