Outrunner Vs Inrunner:Comparative Analysis on DC,BLDC ,Electric Motor And Facts

The term inrunner motor or outrunner motors are the motors where the position of the rotor which is also known as rotor is placed inside or outside of a motor structure.

Comparission between outrunner vs inrunner motor in tabular format:

ParameterInrunner motorOutrunner motor
Accuracy Relatively low accuracy Relatively high accuracy 
Efficiency Relatively higher efficiency due to low inertia.Relatively lower efficiency due to high inertia as more significant momentum generates more torque. 
Torque Relatively low torque generation. Relatively high torque generation. 
Structure The rotor is present inside the stator. The magnets are placed on the rotor, which rotates, and the stator windings are fixed. The rotor is present outside the stator.Relatively larger torque arms, as well as air gap surface, is relatively larger. The magnets are present on the rotor, and stator coils form the core.Here shaft acts as output which is connected to the propeller. 
Physical Size The overall size is small, and diameter is smaller compared to an outrunner. But axially longer in length. The overall size is more extensive, and the diameter is more extensive compared to an inrunner. But smaller in axial length. 
RPM per voltThe inrotor of identical in size of outrotor has a high RPM per volt.The outrotor of identical in size of inrotor has low RPM per volt. As for high voltage, it requires more magnets, which can slow down the speed; that’s why RPM is very much limited. 
Heat transferHigh, as in inrunner, all the magnets are inside the winding there is more efficient heat transfer takes place. And they stay cooler for the long term. Low, as in the outrotor, the heat transfers to the center of the motor, the only way to decapitate the heat is through the shaft; that’s why it gets heated quickly. But with more extensive outrunner motor cooling down, the motor becomes easier. 
Used inEDF jet, car, truck, speed boat, etc. 3D airplane, warbird, trainer plane, helicopter, drone, etc. 
MaintenanceRelatively High maintenance  Relatively low maintenance 
Loss and risksHigh risk of contamination results in low accuracy.Low risks of contamination, resulting in high accuracy. 
Need of gearboxRequired Not required 
Noise Highly noisy Very much silent 
Applications They are used where high speed is required. They are used where high torque is required. 
Number of magnetic polesA fewer number of magnetic poles is present, which results in achieving lower air gap flux. More flux through the yolk A higher number of magnetic poles are present, resulting in high air gap flux—less flux in the yolk. 
Stator winding AC stator winding DC amateur winding 
MagnetsHigh energy magnets are required, which can increase the cost of the motor. Low energy magnets can be used.
Table: Inrunner vs Outrunner motor
outrunner vs inrunner
Image Credit: Motor Structure Roonie.02Motor rotorCC0 1.0

Inrunner vs Outrunner Efficiency

The magnets in the inrunner motor are placed inside the stator, where are the magnets of the outrunner motor are played outside the stator. 

Inrunner motor is more efficient than that of an outrunner motor.

Inrunner motor can generate high speed with high Kv value but produce less torque where outrunner generates high torque, but the low speed with low Kv value, because of high winding resistance.

Image Credit: Outrunner motor Brushed_dc_motor_assembly.jpgIliakriv: Ilia Krivoruk derivative work: Andy Dingley (talk), Brushed DC motor disassembledCC BY-SA 3.0

Every motor is made of two parts: the stator and the rotor. The efficiency of motors can be increased by using a permanent magnet because of their inherent excitation. As inrunner motor, the magnets are present inside the stator; heat transfer is more efficient between the stator and the rotor.

In the inrunner motor, the stator winding is facing directly the rotor, which provides an excellent surface with an extensive area for easy heat transfer. The air is a poor conductor of heat, and as in inrunner motor, there low air flux gap, conduction of heat between stator and rotor efficiently takes place. The relationship between torque and Kv value is inversely proportional, and due to less inertia, low torque is generated in the inner rotor motor, resulting in high efficiency heat transfer.

Image Credit: Outrunner Motor, DnaXFloppy brushless motorCC BY-SA 3.0

Inrunner motor has a high ability to dissipate energy; that’s why it is a very commonly used topology in Motors. Due to the small inertia of the rotating shaft, it can quickly achieve high speed.

What is an Inrunner Electric Motor

Inrunner motor topology is widely in use due to its several advantages.

The construction of the inrunner motor is directly opposite to the outrunner motor, as, in an inrunner motor, the rotor is present inside the stator; the inrunner motor is more efficient than that of an outrunner motor.

The permanent magnet is present on the rotor, which rotates in the stator of the motor stays fixed in a position. Due to lower inertia, high speed can be achieved quickly, where torque generation is much more limited. The working principle of both the inner rotor and outer rotor is similar. Inrunner motor is used where a compact and high output motor is required. A high energy permanent magnet is required, which can also impact the overall cost of the motor.

Image Credit: Inrunner Motor MedvedevStator Winding of a BLDC MotorCC BY-SA 3.0

Advantages of inrunner motor:

  • Highly efficient.
  • High RPM per volt.
  • Smaller in diameter and axially longer in length.
  • High speed can be achieved easily.

Read the comparison of brushed vs brushless motor.

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