If you are looking for a RC car electric motor, you may have wondered about the differences between the increasingly popular 2-pole and 4-pole brushless motors.
The brushless motor is a three-phase motor and needs an ESC to continuously ‘read’ the relative position between the rotor and stator to make it rotate smoothly.
The position can be determined with Hall-effect sensors or by measuring the counter-electromotive force (back EMF) on the three phases.
Number of poles
The poles are the number of permanent magnets installed on the rotor. For the fixed-can motors of RC model cars they are usually 2 or 4 and are mounted in an alternating N-S-N-S position.
The windings are powered by the ESC in a cyclic sequence (switching) according to the direction of rotation of the motor to keep it rotating in the same direction.
Switchings in 2-pole and 4-pole brushless motors
With each revolution of the motor, the number of commutations of the 4-pole motor is double that of the 2-pole motor. The higher number of commutations enables the ESC to read the back EMF signal more accurately and control the 4-pole motor better than the 2-pole motor, making the operation of the 4-pole motor smoother and with more constant torque.
4-pole brushless motors are therefore less prone to cogging (hesitation of rotation at low speed) and are often produced without sensors, which are always used in high-performance 2-pole motors.
However, it is important to emphasise that, in addition to the number of poles, the building quality of the motor and the control electronics (ESC) also have a great influence on the smoothness and performance of a motor.
Purchase cost, reliability and water resistance of 2-pole and 4-pole brushless motors
The difference between the two systems thus described usually results in a lower purchase cost for 4-pole sensorless ESC/motor combos, because they are less sophisticated in their controllin hardware.
Reliability is also generally higher in sensorless systems because the 6-wire cable that carries the signal from the motor sensors to the ESC is not required.
4 poli sensorless motor
Due to its vulnerability, the cable is susceptible to damage, resulting in operational irregularities and power loss.
6 wires cable
A sensorless brushless motor is comparable to a brushed motor in terms of resistance to moisture or minor water exposure. However, sensored motors require completely sealed sensors to achieve waterproofness
2 poles sensored motor
A 4-pole brushless motor, with its increased number of windings, requires more electrical conductor and ferromagnetic material, leading to a weight increase of at least 15% compared to a 2-pole motor with the same power and speed.
Conclusions
Differences between 2-pole and 4-pole brushless motors are reflected in various aspects that may be more or less relevant. The choice must therefore be carefully considered based on individual needs, the application of your RC car and, of course, the available budget.
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