Brushed FPV Drone Motor

The operational principle of a brushed motor is diametrically opposed to that of a brushless FPV drone motor. In the context of a brushed motor, the stator generates a permanent magnetic field that envelops the rotor. The rotor, which functions as an electromagnet, is subject to the influence of the surrounding stator. A pair of brushes, connected to a DC power source, make contact with the commutator ring situated at the base of the rotor. The commutator ring, being segmented, facilitates the periodic reversal of the current flowing through the rotor as it rotates, due to the commutator's alternating polarity. The oscillation of the commutator ring's polarity ensures a continuous rotation of the rotor.

This entire mechanism is housed within a motor casing, which offers superior protection for the sensitive internal components. However, the efficiency of the system is somewhat diminished due to the increased thermal insulation of the internal mechanics. It is feasible to reverse the rotational direction of the motor by inverting the polarity of the DC power supply. Owing to the brushes' contact with the commutator, the lifespan of a brushed motor is significantly shorter when compared to that of a brushless motor. In terms of application, a brushed motor is more aptly suited for micro class multicopters, where its diminutive size, light weight, and straightforward driving mechanism enhance its suitability for micro FPV flight operations.

Brushless FPV Drone Motor

True to its name, a brushless FPV drone motor is devoid of brushes. The brushless motor can be logically partitioned into two distinct components; the rotor and the stator. The stator serves as the central unit into which the rotor is affixed. The stator comprises a network of radial electromagnets that sequentially activate and deactivate to generate a transient magnetic field when an electric current is applied to the windings. The rotor houses a series of permanent magnets that are positioned in close proximity to the semi-permanent stator electromagnets. The attractive and repulsive forces between the stator and rotor magnets are converted into rotational energy. Upon assembly, the rotor shaft is inserted into a pair of ball bearings located within the stator, ensuring a linear and smooth rotation of the rotor.

Although the brushless motor is energized by direct current, it cannot be operated directly. Instead, the brushless motor is connected to control electronics, effectively obviating the necessity for brushes or a commutator. The longevity of the brushless motor is exceptional due to the absence of physical contact between the rotor and the stator. Additionally, the brushless motor exhibits greater efficiency when compared to the brushed motor. The brushless motor is widely utilized in mini and select micro multicopter applications, where emphasis is placed on high power output and efficiency.

Motor Sizing and Identification

The dimensions of a brushless motor are denoted by a four-digit code that specifies the stator's measurements in millimeters, for instance: 2206. The initial two digits in the sequence denote the diameter of the stator, in this instance, 22mm. The subsequent two digits represent the height of the stator, with "06" indicating that the stator unit measures 6mm in height. It is imperative to note that these figures do not describe the external dimensions of the brushless motor itself.

The size of a brushed motor can be identified through a simpler two number system that clearly defines the diameter and height of the exterior can in millimetres. Example: 6×15, the first number “6” is a measurement of the cans diameter and “15” the height of the can.

Mounting Patterns and Thread Size

Mounting patterns and thread sizing is dependent on the type of motor and its application. The mounting pattern defines the positioning of the threaded bolt holes on the base of the motor. Each number describes the diameter of a circle with its centre placed in the middle of the motor shaft. Usually, four holes are placed along the circumference of the circle, if two numbers are given, two holes are placed on each circle. For example, a 2205 with 16×19 spacing will have four M3 size threaded holes distributed evenly on both the circumference of the 16mm circle and 19mm circle. The dimensions of the threaded shaft are given by an ISO screw thread rating, which describes the outer diameter of the shaft.

220X – 240X

Most often a 16x19mm mounting pattern is used, however, 16×16 is becoming increasingly common. The threaded holes are M3. The threaded shaft diameter is usually M5.

180X

Usually a 16×12 mounting pattern, threaded holes are M2 and M5 threaded shaft diameter is typical.

130X – 140X

Commonly 12×12, the threaded holes are M2 and a M5 threaded shaft is typical.

110X

Often 9×9, threaded holes are typically measured as M2. The shaft is not threaded and usually measures 1.5mm in diameter. Motors in this size class also have an additional set of holes on the top of the motor bell. The hole spacing is 5mm and each hole is 2mm in diameter. The purpose of these holes is for secure mounting of the propeller, as a lock nut is absent.

Why doesn’t the Bell fly off?

As discussed earlier, the rotor of a brushless FPV drone motor is compiled of a circular array of magnets and a central shaft. When the motor is assembled, the shaft protrudes from the base of the motor. Here it is either secured by a circlip or tightly bolted in place. Circlips are most commonly used, however, bolts are becoming increasingly popular. Although the circlip has been the primary choice, maintenance can be frustrating due to the difficulty of removal. The circlip is fragile and minuscule in size, causing it to be easily broken or lost.

The Velocity Constant — How fast a Motor Spins

 

kV=RPM per 1 Volt

k = The kV rating of the motor e.g. 2300

V = Voltage input e.g. 16.8v

Example: 2300(kV rating) X 16.8(Voltage) = 38,640(Revolutions Per Minute)

 

The velocity constant (kV) determines how many rotations a motor can make within a minute without a load (no propeller) and at a constant current of 1 Volt. Simply, kV is a representation of how fast the motor can potentially spin. The kV of a motor is defined by the strength of the magnetic field at the stator and the amount of turns in the windings. A motor with a lower kV is best suited for efficiently driving heavy propellers. A high kV motor is optimized for lightweight propellers.

Thrust

Thrust is one of the key factors to consider when choosing a motor. The thrust output of a motor is usually measured in grams and varies depending on how fast the motor is spinning and the propeller that it is rotating. Before a multicopter can begin to accelerate, a certain amount of thrust is required to overcome drag, as well as the pull of gravity.

Weight and FPV Drone Motor Momentum

When selecting a motor, it’s not all about thrust numbers. The weight of the motor should also be considered, as it has a significant impact on the flight characteristics of the multicopter. Due to the moment of inertia, a heavier motor will be more resistant to changes in acceleration than a lighter motor. The primary issue with a heavy multicopter motor being resistant of acceleration is that it will provide inaccurate flight characteristics and poor responsiveness once in the air. If maneuverability is a priority, a lightweight motor is an exemplary choice. On the other hand, an application in which maximum all-out speed is a must; larger motors will be able to provide the higher thrust numbers that are required.

FPV Drone Motor Response Time

Torque is a measurement of how quickly a motor can reach a certain RPM, directly affecting the responsiveness of a motor. Torque allows a multicopter to briskly maneuver through flips and rolls, additionally improving the accuracy of these movements. The amount of torque a motor can output also influences propeller selection. Heavier props will require more torque to accelerate than lighter props. The best gauge for motor torque is the dimensions of the stator. Larger stators tend to be capable of producing greater torque. Although, a larger stator will increase the total weight of the motor.

FPV Drone Motor Efficiency

Motor efficiency is a balancing act, requiring an equilibrium to be struck between the electrical power entering the motor and the mechanical power being produced by the motor as it spins. The importance of motor efficiency varies based on the situation. If high speed is prioritized, short flight times are often seen to be acceptable; FPV quadcopter races may only last for two minutes! In the contrary, long-range FPV multicopters require maximum efficiency to achieve longer flight times, increasing the distance that can be travelled.

Conclusion

Motors are arguably the most influential piece of equipment on a multicopter, having a considerable impact on flight characteristics relative to other components. It is essential that motors are carefully selected with adequate appropriateness for their application.