FPV Drone Motor – A Driving Force!

The selection of an electric FPV Drone Motor has an enormous influence on the flight
characteristics of the multicopter. Tiny variations in the construction of a motor result
in significant impacts regarding the weight, responsiveness and total power of the multicopter

Electromagnetism

The key concept behind the functioning of both brushed and brushless DC motors
is electromagnetism. Both designs intrinsically incorporate the use of an electromagnet,
as a means of converting electrical energy into kinetic energy. When an electromagnet is
electrically charged, a magnetic field is produced. This temporary magnetic field interacts
with that of the permanent magnets located within the motor. The combination of attraction
and repulsion of the electromagnet or permanent magnets translates into rotational
motion of the motor shaft.

Brushless and Brushed, What’s the Difference?

The principle behind brushless and brushed motors is very similar. When an electric
current is passed through the windings of the motor, magnets distributed within the
motor are attracted or repelled. The repetitive repulsion and attraction of the magnets
translates into a revolution of the shaft. This allows the motor to spin an attached
propeller at extremely high speeds, in turn, producing thrust.

Brushed FPV Drone Motor

The internal operation of a brushed motor is contrary to that of a brushless FPV drone motor.
In the brushed motor, the stator provides a permanent magnetic field that surrounds the rotor.
The rotor of the brushed motor is an electromagnet which is influenced by the surrounding stator.
A pair of brushes attached to DC power contact the commutator ring at the base of the rotor.
The commutator ring is divided, therefore its rotation will periodically reverse the direction
of the current flowing through the rotor, as its rotation causes the commutator to reverse
its polarity. The alternation of the commutator ring polarity translates into
uninterrupted revolution of the rotor.

This entire process occurs internally within a motor can, which provides excellent
protection for the delicate components. Although, efficiency of the system is reduced
due to the greater thermal insulation of the internal mechanics. It is possible to reverse
the rotation direction of the motor by inverting the polarity of the DC power input.
Due to the contact of the brushes with the commutator, longevity of the brushed motor
is greatly reduced in comparison to the brushless motor. In terms of application,
a brushed motor is better suited for micro class multicopters, their small size,
low weight and simple driving technique improves their suitability for micro FPV flight.

Brushless FPV Drone Motor

As the name implies, a brushless FPV drone motor lack brushes. The brushless
motor can be effectively divided into two separate components; the rotor and the stator.
The stator is the central unit into which the rotor is mounted. The stator is made
up of a network of radial electromagnets that alternatively power on and off to
produce a temporary magnetic field when a current is passed through the windings.

The rotor holds a collection of permanent magnets which are positioned in close
proximity to the semi-permanent stator electromagnets. Attractive and repulsive
interaction of the stator and rotor magnets is translated into rotational movement.
When assembled, the shaft of the rotor is inserted into a pair of ball bearings located
in the stator that maintain linear, smooth revolution of the rotor.

 Although the brushless motor is powered by DC current, it can’t be driven directly.
Instead, the brushless motor is wired to the control electronics, effectively eliminating
the need for brushes or a commutator. Longevity of the brushless motor is excellent as
there is no physical contact between the rotor and the stator.

The brushless motor is also more efficient than the brushed motor. The brushless motor
is extensively used in mini and some micro multicopter applications,
where high power outputs and efficiency are prioritised.

Motor Sizing and Identification

The size of a brushless motor is identified by a four-digit code that details the
dimensions of the stator in millimetres, for example: 2206. The first two numbers
in the series determine the diameter of the stator, in this case, 22mm. The final two
describe the height of the stator, the last two numbers in this series are “06” therefore
the stator unit is 6mm tall. It is important to remember that these numbers 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 miniscule 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 optimised 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 manoeuvrability 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 manoeuvre 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 prioritised, 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.