BRUSHLESS MOTORS

Hello guys, In the previous tutorial we have learned about propellers and physics behind how the drone flies. In this tutorial, we are going to learn about the drone’s brushless motors and I will guide you to choose correct motors for your drone.

A key concept behind the functioning of motors is electromagnetism.

ELECTROMAGNETISM:

Most of us have learned about electromagnetism in schools and college, now let us know how this concept applies in motors. Here the use of an electromagnet is to convert electrical energy into kinetic energy. When an electromagnet is electrically charged, it produces a magnetic field around it.When this temporary magnetic field interacts with a permanent magnet, attractive and repulsive forces are generated which leads to rotation of a motor shaft.

DIFFERENCE BETWEEN BRUSHED AND BRUSHLESS MOTORS:

Both works with the same principle of electromagnetism

BRUSHED MOTORS

Brushed DC motor consists of armature coil, brushes, commutator, an axle, and a permanent magnet. In brushed motors, the stator is provided with permanent magnets around the rotor. The rotor consists of electromagnets. the commutator is a split-ring device that is around the axle and physically in contact with brushes that are connected to DC power supply.

The efficiency of a brushed motor less due to the greater thermal insulation of the internal mechanism.Brushed motors wear out quickly because of the presence of brushes. These motors are mostly used in micro drones. Their small size, low weight, and simple driving technique make it more suitable for micro drones.

BRUSHLESS MOTORS

As the name implies, these motors don’t have any brushes. A brushless motor consists of two main components motor and rotor. As you can see in the above fig the permanent magnets are attached to the rotor. The stator part consists of an electromagnet which is directly connected to the DC power supply.
The efficiency of these motors is about 85-90%. Whereas a brushed motor is 75-80%. These motors have more life than the brushed motor because there are no brushes to wear out. These motors are mostly used in mini drones and small drones where high efficiency and power are required.

SIZE OF BRUSHLESS MOTORS:

The size of the Brushless motor is identified by the four-digit code which is based on the size of the stator. For example, if the motor size is 2207, it means stator height is 07mm, and width is 22mm.

 

DIFFERENCE BETWEEN 2207 AND 2306 STATOR SIZE

2207 has a narrow stator, so it has smaller inertia which allows for faster RPM changes. Taller stator creates a large surface area which leads to an increase in magnets size, therefore more magnetic fields are created.

2306 has a wider stator, so the coil size increases more than the permanent magnet. Which leads to more torque and an increase in efficiency of the motor.

  Comparing stator volume:                        

• 2306: (23/2) ^ 2 x 3.14 x 6 = 2491.59
  
• 2207: (22/2) ^ 2 x 3.14 x 7 = 2659.58
  

2207 motor has 6.3% of more volume than 2306, so 2207 has more weight than in 2306.
It is impossible to say which motor perform better because it is purely based on the design and material used for motor.
But based on stator size, 2207 has more top-end power and can change RPM effectively at a high throttle range. Whereas 2306 has better power and response at mid and low throttle range.

KV – VELOCITY CONSTANT:
KV represents the speed of motor per every voltage applied to the motor.
i.e., KV = RPM per 1 volt
           K = KV rating of the motor.
                  Eg.2300
           V = input voltage
                 Eg.16.5 V
Example:2300(kv rating)* 16.5(input voltage) = 37,950(Revolution Per Minute)
KV of the motor is defined by the strength of the magnetic field at the stator and no of turns in the winding.
Motors with high KV values suited for lightweight propellers. A low KV value motor best suited for driving heavy propellers.

N AND P NUMBER:

As you shop around for brushless motor you will notice a code as shown in fig.

Where 12N represents the number of the electromagnet in the stator and 14P represents no permanent magnets in the rotor. A typical 22## and 23## motors have 12 poles and 14 magnets.

THRUST:

Thrust plays a crucial role in choosing a motor. The thrust of a motor is usually measured in grams. The value of thrust depends upon how fast the motor is spinning and the propeller which is rotating. Before the drone can accelerate, a certain amount of thrust is needed to pull the gravity and overcome the drag force.

THRUST TO WEIGHT RATIO:

The general thrust to weight ratio is 2:1. You are not going to do any racing with this ratio. The thrust and weight ratio mostly depends upon the weight of the drone.

Diatone Crusader GT has about 8:1 thrust to weight ratio. Some people have achieved 13:1. But there are some limitations in the motor because they spin so fast and spinning them faster makes the motor inefficient.

TORQUE:

Torque is a measurement of how quickly a motor can reach a certain RMP. Torque improves the accuracy of movements like flip and roll.  Heavier propeller requires more torque to spin than lighter propellers.

Factors affecting the motor torque:

• Bigger the stator size higher the torque.
  
• Quality of magnets and copper windings.
  
• The air gap between rotor and stator.
  

EFFICIENCY:

The efficiency is calculated by the ratio of thrust produced by the motor at 100% throttle and power produced by the brushless motor.

Efficiency is measured in grams per watt(g/w). higher the ratio the more efficient is the motor. We are not going to fly with 100% throttle whole time. Better to calculate efficiency of the motor through the whole throttle range from 0 to 100% throttle. Some motor is efficient at the lower end of throttle and some are efficient at higher of the throttle. So choose the right motor as per your requirement.

TEMPERATURE:

Temperature and heat decrease the life span of motors. If the motor is exposed to heat and high temperature for a long time, the magnets in the motor get demagnetized.

The main reason for a motor to get heated is over propping and flying at the higher throttle for a long time. But motor manufacturers have designed the cooling fins to compensate for the issue. So that they help the motor to suck in air into the motor and reduce the heat produced in motors. which increases the life span of motors.

CLOCKWISE AND ANTI – CLOCKWISE MOTORS:

Brushless Motors are typically divided into two types based on their direction of rotation – clockwise and anti-clockwise rotation motors. These motor vary only in the direction by which they rotate the rest of the design parameters are the same. We use both types of motors in our drone design.

 

 

 

 

CONCLUSION:

In this article, we have tried to cover most of the basic concepts of brushless motors. We hope this article helped you to gain some knowledge about the brushless motors and to choose the right motor for your drone. Also you can refer wikipedia for this topic.

Related topics:
Drone Propellers