Where to Start? Decide on Motor Size First

First answer these two questions:

  • What’s your quadcopter’s total weight?
  • What’s the size of the frame?

The aggregate weight of your quadcopter can be your best guess, as you haven’t constructed it yet. It should include everything: frame, flight controller, PDB, wires, motors, battery, payload (for example, HD camera and gimbals), and so on.

If you know the size of the frame, you can determine the right propeller size.

Using the weight and propeller size, you can compute generally how much thrust the motors need to have to lift off and fly the quadcopter at speed.

Thrust to Weight Ratio

A general rule is that the motors should be able to provide twice as much thrust as the total weight of the quad. If the thrust provided by the motors is too little, the quad will not react well to your control and may even experience issues on takeoff.

For instance on the off chance we had a quadcopter that weighs 1kg, the aggregate thrust created by the motors at 100% throttle should be no less than 2kg, or 500g for each motor (which is multiplied by 4 for a quadcopter).

This will give you better control, as well as the space for including additional payload later on (like heavier cameras, or possibly additional batteries to extend flight time).


Motor Size and KV

Brushless motors are typically categorized by a four-digit number – such as **##. where as the “**” numbers are the stator width and “##” is the stator height. Essentially, the wider and taller the motor is, the larger the numbers are and the more torque it can produce.

KV is another essential parameter. It is the theoretical increase of motor rpm (rotation per minute) when the voltage goes up by 1 volt without load. For instance, while running a 2300KV motors with a 3S LiPo battery (12.6V), the motor would turn at around 28980 rpm. (2300 x 12.6V = 28980) This is only an estimation.

In any case, once you mounted a propeller on the motor, the rpm won’t be that high because of the props resistance. Higher KV motors would turn the propeller quicker with less torque, and lower KV motors create higher torque with less rotation. Bigger props are matched with low KV motors, and smaller props with high KV motors.

It’s important to discover a balance between rpm and torque when picking motor and propeller.

By matching high KV motors with excessively large propellers, the motors will try to turn them quickly like it would do with smaller props, and this will draw a lot of current and produced an excessive amount of heat.

N and P

You may infrequently observe something like “12N14P”. The number before the letter N refers to the quantity of electromagnets in the stator, and the number before P refers to the quantity of perpetual magnets in the motor.

Most motors have the same 12N14P arrangement, however, some lower KV motors have more electromagnets and lasting magnets to expand torque and be more productive (and would be more costly).

Frame Size = Prop Size = Motor Size and KV

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For the vast majority of the circumstances, knowing frame size allows us to estimate what kind of motor we should use. This is on the grounds that the frame size limits props size, and prop measurement limits the motor size and KV.

This table below gives you a few thoughts and is based on using a 4S LiPo battery. Frame size is referring to wheelbase (otherwise known as motor to motor distance).

Frame Size Prop Size Motor Size KV
150mm or smaller 3″ or smaller 1306 or smaller 3000KV or higher
180mm 4″ 1806 2600KV
210mm 5″ 2204-2206 2300KV-2600KV
250mm 6″ 2204-2208 2000KV-2300KV
350mm 7″ 2208 1600KV
450mm 8″, 9″, 10″ 2212 or larger 1000KV or lower

RotorDrone - Drone News | Multirotor Motor Guide

RotorDrone - Drone News | Multirotor Motor Guide

Voltage and Current Draw

It’s also essential to understand that voltage will largely affect your motor and propeller choice. Your motor will attempt to turn faster when higher voltage is connected, and it will also draw a higher current.

Understanding Brushed DC Motors

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  • Dimension: 8mm (Diameter) x 23mm (Length)
  • Voltage: 3.2V
  • kV: 13000+
  • Terminal Resistance: 0.63ohm
  • No Load rpm: 37850
  • No Load Current: 130mA
  • Constant Torque: 0.79mNm/A
  • Weight of motor: 6.2g

Comparing between motors

After you have settled on the size and KV of the motors, before picking the best motor for your application, you should consider the accompanying components:

  • Thrust
  • Current Draw
  • Efficiency
  • Weight – Momentum of Inertia

The choice here truly relies upon your preference, how you need your aircraft to perform.

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Higher thrust gives you best speed, additionally you need to look at efficiency, ensuring that it’s not utilizing an enormous amount of power that would exceed what your support equipment (battery, speed control).

Likewise your choice of motor and propeller will influence your selection of batteries as well. If your quad draws a lot of current at full throttle, your battery’s maximum discharge rate must have the capacity to keep up so that it can supply the power needs, and so that they don’t overheat and puff up ( this is where the C rating comes in).

More tips on Motor Efficiency

  1. multirotor is more productive and efficient when it’s as light as could be expected under the circumstances. You can find the right balance when choosing LiPo batteries for your multicopter.
  2. Battery and weight are the key factors we have to consider with regards to general power effectiveness. At the point when picking motors, aside from motor KV and thrust, we likewise need to take a look at motor productivity.
  3. The same applies to the brushless motor: the higher proficiency the better. A 70% proficient motor produces 70% power and 30% heat. A 90% effective motor produces 90% power and 10% heat.

Features of Motors to consider

  • Solid/Hollow shaft
  • Type of Magnets (N52, N54)
  • Arc Magnets
  • Smaller air gaps
  • Soldering tabs on motor
  • Speed control integration
  • Cooling design

Difference between Brushed DC Motors and Brushless Motors

“brushed” DC motor has a rotating armature (a set of wound wire coils) which acts as an electromagnet with two poles. A rotary switch called a commutator reverses the direction of the electric current twice every cycle, to flow through the armature so that the poles of the electromagnet push and pull against the permanent magnets on the outside of the motor.

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“brushless” DC motor does not use brushes. It uses a permanent magnet and accomplishes the switching by electronically switching the polarity. In order to accomplish this in a controlled manner, a speed feedback mechanism and an electronic controller are required. The controller can be mounted on the motor or may be a separate item.

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  1. Application
    Brushless motor: widely used in the machine which requires high rotation speed and controled power.
    Brush motor: it is widely used in things like fan motor, power tools etc.
  2. Lifespan
    Brushless motor: the life span is more than one thousand hours
    Brush motor: the life span is under one thousand hours.
  3. Energy saving:
    Brushless motor is far more efficient and energy saving than the brush motors. While brushed motor, require maintenance to change carbon brushes in a timely manor, otherwise, the motor might get damaged.

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Brush DC motors are mechanically commutated motors that are good for high speed applications. Brush DC motors are easy to produce and cost effective when long life is not required.

Why a Brushed DC Motor?

The Brushed DC Motor is the classic motor that is used in applications like motorized toys, appliances, and computer peripherals. This type of motor is inexpensive, efficient, and useful for providing high speed and power in a relatively small package.

How Does the Brushed DC Work?

This type of DC motor has a split ring device called a commutator around the middle. When DC power is applied, the electromagnetic energy pushes the armature away, causing rotation.

Brushed Motor Pros

  • Two wire control
  • Replaceable brushes for extended life
  • Low cost of construction
  • Simple and inexpensive control
  • No controller is required for fixed speeds
  • Operates in extreme environments due to lack of electronics

Brushed Motor Cons

  • Periodic maintenance is required
  • Speed/torque is moderately flat. At higher speeds, brush friction increases, thus reducing useful torque
  • Poor heat dissipation due to internal rotor construction
  • Higher rotor inertia which limits the dynamic characteristics
  • Lower speed range due to mechanical limitations on the brushes
  • Brush Arcing will generate noise causing EMI

How Can You Find the Right Brush DC Motor for You?

There are many different types of brush motor that are flat, or rectangular for feeding and loading, and round ones are mainly used for spindles. You can also select a brush motor according to rated load/rotation speed, according to your required torque/speed characteristics.

Selecting by Rated Load / Rotation Speed

The typical torque/speed characteristics for each motor size are shown below for your reference when selecting a motor.

Rated Voltage (V) Voltage Range (V) Rated Load (mNm) Starting Torque (mNm) Rated Load Speed (rpm)
PYN13 3.0 0~4.0 0.1 (1gf.cm) 0.4 17,900
PNN3 1.5 0~3.0 0.03 (0.3gf.cm) 0.09 8,200
PNN7 1.5 0~3.0 0.1 (1gf.cm) 0.23 5,600
PNN13 3.0 1.0~4.0 0.15 (1.5gf.cm) 0.5~0.6 8900~12000
PKN7 2.0 0~4.5 0.2 (2gf.cm) 0.4~0.6 3790~7050
PKN12 3.0 0~4.5 0.2 (2gf.cm) 0.63~0.9 7250~10540
M1N6 3~5 1.0~6.0 0.2~0.3 (2~3gf.cm) 0.67~2.07 5980~15600
M1N10 2~5 0.5~8.0 0.2~0.3 (2~3gf.cm) 0.78~1.90 3010~11220
PPN7 2.5~6.0 1.0~7.5 0.1~0.5 (1~5gf.cm) 0.68~2.88 2600~11600
PPN13 2.0~9.6 1.0~11.0 0.2~1.47 (2~15gf.cm) 1.37~4.08 2700~9700
PWN10 6.0~12.0 5.0~12.0 1.96 (20gf.cm) 5.2~9.5 4870~8400
PAN14 12.0 9.0~14.5 10.0 (102gf.cm) 35.40 9,730
MXN13 6.0~12.0 3.0~14.0 2.9~4.9 (30~50gf.cm) 8.83~13.73 1900~4520
MDN1 2.0 0.7~6.0 0.29 (3g.cm) 0.8~1.1 1360~2250
MDN2 2.0~5.0 0.7~6.0 0.39~1.47 (4~15g.cm) 1.2~2.8 2750~2900
MDN3 2~3 0.7~6.0 0.39 (4gf.cm) 1.2~2.8 1480~2590

Brushless DC Motors

Traditionally, many motor needs have been met using brushed DC motors. These motors use the brushes to move the commutator, which creates the rotational torque needed for it to work. In a motor that is brushless, the commutation is done electronically. There is no need for brushes, as the torque is a function of the electronic action of the brushless motor on the commutator.

Why Use a Brushless Motor?

With a brushless DC motor, also called a BLDC motor, there is never any need to be concerned about the condition of the brushes, which could require that the motor be taken out of service and restored. Brushless motors can be just as effective for high –speed operation as a brushed motor, if not more, and because there are no brushes to replace, a brushless can have a life expectancy in excess of 10,000 hours.

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For a project where a motor is only going to be used for a short time, a brushed DC motor may be sufficient and cost effective. But if it is going to be in continuous use, especially if it’s going to be required to take on a lot of power, a brushless motor is a much better choice.

Brushless motors can be used in a wide variety of applications. Low power brushless motors can be used to power radio controlled model airplanes, while high power brushless motors can be used for electric vehicles and industrial machinery.


BLDC Motor Construction and Operating Theory

To understand why a BLDC motor is so effective, it’s important to have a good understanding of how it works. There are actually two different types, with different benefits and drawbacks. While either one will probably be effective for most jobs, you may wish to familiarize yourself with both types, just in case one would be more appropriate for your project or application than the other.

Any BLDC motor has two primary parts; the rotor, the rotating part, and the stator, the stationary part. Other important parts of the motor are the stator windings and the rotor magnets.

There are two basic BLDC motor designs: inner rotor and outer rotor design.

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In an outer rotor design, the windings are located in the core of the motor. The rotor magnets surround the stator windings as shown here. The rotor magnets act as an insulator, thereby reducing the rate of heat dissipation from the motor. Due to the location of the stator windings, outer rotor designs typically operate at lower duty cycles or at a lower rated current. The primary advantage of an outer rotor BLDC motor is the relatively low cogging torque.

 Outrunner Motor

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In an inner rotor design, the stator windings surround the rotor and are affixed to the motor’s housing as shown here. The primary advantage of an inner rotor construction is its ability to dissipate heat. A motor’s ability to dissipate heat directly impacts its ability to produce torque. For this reason, the overwhelming majority of BLDC motors use an inner rotor design. Another advantage of an inner rotor design is lower rotor inertia.

Inrunner Motor

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BLDC Motor Advantages:

If you’re still not sure whether or not this motor is right for you, here is a basic breakdown of some of the primary advantages of the BLDC motor.

  • High Speed Operation – A BLDC motor can operate at speeds above 10,000 rpm under loaded and unloaded conditions.
  • Responsiveness & Quick Acceleration – Inner rotor Brushless DC motors have low rotor inertia, allowing them to accelerate, decelerate, and reverse direction quickly.
  • High Power Density – BLDC motors have the highest running torque per cubic inch of any DC motor.
  • High Reliability – BLDC motors do not have brushes, meaning they are more reliable and have life expectancies of over 10,000 hours. This results in fewer instances of replacement or repair and less overall down time for your project.

BLDC Motor Pros

  • Electronic commutation based on Hall position sensors
  • Less required maintenance due to absence of brushes
  • Speed/Torque- flat, enables operation at all speeds with rated load
  • High efficiency, no voltage drop across brushes
  • High output power/frame size. Reduced size due to superior thermal characteristics. Because BLDC has the windings on the stator, which is connected to the case, the heat dissipation is better
  • Higher speed range – no mechanical limitation imposed by brushes/commutator
  • Low electric noise generation

BLDC Motor Cons

  • Higher cost of construction
  • Control is complex and expensive
  • Electric Controller is required to keep the motor running. It offers double the price of the motor.

Advantages between outer rotor and inner rotor motors?

The advantage of an outer rotor motor is torque. These smaller packages can produce more torque than there equivalent inner rotor size motors. This is accomplished by the larger moment arm of the rotating outer rotor magnet. One disadvantage is speed capability. If high speeds exceeding 6,000rpm are required, it is recommended you use an inner rotor construction motor.