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How do Brushless Motors Work?

The brushless motor aka the bread and butter of your miniquad: their light, responsive and most importantly fast, but have you ever considered how one of these bad boys works.

The Brushless Motor

Now you probably know that your motors are designed to do one thing and that is to spin in a specific direction as fast as possible. You bolt your propeller onto the motor which is effectively like putting two little wings and flying them through the air at high speed. This forces air in a specific direction called thrust depending on which direction they spin. The faster they spin the more thrust is generated. Simple.

Now without going into University level physics we hope to give you a basic understanding of how a brushless motor works.

To put it simply a brushless motor contains a series electromagnets (coils). An electromagnet is a coil of wire which creates a magnetic field when a current is pass through it. These coils are connected together in specific pairs and then actived in phases to create magnetic feilds in specific parts of the motor.

This is where your ESC comes in, it controls the motor by activating and deactivating specific sections of electromagnets in the motor at very specific times to cause the rotor of the motor to spin due to the magnetic force. These electromagnets are connected into three main sections which is why all brushless motors have 3 wires coming out of them.

What makes up a Brushless motor

scorpian motor diagram

  1. Rotor
  2. Screw hole for mounting
  3. Further mounting bracket
  4. Magnet these are flipped every magnet so it goes N.S.N.S.N.S.N.S…
  5. Flux ring (Casing)
  6. Rear locking ring which maintains magnet spacing and also helps to strengthen the rear end of the flux ring
  7. Stator plate, which a coil of wire is wound around. The stator plate is expoxied in order to prevent the wires shorting through the motor. – 0.20mm stator plates are used on the 22mm motors and 0.35mm stator plates on larger sizes.
  8. High Temperature 180 C (356 F) rated wire is used for winding the motors to minimize the risk of burning up the motor. If the wire is heated too much and melts then the circuit is broken; tt can no long form an electro magnet and so the motor stops.
  9. Shielded ball bearings are used to support the motor shaft
  10. High temperature adhesives are used to secure the stator windings and prevent them from shifting and shorting.

Rotor – the part of the motor that rotates and has the magnets mounted in a radial pattern

Stator – the part of the motor which houses electromagnets (coils)

Inrunner and Outrunner motors

There are the two different categories of brushless motors:

OutRunner: This is where the outer part of the motor spins while the inner part stays stationary. This is the type you will predominantly find on multirotors and most fixed wing aircraft. This is because Outrunners have lower KVs and better cooling than inrunners, which optimises them for use on aircraft aldo Outrunners are able to produce more torque which allows them to drive larger propellers used on aircraft and multorotors.

Outrunner-Anatomy-sm1

Inrunner: This is where the inner part spins while the outer part stays stationary
(more like a traditional motor). Inrunner motors have the rotating part on the inside while the outer shell remains stationary. Inrunner motors are often used with R/C cars and boats because the have gearing which allows them to run a motor with a higher kv. Though this gives it less torque it doesn’t matter due to the gearing.

Inrunner-Anatomy-SM

How are different Motor identified?

When it comes to motors and their applications there are two main factors. Their size and the speed that they run.

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Usually the size of the motor will give you a good idea of the kind of multi-rotor which you using it on.

Class of Quad Number of Cells Capacity (Mah) Discharge rate (C) ESC Motor
120-150 class 3-4S 450-850mah 45-65C 10-20A 11xx-14xx 3000-5000kv
180 3-4S 850-1300mah 30-65C 12-20A 13xx-18xx 2300-3200kv
210-250 Acro 3-4S 1300-1800mah 40-65C 15-30A 18xx-22xx 2100-2600kv
Racing 5” props 3-5S 1000-1500mah 75+C 20-30A 22xx-23xx 2100-2600kv
300 GT quads 3-5S 1800-3000mah 40-65C 20-40A 22xx-23xx 2000-2400kv
400+ 3-8S 2200-8000mah 20-65C 20-40A 22xx-50xx 400-1800kv

kV of a Motor:

A motor is simple right? As you apply voltage across it, it spins. If you increase the voltage it spins faster right? The measure of how much faster it spins as you increase the voltage is called the kV, where ‘k’ stands for RPM and ‘V’ stands for voltage. It is important to note this value is without any ‘load’ on the motor, i.e. no prop.

The kV rating of a motor is possibly the single most important specification when choosing a motor for your quad, therefore it is very important you do research to find out the best kV for you usage.

It is worth noting that KV is inversly proportional to torque! aka when one goes up the other goes down.

Here is a rough guide to choosing kV for different sized quadcopters.

Class of Quad Number of Cells Capacity (Mah) Discharge rate (C) ESC Motor Specs
120-150 class 3-4S 450-850mah 45-65C 10-20A 11xx-14xx 3000-5000kv
180 3-4S 850-1300mah 30-65C 12-20A 13xx-18xx 2300-3200kv
210-250 Acro 3-4S 1300-1800mah 40-65C 15-30A 18xx-22xx 2100-2600kv
Racing 5” props 3-5S 1000-1500mah 75+C 20-30A 22xx-23xx 2100-2600kv
300 GT quads 3-5S 1800-3000mah 40-65C 20-40A 22xx-23xx 2000-2400kv
400+ 3-8S 2200-8000mah 20-65C 20-40A 22xx-50xx 400-1800kv

Magnet Configuration:

A less important specification is the magnet configuration, this tells you how many permanent magnets there are in the rotor and also how many electromagnets there are in the stator.  The naming convention is as follows.

XXNXXP

The two numbers before the N state the number of electromagnets there are in the stator, and the number before the P state the number of permanent magnets there are in the rotor.

By far the most common configuration is 12N14P, although you will see some motors (such as pancake motors for AP Ships) with configurations with larger numbers.

Efficiency

One other important spec is the efficiency of a motor, often forgotten by mini quad pilots. For our hobby efficiency can be written as:

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There are many sources of loss of power on a multirotor, all these losses decrease the efficiency of our power system. However these losses can all be grouped into 3 categories (this is forgetting losses from power wires but they tend to be negligible)

  1. The ESC- Generally 95% efficient
    1. Losses come from the FETs which ‘chop up’ the current into little pieces to make the motor spin. This constant switching wastes some of the current.
  2. The Motor- Very roughly 80% efficient
    1. Losses come mainly from Ohmic losses, Iron losses (more on these later) and finally friction between the bearing and such.
  3. The Prop- Roughly 80-90% efficient
    1. These mainly come from the propeller ‘whirling’ the air into a helical motion, which cannot be used as useful thrust. And also the fact that thrust is lost by the fact there is no ‘clean’ separation between the ‘fast’ air accelerated by the prop and the rest of the air that has yet to be accelerated.

 

Ohmic Losses:

When current flows through a wire, the wire heats up a little, this is lost energy from the electricity flowing through it.  On a brushless motor these are from the current flowing through the stator of the motor. This can be quite sizeable losses, especially if you push your motor hard.

 

Iron Losses (core losses):

These are losses caused by the magnetism of the motor, explaining these can be quite difficult unless you are an electrical engineer! But put simply they are caused by the magnetic fields in the stator constantly switching polarity rapidly.

Conclusions

Hopefully we have been able to explain some of the finer details of Brushless motors to you.

If there is anything we should add to guide, feel free to comment below?

 

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4 comments

  1. Really nice art! Keep going!

  2. Great article, although the author and editor need to fix the grammatical errors.

    • Thanks, and apologies for the errors, some of our writers are on holiday, and sometimes a couple of things slip through the gap.

  3. The motor and propeller efficiencies sound waaay off. Following your nnumbers, efficiency of quad should be .95*.2*.2 = 4% tops. Might as well try flying an electric heater.

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