Choosing Motors: In a Prop-crack

Choosing motors can be tedious considering the vast options that is available for your purchase. In many cases the choice of motors can depend on several factors. One of the biggest challenges in creating an energy efficient plane, involves understanding motor performance. In this article, it will be compiled to help the many young engineers that might end up in this page. Firstly the method described here is not the professional level method which involves several layers of theoretical testing and practical performance study. I will highlight some of these as we go.

Factors Affecting Motor Performance (Not limited to)

  1. Manufacturing Methods
  2. Materials /Quality
  3. Design /Aerodynamics /Flow quality
  4. Cooling
  5. Magnet Quality
  6. Shaft and Bearings
  7. Electronic Controller (ESC)
  8. Environment
  9. Type of Propeller
  10. Power Supply
  11. Wire quality/ Thickness / Diameter
  12. Motor Weight

Factors Affecting your choice.

These are the factors you need to consider before you purchase a motor. One may have an higher importance than the other.

  1. Cost
  2. Thrust/Weight Ratio
  3. Propeller
  4. ESC limitations
  5. Application (Multicopter/Plane/Heli)
  6. Energy Efficiency
  7. Brushed/Brushless
  8. Quality
  9. Performace Specification Available ( The more data you have, the better!)
  10. Type: Inrunner/Outrunner

 

How to Read motor Specifications on a Website?

Specs:
RPM/v: 820kv  This is known as KV of motor, or rpm/volt. The amount of rotations per minutes when you input 1 volt. So if you are using a 2 cell battery, it would be 7.4V input. 
Dimensions: 51 x 23mm
Shaft: 6.0mm  This is shaft diameter, this is important when you select the propeller. Usually 3mm shafts are also common. Check if your prop comes with necessary adapters. 
Voltage: 11.1~14.8V    This defines your motor operating voltage, also means you can use a 3 cell battery to 4 cell battery. 
Weight: 194g
No-load current: 2.8a @ 11.1v   This is when the motor is spinning without the propeller, the lowest current draw occurs at this point.
Max Current: 60A This is the maximum amount of current that your motor will draw from the batteries. Therefore, your ESC must be of higher A rating, example 70A. 
Max Watts: 680W
Mounting Holes: 65mm  This data should be checked against your design. Your mount should be able to fit this dimension. Check manufacturer data for further specifications. 

Manufacturer Data 

Like mentioned earlier, it is important to have sufficient data about the motor you are buying. Many go for the cost of the motors, but i usually recommend going for motors with good supporting data. They make life much easier, especially when you are using the motor for projects. The most important data specification the manufacturer can provide you with is the PROP-BATTERY-THRUST chart. These charts are more reliable when you have a well known manufacturer. Some Chinese manufacturers doctor results. Ultimately you can conduct your own tests (within safe limits) using a thrust test stand  and a POWER meter. If you are using a multi-meter, make sure if can withstand the motor current draw.

A simplified version of PROP-BATTERY-THRUST chart looks something like this,

12×8 – 11.1v – 1750g Thrust
11×5 – 14.8v – 2250g Thrust
10×5 – 14.8v – 1850g Thrust

 

Motors can be categorized into three different categories.

  • Low cost, low quality motors.
  • High cost high quality motors with good power to weight ratio along with documentation.
  • Lastly, custom made motors which suit each specific application.

 

  • Chinese made Turnigy, Quanam, Hextronik, Trackstar and Hobbyking fall under the first category.
  • Maxon, Minimotor, portescap, faulhaber, Neumotors fall under the second category.
  • Hacker, Strecker and plettlenberg are considered as the most favorite custom make electric motor companies.

 

Number of Poles  / Number of Turns in a Motor.

While selecting electric brushless motors, the number of poles and number of turns are considered when selecting a motor. The number of pole coincides with the number of times the magnetic field reverses as it goes around the motor. The poles are formed by coils of the wire around the stator. More poles will make the motor more efficient and give a higher torque. This also means the rpm/volt or Kv of the brushless motor will be lower. The number of turns is used as selection criteria for some brands of motors.  The turns coincide with the amount of the wire wrapped around the stator in the motor. The lower the number of coils, the higher the rpm. Fewer turns will result in higher Kv with low torque suitable for a fast flying plane.  A motor with high number of turns is intended to spin a larger propeller at low rpm with larger torque.

Brushless Motors

Brushless motors use electronic communication to create a rotating magnetic field vector that pulls a permanent magnet or electromagnet. In a direct current motor the inner part is the rotating element which has a coil generating a rotating magnetic field. The surrounding part is a stator which is magnetic. This creates a fixed magnetic field. The rotation will change coil polarities and creates an oscillating current. When DC motors are brushed, they may be susceptible to friction, sparks and electrical noise as current increases. They also have trouble getting rid of heat. Speed control for the motors is achieved through using PWM or pulse width modulation signals. Sometimes varying the input constant voltage also allows the control of the speed.  The brushless DC motor is preferred as it generates less noise, has fine speed control and high torque to weight ratio. It has higher efficiency due to lower mechanical friction, resulting in longer lifetimes.

 

Inrunner/Outrunner

There are two categories of brushless motors however with multirotors and most fixed wing aircraft you will only encounter out-runner motors are pictured above. The difference between the two is that out-runners have the rotor on the outer area of the motor, and in-runner motors have the rotating part on the inside while the outer shell remained stationary. In-runner motors are often used with R/C cars as they can spin much faster than out-runner motors. However out-runner motors are able to produce more torque which allows them to drive larger propellers used on aircraft and multorotors.

  • Rotor – the part that rotates and has the magnets mounted in a radial pattern
  • Stator – the part that does not rotate, and has electromagnets

Image result for stator and rotor

So how do you select your motor?

In summary, you should be considering, in chronological order

  1. Thrust to Weight Ratio, your motor must be able to lift the aircraft along with its components. For multicopters 1.5:1 or 2:1 is the minimum.( e.g 4×500 grams of thrust / 500 gram quad = 4:1 thrust ratio.) Note that, the higher you go, the more power the motor requires. This means, you fly for lesser time. So do your calculations on flight time for about 50 percent throttle current draw. (e.g, your MAX Current draw is 60A, take 60A divide by 2, = 30A., now divide by the battery Ah to get your flight time.)
  2. The cost, once you have a few motors on your mind, go through the cost factor. You may not want to spend too much on your first trial quadcopter. With more spare motors, you can maintain your quad. Never just buy 4 motors for your quad copter, always buy 2 more spares.
  3. The amount of data available about the motor, as mentioned before, data is everything. Without it, you are just building a TOY.
  4. The type of battery and ESC you are using along with the motor, ensure that the motor is compatible with other peripherals. This will ensure maximum performance. Some motor manufacturers give specific recommendations.
  5. In-runner motor if you have a gear box, Out runner motor for direct drive. This is basically more relevant to your design. When you are looking towards efficiency, or altering flight speeds,gearbox can be used. However in most cases everyone tries to go for direct drive to save on weight.

Parts of a motor. The common Outrunner.

 

 

Author: adlabs2

Arrowdynamic Laboratories Pte. Ltd. (ADLABS) was founded in 2008 anticipating imminent technological pervasion. ADLABS operates with two wings, Education and Engineering. Education involves the development of skills training programs and STEM-based enrichment programs for everyone. The engineering wing conducts research and development activities on new prototypes with potential to form into a new branch startup. Our engineering team also offers specialised services for startups, companies and institutions.