Selecting Motors for Your Multirotor

by | May 3, 2014 | design, theory | 0 comments

Selecting the best combination of components for your quadcopter is a long and complex process; if you have spent much time reading the other guides on this site, you are probably getting sense of this fact. And as one of the most important components on your quadcopter, selecting motors that will work well for your particular application (acrobatic sports flying, or aerial videography, or heavy lifting, et cetera) requires a fair bit of research and consideration. On the up side, when shopping for motors, you should easily be able to find all of the information you will need to determine which motors will work well for you and which will not.

Before we begin discussing what all of the different specifications provided by motor manufacturers and sellers mean, which we will use when selecting motors, I want to take a moment to go over some basic background information about quadcopter motors.

Brushless Motors vs Brushed Motors

There are two major types of DC electric motors, brushed motors, and brushless motors. In quadcopters and other multirotor aircraft, we always use brushless motors, but what exactly are brushless motors and how do they differ from brushed motors?

Well, first of all, brushless motors and brushed motors use the same physical principles to work. Inside both types of motors there are two different kinds of magnets, permanent magnets, and electromagnets. When power is applied to the motor, current is directed through the wire windings that make up the electromagnets. This current induces a magnetic field around the wire coils that attracts and/or repels the permanent magnets. This magnetic attraction/repulsion causes the motor shaft to turn. As the shaft turns, the current applied to the electromagnets is switched in such a way that the motor shaft is made to continuously rotate.

The difference between brushed motors and brushless motors lies in the way the permanent magnets and electromagnets are configured and in the way electrical current is applied to the electromagnets.

In brushed motors, the permanent magnets are arranged on the motor’s outside casing, which stays stationary, and the wire coils that form the electromagnets are on the motor shaft which rotates. In order to transmit electrical current to the electromagnets on the rotating shaft of the motor, brushed motors have, you guessed it, brushes, which are basically bits of wire that reach from the outside of the motor to the motor shaft and transmit electrical current.

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brushed-motor-inside-diagram

Brushed motors are constructed with the permanent magnets on the exterior casing and the electromagnets on the rotating motor shaft. They contain “brushes” to transfer electrical current to the rotating electromagnets.

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In brushless motors, the permanent magnets are on the motor shaft and the electromagnets are arranged around the motor casing. Since in brushless motors the electromagnets remain stationary, no brushes are necessary to transmit electrical current to the wire coils; a power source can simply be connected directly to the electromagnets.

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brushless-motor-inside-diagramIn brushless motors, the permanent magnets are mounted on the rotating motor shaft with the electromagnets arranged on the outside casing. This construction means that no brushes are required.

[/full_width] The motor pictured above is a three-pole motor, since it has three electromagnetic coils. Most brushless motors have far more coils than this though; the ones used in the tutorials on this site have fourteen coils, so they are called fourteen-pole (or 14-pole) motors.

Both types of motors have their distinct advantages and disadvantages in various aspects of their construction and operation.

Characteristic Brushed Motor Brushless Motor
Wiring/Control Only a power source is needed and can be connected directly to the motor. Speed is controlled by varying the voltage applied. No specialized control circuitry is required. Require a controller with the ability to monitor the position of the motor shaft and control the electromagnets accordingly.
Efficiency 75-80% 85-90%
Cost Simple and inexpensive More expensive and also require the purchase of a controller (ESC).
Durability Less durable due to wear on brushes/motor shaft. Lack of brushes mean more durability and longer life span
Torque Less torque More torque per weight and per watt
Speed Mechanical electrical contacts limits speed Higher speed due to lack of mechanical electrical connections.

From these pros and cons, you can probably tell why we use brushless motors for building multirotor aircraft. Brushless motors are more powerful, more efficient in their use of battery power, allow us to better control their speed, and are more durable. The downside is that brushless motors are much more expensive than brushed motors and require controllers, which are the ESCs.

Motor Specifications

When shopping for motors, you should easily be able to find a range of specifications supplied by the motor seller or by the manufacturer, or both. In this section I will define these specifications and in the next section we will cover how to use these specifications to choose motors.

  • Kv
    Kv means RPM/V, which we can use to determine the rotational speed of the motor. It is based on the construction of the motor. Without going into too much detail, Kv is related to the way the electromagnets inside the motor are constructed. Therefore, Kv is also related to the torque of the motor. In general, motors with lower Kv have more torque and vice versa.
  • Weight
    This one is easy, we generally want to go for lighter motors since weight is extremely important for quadcopter performance. That said, we do want to go for well-built durable motors.
  • Max Current (A)
    This is the maximum current the motor will draw at full throttle. This is an important figure for choosing an ESC, which is the subject of the next step.

Selecting Motors

Choosing a motor then is a decision that you must make based on the type of flying you intend to do. It is also a decision that you should make in conjunction with choosing a prop.

For acrobatic sports flying you should choose faster (higher Kv) motors and smaller props. The smaller props, as discussed in more detail in the Selecting Props tutorial, can spin up and slow down more quickly larger props, which means a quadcopter using smaller props is more agile. Also because smaller props move less air, they need to spin faster in order to generate the same lift.

For aerial photography though, you should choose lower Kv props with larger props. Larger props are more efficient and generate more lift for moving heavy payloads like a camera. Larger props also offer a more stable configuration.