Brushed and Brushless UAV Motors
There are two types of motors that one can get to power the RC Drone. These two categories are the brushed and brushless. When buying a ready to fly (RTF) or a bind and fly (BNF) kit the type of motor being used will have been determined by the manufacturer for you, but when building your own or upgrading a kit then you will need to make your own decisions as to which is best for your craft. In this article we will discuss some of the pros and cons of each as well as some basic facts and principles so that you are able to make a more educated decision as to what motor is best for your needs.
As stated above there are two basic types of motors that you will be choosing between when getting motors for your craft, brushed and brushless however the principles upon which they work is the same for both motor types. Both are DC (direct current) motors that convert the electrical energy provided by the battery of the craft into mechanical energy that is used to turn the propeller that is controlled by that motor. This is accomplished when the motor winding’s have a current supplied to them by the battery that is used to generate an electromagnetic field that pushes against the fixed magnets in the motor causing the motor shaft to spin.
In a brushed motor the winding’s of the motor are fixed onto the shaft inside the case and spin when the current is supplied to the motor. The magnets inside the motor don’t move and power is supplied to the motor winding’s via the positive and negative brushes that push against the commutator on the shaft. This is one of the key weaknesses of the brushed motors as the friction between the brushes and the shaft will reduce the efficiency as well as create wear on the brushes that my need to be cleaned and/or replaced do to wear over time.
Brushed Motor Pros
- Brushed motors may be re-buildable for extended life
- Brushed motors operate in extreme environments due to lack of electronics
- Brushed motors are cheaper to produce and buy
- The wiring of brushed motors is simpler as they only have two wires to power them and they don’t require any electronic commutation which means that you can use less complicated and less expensive Electronic Speed Controllers (ESC)
Brushed Motor Cons
- The brushes and commutator must be cleaned periodically and eventually will wear out
- Friction from the brushes will cause the motor to slow down, will lower the power to weight ratio and will result in shorter battery life which means shorter flight times
- Brush arcing will generate noise causing electrical magnetic interference (EMI)
In a brushless motor there is no commutator or brushes, as the design basically turns the motor inside out, changing the way that the current uses the magnets to push the shaft so that it rotates. The key advantage to this is that you remove several of the parts of the motor that were prone to wear and tear and you reduce friction points which increase the efficiency of the motor.
Brushless Motor Pros
- Brushless motors are much more efficient than conventional brushed motors
- Brushless motors do not have brushes or commutator that require maintenance or replacement which also increases the lifetime of the motor.
- Brushless motors generate less noise causing electrical magnetic interference (EMI)
- There is no friction being generated by brushes to slow the motor down which means that you have a better power to weight ratio, longer battery life and flight times and possibly higher speeds.
Brushless Motor Cons
- Brushless motors are more expensive to produce and buy
- The wiring of brushless motors is more complicated as they only have at least three wires to power them and they require electronic commutation which means that you must use more complicated and expensive Electronic Speed Controllers (ESC)
Terms used to explain brushed and brushless motors
Armature: The armature (or arm) is what spins in the motor and makes your rc helicopter move. It is made up of the commutator, laminations, shaft and winds. Electricity flows through the wires from the ESC to the end bell. It then travels through the brushes to the commutator, and into the windings on the arm. Since the windings are wrapped into a coil, they create a magnetic field when current is passed through them. This magnetic field is repelled and attracted to the magnets in the can causing the arm to turn.
Brush: Made of a silver, copper, or graphite compound and at the end of the shunt wire. They are what makes contact with the commutator and transfer the electric current to the commutator.
Commutator: Typically referred to as the Comm. The comm takes current from your brushes, which ride on this part of the arm, and sends it to the windings. The comm is not one solid piece, but is actually made up of 3 separate pieces. This allows the current to be switched to the different windings of the arm as it spins. Because it rubs against the brushes as the arm spins, the comm needs to be cleaned and shaped regularly.
Continuous / Burst Current: Continuous current measures how much current a motor can handle continuously, for an extended period of time. Burst current measures how much current a motor can handle for a short amount of time, about a few seconds.
Current Rating: This is the maximum current that a given motor can handle, measured in amps.
Endbell: The part of the motor that consists of the brush hoods and the tabs. The endbell holds the bearing that supports the short end of the shaft.
Kv Rating: The Kv number is the RPM per volt supplied to the motor. The KV number’s useful because it let’s you figure out how many volts you need to achieve a certain RPM, or vice versa. For example, a 1200 Kv motor, supplied with 3 volts, will run at a nominal 3600 rpm. The Kv rating always assumes no load on the motor, so the actual RPM that your achieve will be less than the one you calculate. Note that Kv is the voltage constant (capital-K, subscript v), not to be confused with the kilovolt, whose symbol is kV (lower-case k, capital V).
Laminations: The part of the armature the winds are wrapped around. These are usually about half a millimeter thick, and are stacked on top of each other. The laminations are sometimes shaped to provide a stronger field. They are usually made of iron ferrite.
Magnets: Provides the opposing force that the armature’s magnetic force pushes against.
RPM: This is a measure of angular speed, or how fast something is rotating. A motor’s RPM is simply how fast it can rotate per minute.
Torque: Torque is a measure of angular force, or how much “push” a rotating shaft has.
Volt: This measures electric potential, or how much “push” the electrons from a battery have. A greater voltage means that more energy is being applied to a given amount of charge.
Watt: This is a measure of power, or how fast energy is used.