To detect where the rotor is at certain times, BLDC motors employ along with controllers, rotary encoders or a Hall sensor. In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature windings are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy.
The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit. A BLDC Motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor revolve at the same frequency. Brushless motor does not have any current carrying commutators. The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.
Outrunner — The field magnet is a drum rotor which rotates around the stator. In runner — The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the out runner style, but is capable of spinning at very high rpm. The Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force.
In the BLDC motor, the current carrying conductor is stationary and the permanent magnet is moving. When the stator coils get a supply from source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape.
Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate. A brushless DC motor is essentially flipped inside out, eliminating the need for brushes to flip the electromagnetic field. In brushless DC motors, the permanent magnets are on the rotor, and the electromagnets are on the stator. A computer then charges the electromagnets in the stator to rotate the rotor a full degrees.
Brushes eventually wear out, sometimes causing dangerous sparking, limiting the lifespan of a brushed motor. Brushless DC motors are quiet, lighter and have much longer lifespans. Because computers control the electrical current, brushless DC motors can achieve much more precise motion control. Because of all these advantages, brushless DC motors are often used in modern devices where low noise and low heat are required, especially in devices that run continuously.
It uses direct current coupled with a switch system to achieve an alternate three-phase current. This output current can then be modified by changing the rate at which the switches open and close in the circuit.
Brushless ESCs need information on the current position of the rotor to be able to start the motor and choose a direction for the rotation. To determine its position, the ESC uses information from the last unpowered electromagnet to measure its induction.
This induction varies depending on where the closest permanent magnet is and the closer it is to the electromagnet, the stronger is the magnetic field induced. The throttle is used to vary the speed of the motor. There are several signal delivery protocols with different performances, the most common ones being Oneshot, Multishot and Dshot.
The difference between them is the frequency of the signals delivered. Shorter frequencies allow a faster reaction time. Furthermore, the Dshot protocol is different from the two others because it sends a digital signal instead of an analog signal. This makes the signal more reliable since it is less sensitive to electrical noise and is more precise with its higher resolution. Every brushless motor is made of two main parts, a stator and a rotor.
The stator is static, it does not move, and it holds the electromagnets. The rotor is the rotating component that holds the permanent magnets. There are two types of Brushless DC motors: Inrunner and outrunner models. For inrunner motors, the rotor rotates 'inside' the stator, or further inwards relative to the motor casing.
Outrunner motors have the opposite set-up as the rotor rotates 'outside' of the stator or further outwards, see figure 6. Both models have their pros and cons with different applications. When comparing an inrunner and an outrunner of the same size, it is easy to see that the diameter on which the forces are applied is different.
This happens because the electromagnets take a lot more space than the rotor carrying the permanent magnets. If the electromagnets are located inside, the diameter is bigger compared to if they are located on the outside figure 6. A larger diameter means more torque because the force is applied further from the center of rotation, while a smaller diameter would be better for high RPM.
Thus inrunners run best at high speed but generate less torque while outrunners work best with larger propellers because they can output more torque, but spin at slower speed. For the reasons described above, eVTOLs often use outrunners for vertical thrust due to their high torque. Inrunners are more commonly used for ducted fan jets or fixed wing aircraft and for horizontal movements requiring high RPM. This value determines at which speed the motor will spin if 1 V is sent through it.
Therefore, an inrunner typically has a higher Kv value than an outrunner of the same size because, as stated above, the inrunner has a smaller rotor which spins faster for the same voltage. You will often see the Kv value displayed first when browsing brushless motors. When talking about the performance of a drone, we often think about the flight time.
The only solution is to carefully select the electrical components that will use the battery charge most efficiently. Many factors can optimize the energy consumption of a battery. To demonstrate this idea, we present a comparison of two similar BLDC motors with different price ranges.
To compare the motors' performance, we used the RCbenchmark Series test stand. It is capable of measuring thrust, torque, voltage, current, RPM and efficiency. The data acquisition is done using the RCbenchmark software. The two motors tested are Ts of Kv and Kv respectively. Both motors spin the same 7-inch propeller with a pitch of 4. The first graph in figure 7 displays the efficiency of both motors compared to the throttle.
The second graph shows the efficiency at specific speeds. Clearly, the smaller Kv motor requires more throttle to run efficiently.
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