The basics of BLDC motors

Understanding and application of the principle of the motors DC brushless high efficiency

A motor converts electrical energy into mechanical energy supplied. The brushless DC motors (BLDC) They provide high efficiency and excellent controllability and are widely used in many applications.

The simplest type of engine is the DC brushed motor. In this type of engine, the electric current passing through coils arranged within a fixed magnetic field. The current generates magnetic fields in the coils; this rotates the coil assembly, since each coil is pushed away from the similar pole and pulled towards the unlike pole of the fixed field. To keep the rotation, you must constantly turn the tide, so that the polarity of the coil is constantly tipping over, causing the coils continue to “run after” the opposite poles. The power supply to the coils is provided through brushes Fixed conductive who come into contact with a switch rotating; It is the rotation of the switch which causes the reversal of the current through the coils. The commutator and brushes are the key components that distinguish the brushed DC motor by other engine types. The figure 1 It illustrates the general principle of the brushed motor.Figure 1: Brushed DC motor operation.

The fixed brushes supply electrical energy to the rotary switch. When the switch wheel, continually reverses the direction of current in the coils, reversing the polarity of the coil so that the bobbins maintain the rotation to the right. The commutator rotates because it is connected to the rotor on which the coils are mounted.

Types of common engine

The engines differ according to the type of power source (CA o CC) and their method of generation of the rotation (Figure 2). Below, examine briefly the characteristics and uses of each type.Figure 2: various engine

The DC brushed motors, characterized by a simple design and simple control, They are widely used for opening and closing the tray of CD / DVD / BLU-RAY. In cars, They are often used to retract, extend and position the side mirrors to power supply. The low cost of these engines makes them suitable for many uses. one drawback, however, It is that the brushes and the switches tend to wear out relatively quickly as a result of their continuous contact, requiring frequent replacement and periodic maintenance.

A stepper motor is driven by pulses; rotates through a specific angle (step) with each pulse. Since the rotation is controlled accurately by the number of pulses received, These motors are widely used to implement positional adjustments. They are often used, eg, for controlling the supply of paper in printers, since these devices feed the paper in fixed steps, which are easily related to the pulse count. A pause can also be easily controlled, since the rotation of the motor is interrupted instantly when the pulse signal is interrupted.

With synchronous motors, the rotation is synchronous with the frequency of the supply current. These motors are often used to drive the rotating trays in microwave ovens; reduction gears in the motor unit can be used to obtain the appropriate speed of rotation. Even with induction motors, the rotation speed varies with the frequency; but the movement is not synchronous. In the past, these engines were often used in electric fans and in washing machines.

Because BLDC motors run?

As their name suggests, brushless DC motors do not use brushes. With brushed motors, brushes provide current through the switch in the coils on the rotor. Then, how does a brushless motor to pass current to the rotor coils? It does not do it, because the coils are not found on the rotor. instead, the rotor is a permanent magnet; and coils do not rotate, but instead they are fixed in position on the stator. Since the coils do not move, there is no need of brushes and a commutator. (See figure 3.)

Figure 3: a BLDC motor.

Since the rotor is a permanent magnet, it does not need power, eliminating the need for brushes and commutator. The current to the fixed coils is controlled from the outside.

With the motor brushed, the rotation is achieved by controlling the magnetic fields generated by the coils on the rotor, while the magnetic field generated by the stationary magnets remains fixed. To change the speed of rotation, It changes the voltage to the coils. With a BLDC motor, It is the permanent magnet that rotates; the rotation is achieved by changing the direction of the magnetic fields generated by the surrounding stationary coils. To control the rotation, It adjusts the magnitude and direction of the current in these coils.

Advantages of BLDC motors

A BLDC motor with three coils on the stator will have six electrical wires (two for each coil) which extend from these coils. In most of the three implementations of these wires will be connected internally, with the three remaining wires that extend from the body of the motor (in contrast with the two wires that extend from the engine previously described brushes). The wiring in the houses of the BLDC motor is more complicated than the simple connection of the positive and negative terminals to power; I will examine more closely how these later engines. Now I take care of advantages of BLDC motors.

A big advantage is the’efficiency, in these engines they must be continually at the maximum rotational force (couple). I brushed motors, Unlike, reach the maximum torque only at certain points of the rotation. For a brushed motor produces the same torque of a brushless model, you need to use larger magnets. This is the reason why even small BLDC motors can provide a remarkable power.

The second big advantage, linked to the first, and the controllability. The BLDC motors can be controlled, using feedback mechanisms, to accurately deliver the torque and the rotational speed desired. The precision control in turn reduces the power consumption and heat generation and, in cases where the motors are battery powered, lengthens battery life.

The BLDC motors also offer high durability and a low electrical noise generation, thanks to the lack of brushes. With brushed motors, the brushes and commutator wear out due to the continuous contact in motion and also produce sparks in case of contact. The electrical noise, in particular, It is the result of strong sparks that tend to occur in areas where the brushes pass over the blanks of the switch. This is why the BLDC motors are often considered preferable in applications where it is important to avoid electrical noise.

Ideal applications for BLDC motors

We have seen that the BLDC motors offer high efficiency and controllability, and they have a long operating life. So what are? Because of their efficiency and longevity, They are widely used in devices that operate continuously. They have long been used in washing machines, air conditioners and other consumer electronic devices; and more recently, They are appearing in fans, where their high efficiency has contributed to a significant reduction in energy consumption.

They are also used to guide the vacuum machines.

The BLDC motors are also used to rotate the hard disk drive, where their duration maintains operational drives so reliable long-term, while their energy efficiency helps reduce energy in an area where this becomes increasingly important.

Towards the wider use in the future

We can expect to see the BLDC motors used in a wider range of applications in the future. Eg, They are likely to be widely used to guide service robots: small robots that provide services in fields other than manufacturing. You might think that the stepper motors would be more suitable for this type of application, where the pulses could be used to control the positioning with precision. But the BLDC motors are better suited to the force control. And with a stepper motor, maintain the position of a structure such as a robotic arm would require a relatively large and continuous current. With a BLDC motor, everything that would be required is a current proportional to the external force, allowing a more efficient control from the energy point of view. In addition to their improved efficiency, The BLDC motors are also ideal for drones. Their ability to provide a precise control makes them particularly suitable for the drones multicopter, in which the trim of the drone is controlled precisely controlling the speed of rotation of each rotor.

We have seen how the BLDC motors offer excellent efficiency, controllability and longevity. But a careful and proper control is essential to fully exploit the potential of these engines.

The connections are more complicated

The figure below shows the appearance and the internal structure of a typical kind of BLDC motor. It should be noted that the permanent magnet of this motor is connected to its rotor and the coils are positioned all'intermo. Since the rotor of the BLDC motor does not use coils, It needs no power supply.

But the BLDC motors are more difficult to drive than brushed motors. With a brushed motor, all you have to do is connect the power source to the positive and negative motor leads. The BLDC motor has a different number of cables, and the connection is more complicated.

Video head of an old VCR note the pole pieces and welding on the base of the four poles plus U-V-W City, in the upper a Hall sensor used for the actual rotation control. With time and the evolution of technology, the use of the common terminal is lost in favor of a more complex and efficient steering system.

the magnetic field control

To rotate a BLDC motor, it is necessary to control the direction and the timing of the current in the coils. The figure 4 (a) illustrates a stator (coil) and a rotor of the BLDC motor (permanent magnets). Using this illustration to see how it will rotate the rotor. In this example, I use three reels, while in practice it is more common Dispense according six or more. But here for simplicity of explanation of the principle of operation using only three reels, spaced at 120 °. So how does the motor to rotate in our illustration? Let's look at what happens inside.

Figure 4 (a): of the motor rotation Principle BLDC.

We label the coils U, V e W. Remember that the passage of a current through a coil generates a magnetic field. Since there are three reels, There are three routes through which we can pass current; We can call these phases U (current in the coil U), fase V (in the coil V) e fase W. Let's first look at the phase U. If the current moves only through U, the magnetic flux is generated as shown by the arrow in Fig. 4 . In reality, all three coils are interconnected, through a single conductor wire from each, and you can not generate the U-phase in isolation. The figure 4 (c) shows what happens when the current moves through the U and W coils (phase “UW”), with arrows showing the flux generated at each coil again. The large arrow in Figure 4 (d) It is the resulting stream, the result of the combined magnetic fields of U and W.

Figure 4 (c): of the motor rotation Principle BLDC. The current flowing through U and W. The two arrows show the flow generated by U and W coils, respectively.

Figure 4 (d): of the motor rotation Principle BLDC. The large arrow shows the resulting stream, the flow of the product sum of U and W.

The rotation is maintained continuously passing the flow so that the permanent magnet constantly chase the rotating magnetic field induced by the coils. In other words, energization of U, V and W must be continuously switched in such a way that the resulting flow continues to move, producing a rotating field which affects continuously on the rotor magnet.

The figure 5 shows the relationship between energized phases and flow. As you can see, the sequential passage through various input combinations will rotate the rotor clockwise. The rotation speed can be controlled by controlling the speed with which the phases change. We use the name “conduction control to 120 degrees” for the control method described herein.

Figure 5: The change of the resulting flow continuously move the rotor magnet, causing the rotation.

The sinusoidal control enables a uniform rotation

With the conduction control to 120 degrees, There are only six directions of flow resulting for motor control. Eg, switching from mode 1 a 2 (see Fig. 5) It moves the direction of the resulting flow of 60 °, pulling the rotor accordingly. Passing from the mode 2 a 3 moves the direction of the other flow 60 °, pulling the rotor again. The repetition of this process generates a continuous rotation, but it is a rotation a bit 'jerky. In some cases, this shot creates unwanted vibration and mechanical noise.

As an alternative to the conduction control to 120 degrees, we can use the sine wave control to achieve a smooth, quiet operation. With a conductive control 120 degrees, the motor is controlled by passing continuously through six fixed resulting flows. And as you can see in Figure 4 (c), U and V generate both flows of equal magnitude. However, controlling the current in the most closely U, V e W, we can generate different magnitudes of flow of each coil, allowing us to more precisely vary the resulting stream. (See Figure. 6.)

By carefully regulating the flow of current in each of the three phases, then, we can achieve a continuous change in the resulting stream, resulting in more uniform rotation of the motor.

Figure 6: sinusoidal control.

By controlling the current in all three phases, the magnitude and direction of the resulting flow can be controlled more accurately with a conductive control 120 degrees, so as to obtain a more regular rotation. The resulting stream is no longer limited to six discrete directions.

Control via inverter

We see again the nature of the current in the U, V e W. For simplicity, let's see how it works with the conduction control 120 degrees. Looking back at Figure 5, we see that in Mode 1 the current flows from U to W; mode 2, da U a V. As shown by the arrows in the figure, each change in the combination of energized coils causes a corresponding variation in the flow direction.

Now view mode 4. Here we have the current that moves from W V; this is the inverse of the mode 1. With a DC brushed motor, this type of current reversal would be obtained by using brushes and commutator. The BLCD engines can not, by definition, use brushes or other mechanical contacts to achieve this reversal.

Using the inverter circuit to adjust also the voltage in each coil, we can also control the amplitude of the current. A typical way to adjust the voltage with the modulation of the pulse width (PWM). In this approach, We modify the tension by extending or reducing the ON time of the pulses (also known as “duty cycle”: the ON time expressed as a range switching ratio ON + OFF). Increasing the duty cycle has the same effect of the raising tension; reduce the work cycle has the same effect of lowering the current. (See Figure. 7.)

PWM can be implemented using MPU with dedicated hardware PWM. While the conduction control 120 degrees requires only the voltage control in two stages and can be implemented in a relatively simple manner in the software, the sinusoidal control uses the control of the three-phase voltage and is considerably more complicated. The appropriate inverter circuitry is therefore essential to operate the BLDC motors. Note that the inverters can also be used with AC motors. But when a term like “type of inverter” It is used with reference to consumer electronics, usually it refers to a BLDC motor.

Figure 7: Uscita PWM vs. Output Voltage. The variation of the work cycle (the activation time within each switching period) change the effective voltage.

BLDC motors and position sensors

As we have seen, We drive the BLDC motors constantly changing the directionality of the flux produced by the coils. The permanent magnets on the rotor continually chasing the rotating magnetic field, by rotating the rotor.

Till now, however, we did not examine another important feature of these engines: their positional sensors. Since the control of the BLDC motor must be coordinated with the position of the rotor (magnets), typically these engines also include sensors to detect this position. The application of the current when the rotor position is unknown can cause the rotation in the wrong direction. The use of sensors prevents this problem.

The chart 1 lists the typical types of engines used in these sensors. Several types of sensors are used for different control methods. The Hall elements, with signal inputs spaced 60 °, They are best for engines that use a conduction control 120 degrees, where all that is necessary is to determine which phase excite. more accurate sensors, as a resolver and optical encoder, They are more appropriate with engines that use vector control (look down), where the flow is controlled more finely.

Although these sensors provide obvious advantages, also has drawbacks. Some sensors have a low tolerance to dust and require regular maintenance. Others work correctly only on a limited range of temperatures. The use of sensors and the implementation of all the associated circuits increase the costs of production; and the high-precision sensors are obviously more expensive. I “motori sensorless BLDC” currently on the market completely eliminate the use of the sensor, so as to reduce the costs of parts and maintenance.

Table 1: types and characteristics of the position sensor

The vector control maintains high efficiency

As we have seen, the sinusoidal control delivers a uniform rotation using the three-phase current to control the flow gently. When the conductive control 120 degrees excite only two of the three phases (The, V e W) at any time, the sinusoidal control is considerably more complicated, exactly because it must distribute different amounts of current in all three phases.

One way to reduce this complication is with vector control , in which the calculations are used to convert the coordinate space, allowing you to manage the three-phase AC DC values ​​as values 2 phases. This approach can work, however, if high-resolution positional information is available for these calculations. One way to obtain this information is through the use of high-precision sensors (optical encoders, solve, etc.). Another way “sensorless” It is the estimation of the position based on the magnitude of the current in each phase. In both cases, the conversion of the coordinate space allows direct control of the electric current correlated to the torque, enabling a highly efficient operation with little wasted power.

The implementation of vector control requires intensive processing mathematics, including the ability to quickly solve trigonometric functions necessary to convert the coordinate space. The MCUs used to control these motors will generally include an FPU (floating point unit) and must be able to provide a substantial processing power.

With this I think I have provided a broad overview of BLDC motors, certainly it not all has been said and will not be completely exhaustive but, now anyone will tell you about these engines will find you completely unprepared.

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