Operation of BLDC Motor

Unlike a brush DC motor, a brushless DC (BLDC) motor, as the name suggests has no brushes. Because there are no brushes, a BLDC motor produces little electrical and acoustic noise, and does not suffer from the wear of brushes and the need to replace them periodically. Thus a BLDC motor is more reliable than a brush DC motor.

In a BLDC motor, the rotor is made of permanent magnets, and the stator is constructed of coils. There is no wiring to the rotor. Because the rotor is lighter than that in a brush motor, a BLDC motor can operate at much higher speeds than a brush motor.

In addition, a BLDC is more efficient than a brush motor due to the absence of brush friction, and is thermally better suited to dissipate heat since the powered coils are located on the exterior portion of the motor.

Brushless motors are normally used in machinery applications that require fast response, low heat generation, and long life. Hence, they are used in high-end machine tools and robots and computer disk drives.

In a BLDC motor, unlike a brush motor, commutation is not mechanically done, but is performed through electronic means in which the stator fields are electronically commutated, depending on the position of the rotor.

In most cases, the rotor position is obtained from non-contact, Hall-effect type, proximity sensors that are mounted on the stator, but encoder feedback also can be used.

Some BLDC motor drivers perform sensorless control, which is based on using the EMF voltage over an unpowered coil to determine when to perform commutation of the current in the remaining coils.

BLDC motors are available in single phase, two-phase, and three-phase configurations, where the phase refers to the number of independent windings on the stator, with the three-phase being the most common configuration for industrial motors.

For a typical three-phase winding, the phases are wired in either a delta or a Y configuration. The Y configuration is more commonly used and is electrically more efficient.

A three-phase BLDC motor cable typically has three wires, one for each motor phase, in addition to the five wires for the three Hall Effect sensors (one wire for each sensor output plus ground and supply voltages wires).

The brushless amplifier drives two of the three motor phases with DC current during each 120° rotation segment of the rotor.

Operation of BLDC Motor

To illustrate the operation of a BLDC motor, let us consider a simplified three-phase BLDC motor with a two-pole rotor as shown in Figure 1. The stator coils are labeled phase A (ɸA), phase B (ɸB), and phase C (ɸC) and wired using a single circuit. Real motors have multiple circuits that are wired in parallel to each other, and a corresponding number of multi-pole rotors.

operation of bldc motor
Figure 1. Schematic of an simplified three-phase
BLDC motor.

In the simplified schematic of Figure 1, one electrical revolution of the motor corresponds to one mechanical revolution. If two electrical circuits were used, then there are two electrical revolutions per one mechanical revolution.

The figure also shows three Hall-effect sensors labeled A, B, and C that are placed on the stator. Each sensor outputs a high signal for 180° of electrical rotation and a low signal for the other 180°. The sensor output is high when the north pole of the rotor is pointing towards the sensor. Using this sensor arrangement, there are six combinations of the sensors’ output, with one combination or state for each 60° of electrical rotation.

Each combination is indicated in Figure 1 using the notation [CBA], where the least significant bit corresponds to sensor A output, and the most significant bit corresponds to sensor C output.

In practice, the particular labeling of each sensor (i.e., whether it is A or B) is not important. What is important is the association of the sensors output states with the position of the rotor.

In a BLDC motor, the phases are electronically commutated as the rotor moves from one sensors state to another. For each combination of the sensors’ output, two of the three phases are activated such as to produce an angle close to 90° between the stator and rotor flux vectors. There are six possible stator flux vectors shown as arrows in Figure 1.

A particular stator flux vector is generated for a given position of the rotor and a desired direction of rotation.

For the rotor position shown in Figure 1, the stator flux vector should be horizontal and pointing toward to the left for CCW rotation or horizontal and pointing to the right for CW rotation.

Figure 2. Illustration of phase activation to produce a
particular stator flux vector.

Figure 2 shows the generation of this stator flux vector for CCW rotation through activation of phases C and B and leaving phase A floating, with phase C connected to high voltage and phase B connected to low voltage.

Table 1. Commutation sequence
for CW and CCW rotation.

The commutation sequence for all possible sensors states are listed in Table 1 for both CW and CCW rotation. The reader can verify the commutation sequence for other positions.

Using this commutation sequence, on can create a drive timing diagram for CW rotation of the rotor as shown in Figure 3. The 0° electrical position corresponds to the 12 o’clock position.

Figure 3. Drive timing diagram for CW rotation.

A brushless motor requires a special driver that can provide the proper excitation voltages to the stator coils. The driver is of the bipolar type and is commonly referred to as a three-phase bridge driver (see Figure 4).

Figure 4. Three-phase bridge driver for driving a BLDC motor.

Such a driver consists of three parallel half H-bridge legs. As seen in Figure 4, closing switch S1 on the first leg and switch S4 the second leg and keeping the remaining switches open causes power to be applied to phases A and B of the motor with Phase C floating.

To reverse the current flow for phases A and B, switch S3 on the second leg is closed with switch 2 on the first leg. The particular switches to close are determined from a commutation table (such as Table 1), which provides the commutation sequence for correctly driving the motor. Obviously, the switches in Figure 4 are a representation for transistors in actual implementation.

An example of a brushless DC-motor is the motor that powers the small cooling fans in personal computers. Since BLDC are very light, and produce little electrical and acoustic noise, they are preferred to use for this application.

Figure 5. Components of a
BLDC fan.

These fans are typically constructed using a two-phase BLDC motor. A layout of the components of a BLDC fan is shown in Figure 5. The rotor has surface-mounted permanent magnets, while the stator has two-phase coils.

The fan uses a single Hall-effect sensor that is mounted on the stator circuit board. When the rotor axis is aligned with the sensor, the sensor sends out two complementary 50% duty cycle waves. These cycles are fed to the transistor gate input that controls the current flow through each of the two coils. Since the two square waves are complementary, only one coil will be active at a time.

 Increasing the voltage supplied to the motor causes the motor to increase its speed. Some BLDC fans come with an output that indicates the speed of the fan.

The torque–speed characteristics of a BLDC motor are different from a brush motor. Typical characteristics are shown in Figure 6.

Figure 6. Torque–speed
characteristics of a
BLDC motor.

The figure shows that a brushless motor has a constant or slowly decreasing torque over a wide speed range up to the rated speed. After the rated speed, which is normally above 3000 rpm, the torque starts decreasing more rapidly.

In the constant-torque region, the motor horsepower increases linearly with speed.

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  9. BLDC Motor Working Principle
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