How to connect brushless motor?

How to power and control the brushless DC motor? This article will introduce some professional knowledge of brushless motors.

This passage is going to talk about the followings of brushless motor:

(1) Basic knowledge of brushless motor

(2) BLDC motor control features

(3) Control schemes for electronic commutation of brushless motor

(1) Basic knowledge of brushless motor

All motors, whether mechanical or electronic, follow the same basic method of converting electrical energy into mechanical energy. The current passing through the winding generates a magnetic field, which in the presence of a second magnetic field (usually caused by a permanent magnet) will generate a force on the winding. When its conductor is at 90° to the second field, the force reaches Maximum value. The increase in the number of coils will increase the output of the motor and smooth the power output.

Brushless DC motors overcome the need for mechanical commutators by reversing the motor setting. The winding becomes the stator and the permanent magnet becomes part of the rotor. The stator is usually composed of steel laminations that are axially slotted to accommodate an even number of windings along their inner circumference. The rotor consists of a shaft and a hub with permanent magnets arranged to form two to eight pole pairs alternating between "N" and "S". Figure 1 shows an example of a common magnet arrangement. In this case, two pairs of magnets are directly bonded to the rotor hub.

In BLDC motors, permanent magnets are fixed on the rotor. A typical configuration consists of two pairs and eight pairs, alternating between "N" and "S" poles.

Because the windings are fixed, a permanent connection can be established to excite them. In order for the fixed winding to move the permanent magnet, the winding needs to be energized (or commutated) in a controlled sequence to generate a rotating magnetic field.

Since the rotating magnetic field generated by the stator causes the rotor to rotate at the same frequency, the BLDC motor is called a "synchronous" type. BLDC motors can be one-phase, two-phase or three-phase. The three-phase BLDC motor is the most common motor.

(2) BLDC motor control features

So far, the most common configuration used to sequentially apply current to a three-phase BLDC motor is to use three pairs of power Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) arranged in a bridge structure. Each pair of switches controls one phase of the motor. In a typical configuration, the high-side MOSFET is controlled by pulse width modulation (PWM), which converts the input DC voltage into a modulated drive voltage. Using PWM can limit the starting current and provide precise control of speed and torque. The PWM frequency is a compromise between the switching loss that occurs at high frequencies and the ripple current that occurs at low frequencies, which can damage the motor in extreme cases. Generally, the PWM frequency used by designers is at least an order of magnitude higher than the maximum motor speed.

(3) Control schemes for electronic commutation of brushless motor

There are three control schemes for electronic commutation: trapezoidal, sinusoidal and field-oriented control. The trapezoidal technique (described in the example below) is the simplest. In each step, both windings are energized (one winding is "high" and one winding is "low") while the other winding is floating. The disadvantage of the trapezoidal method is that this "stepped" commutation causes torque "fluctuation", especially at low speeds.

Sinusoidal control is more complicated, but it can reduce torque ripple. In this control mode, all three coils are kept energized, and the drive current in each coil changes sinusoidally with each other, at 120° to each other. Compared with trapezoidal technology, the result is smoother power transfer.

The field-oriented control relies on measuring and adjusting the stator current, so the angle between the rotor and stator flux is always 90°. Compared with all other technologies, this technology is more effective than the sinusoidal method at high speeds and has better performance during dynamic load changes. There is almost no torque fluctuation, and smoother and more accurate motor control can be achieved at low speed and high speed.