Lead Angle/Conduction Angle of Brushless DC Motor

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Lead Angle/Conduction Angle of Brushless DC Motor [Copy link]

In the control of brushless DC motors (BLDC), Lead Angle is often used, whether for sensored or sensorless motors. Because the motor coil is an inductive load, the current in the coil will have a certain time delay relative to the loaded voltage on the coil, which will affect the efficiency of the motor and generate noise and vibration. For the trapezoidal wave/square wave control of BLDC, debugging and selecting the appropriate lead angle can significantly improve the efficiency and vibration and noise level of motor control without changing the basic control algorithm. Especially for sensored motors, the lead angle in the control sequence is equivalent to adjusting the sensor position inside the motor, thereby achieving the inconvenient and difficult to adjust sensor physical position adjustment through a simple and easy software method.

1. Three-phase BLDC control principle (trapezoidal wave)

The basic principle of the brushless motor trapezoidal wave control algorithm is shown in the figure below. First, the AC power is rectified into a DC voltage, and the latter stage is the inverter, which contains 6 switching devices (FETs) - U, V, W of the upper bridge arm and X, Y, Z of the lower bridge arm.

By controlling these FET switching devices in a certain order, such as — 1: U->Y, 2: U->Z, 3: V->Z, 4:V->X, 5: W->X, 6: W->Y (assuming the motor direction is forward), the current flowing through the motor coil will follow this order — 1: U phase to V phase (U->V), 2: U phase to W phase (U->W), 3: V phase to W phase (V->W), 4: V phase to U phase (V->U), 5: W phase to U phase (W->U), 6: W phase to V phase (W->V). There are 6 steps in total. This cycle is shown in the figure below.

Figure 1

Similarly, if the motor direction is reverse, the switching sequence is 1: U->Z, 2: U->Y, 3: W->Y, 4: W->X, 5: V->X, 6: V->Z. This is the trapezoidal wave/square wave control algorithm of the brushless motor BLDC.

2. Three-phase BLDC control timing

The control timing of the brushless motor depends on the motor rotor position. To facilitate the explanation of the lead angle/conduction angle, a motor with a Hall sensor is used as an example (the same below). According to the position feedback of the Hall chip as an interrupt, each time a valid Hall position pattern is detected, the commutation step (Step) begins, and then the next position feedback interrupt is detected. As shown in Figure 2 below. According to the above 6 steps, the commutation is continuous: 1: (U->V), 2: (U->W), 3: (V->W), 4: (V->U), 5: (W->U), 6: (W->V). Repeat the cycle.

Figure 2

Each time a Hall position pattern is detected, commutation is performed and the corresponding PWM pattern is output. Then, the position feedback signal is sampled and monitored until a valid Hall position pattern is detected again.

The black area in the PWM switch signal (U~Z) in the figure is the effective level, which contains the PWM carrier (from a few K to tens of KHz, with variable duty). The black narrow columnar waveform in the phase voltage (U phase~W phase) in the figure is at this phase switching moment (switching from one step to the next PWM Pattern), and the phase switching voltage jumps due to freewheeling and other reasons (inductance of the motor coil).

3. Lead angle

Lead Angle is often used in BLDC control of brushless DC motors. Because the motor coil is an inductive load, the current in the coil will have a certain delay relative to the loaded voltage on the coil, which will affect the efficiency of the motor and generate noise. Taking the U-phase voltage of the motor as an example, the PWM signal U, X and U-phase voltage waveforms in the above figure are enlarged, as shown in the figure below. The U-phase is expanded to see the actual voltage waveform, which contains the PWM carrier. Ignore the PWM carrier and look at the envelope diagram, as shown in the bottom waveform of Figure 3 below.

Figure 3

Here, the green dotted line is the commutation point, which is defined as the lead angle/conduction angle 0 point. This point is 30 degrees to the right (behind) of the midpoint of the phase voltage. The Lead Angle lead angle/conduction angle, as the name implies, is how many degrees ahead of the lead angle 0 degree to the left (forward).

Generally speaking, especially for the BLDC trapezoidal wave control algorithm for brushless motors, when the motor is at high speed, it is necessary to insert a certain Lead Angle/conduction angle into the six PWM input signals (U~Z) before each commutation point. The following figure shows the input signals corresponding to different Lead Angles/conduction angles, from 0 degrees, 7.5 degrees, 15 degrees to 30 degrees.

Figure 4

The lead angle/conduction angle adjustment depends on the motor parameters, motor voltage, motor speed, etc. Generally speaking, for the same motor and given voltage, the higher the motor speed, the higher the lead angle/conduction angle needs to be. The key method to adjust the appropriate Lead Angle is to maximize the motor efficiency (output power/input power) at the operating speed (or range) of interest when the motor is loaded, and minimize the motor noise and vibration.

For motors with Hall sensors, adjusting the physical position of the Hall sensor (corresponding to the electronic angle) can also achieve a similar effect. Generally speaking, the Hall sensor is located on the PCB board inside the motor and is not easy to adjust. Conversely, by adjusting the Lead Angle/conduction angle, as long as the relationship between the physical angle and the electronic angle (depending on the number of pole pairs of the motor rotor) is well matched, it can also play a similar role in correcting the physical position of the Hall sensor inside the motor. Thus, the inconvenient and complicated adjustment of the physical position of the sensor inside the motor can be achieved through a simple and easy software method (adjustment of the lead angle/conduction angle).