Figure 1 for three-phase brushless dc motor drive part of the schematic
mainly include hall information collection, and according to the hole, do the corresponding modulation signal of three-phase inverter, three-phase inverter PWM switching sequence has the PWM duty cycle is the main content of the modulation, different modulation mode has a great influence on the BLDC the running performance of motors, in recent years, along with the motor control system is more and more sophisticated, the original common square wave pulse width modulation (PWM) based on 120 degrees, the sine pulse width modulation (变频调速)And space vector pulse width modulation (SVPWM), reduce motor pulse, current waveform distortion is reduced, but the latter two algorithms is more complex, brushless motor, this article will be to introduce three kinds of modulation method one by one to its features, principle and calculated in detail. Various semiconductor LC08000M chip integrated the three kinds of modulation mode, suitable for application in BLDC drive motors.
1. 120 degrees square wave pulse width modulation
use value (hallEach electric cycle 6 times change), change the UVW phase current flow, but the same value within current flowing into the same hall, at any time only one phase of the bridge under the bridge and another phase conduction, this control method is simple, but has a maximum torque Angle of 60 degrees, the efficiency is lower, at the same time with the rotation noise.
brushless dc motor PWM input frequency problems
figure 2: state in contrast to the PWM, three Hall electromotive force, the corresponding relation of three phase current
in the bridge bridge PWM switch control in a different order, we can make the following five kinds of mode of supply.
LC08000M in order to reduce the commutation torque ripple was studied and adopted PWM value transition, this process can effectively reduce the rotation noise.
brushless dc motor PWM input frequency problems
figure 3: LC08000M square-wave 120 & deg; Pulse width modulation PWM hall in relation to the corresponding figure
2. Sine pulse width modulation (变频调速)
superposition in MOS tube into an equivalent dc voltage can be controlled by PWM switch to sine voltage, due to the neutral point is zero, so also for sinusoidal phase voltage of the motor, so as to make the motor into the phase line current is sinusoidal variation rule, eliminates the torque ripple. According to the area of the equivalence principle, sine wave can be equivalent into PWM wave. As shown in figure 5, by the way, we constantly adjust the PWM duty ratios sine wave voltage effect.
brushless dc motor PWM input frequency problems
figure 4: the equivalent figure of sine wave and the PWM wave
sine pulse width modulation) need to know & omega; T values in detail, and we only can be read from $hall element to 60 & deg; 120°180°240°360°The six general location information, so we need a few times past hall values change interval calculated within 60 degrees of Angle. In static starting case, we can't calculate the Angle of information, so start the case, we will use 120 degrees square wave pulse width modulation mode to start, but get a stable rotating electric motor, we can calculate the Angle, you can switch into a sinusoidal pulse width modulation method.
calculate the Angle method, as shown in figure 6 -1 first calculate each 60 & deg; Need time, divided by the PWM cycle time can be calculated & 60 deg; The number of PWM, resulting in 60 & deg; Within every increase a PWM when the value of the Angle increase, plus value corresponding to the large Angle by hall to get the current Angle; With 120 UVW three-phase differences & deg; Phase.
