CHAPTER 1 CONTROL METHOD
Application Note U17209EJ1V0AN
16
The 120° control method for a BLDC motor without a sensor is described below.
To control a BLDC motor, the rotor position must be known.
To control a BLDC motor without a sensor, the rotor position is estimated using induced voltage.
The induced voltage is in phase with the driving voltage waveform and its waveform is close to a sine wave, as
shown in Figure 1-1. Figure 1-2 illustrates how the polarity of the stator of the motor is switched and how the magnet
rotor revolves.
As shown in Figures 1-1 and 1-2, a three-phase DC motor rotates its rotor by switching the three driving current
patterns on the three coil phases.
During period <1> in Figure 1-1, for example, transistor TrU
1 in Figure 1-3 is turned on by the U-phase driving pin,
and TrV2 is turned on by the V-phase driving pin, causing the current to flow from the U-phase driving pin toward the V-
phase driving pin. At this time, the W-phase coil seems to be disconnected from the driver circuit and induced voltage
is generated.
This induced voltage is used to detect the rotor position.
To control the number of revolutions of the motor, the voltage applied to the motor is controlled to change the
current flowing through the coil. To change the voltage, a waveform that is controlled by PWM is applied to the
transistor.
The voltage is changed by applying a waveform (PWM waveform) in proportion to the voltage to be applied, to the
transistors on the lower arm side (TrU
2, TrV2, and TrW2) while the transistors on the upper arm side (TrU1, TrV1, and
TrW 1) are on.
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