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The primary distinction between brushless DC (BLDC) motors and brushless AC (BLAC) motors is their baông xã EMF waveform. BLDC motors typically have sầu a trapezoidal bachồng EMF, while BLAC motors (also referred lớn as permanent magnet synchronous motors, or PMSM) have sầu a sinusoidal back EMF. The shape of a motor’s back EMF is important, because it determines what type of drive sầu current and commutation method should be used.
BLDC motor constructionIn a BLDC motor, the rotor consists of permanent magnets, & the stator has steel laminations with windings through axial slots. The windings are wound in a trapezoidal fashion and produce a trapezoidal baông xã EMF.
For the best performance, the drive current should match the baông xã EMF wavekhung, so BLDC motors should be driven with trapezoidal waveforms (direct current). Trapezoidal drives are sometimes referred khổng lồ as square-wave sầu drives, although true square waveforms are rarely used due to lớn their sharp transition between positive và negative values. Instead, modified square, or quasi-square current is used.
Motor terminology can be tricky. A comtháng way lớn categorize motors is by their input đầu vào voltage: AC or DC. But many engineers argue that brushless DC (BLDC) motors are actually AC motors, because they are driven by DC voltage that switches from one stator coil lớn the next, generating an alternating (hence, AC) trapezoidal or quasi-square wavekhung.
Another way to classify motors is by their back EMF profile. BLDC motors are referred to as trapezoidal motors, since they produce trapezoidal baông xã EMF, và BLAC motors (aka PMSM) are referred to as sinusoidal motors since they produce sinusoidal bachồng EMF. But, as discussed below, the back EMF of a BLDC motor is not truly trapezoidal in shape. In reality, its shape is more sinusoidal. This is why BLDC motors can use trapezoidal or sinusoidal commutation.
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Trapezoidal (aka six-step) commutation is common in high-speed applications or when higher starting torque is required. Trapezoidal commutation is also less costly than other methods, due lớn its simple control algorithms. In most applications, the rotor position is determined by three Hall-effect sensors that are mounted on the stator, 120 degrees apart. When the rotor passes over the sensors, they produce either a high or a low signal to indicate which rotor pole (N or S) is passing over. The change from high khổng lồ low (or low lớn high) of the three Hall sensors gives rotor position information every 60 degrees, meaning that six steps are needed in order khổng lồ complete one electrical cycle—thus, the term “six-step commutation.” The correct commutation sequence is determined from the combination of the Hall sensor signals.
Trapezoidal commutation can also be performed based on the motor’s baông chồng EMF, which allows the elimination of Hall sensors. In a typical three-phase BLDC motor with trapezoidal current, one winding is positive, one winding is negative, and one is open. The open winding can be used to lớn detect the zero-crossing point of the back EMF, which corresponds lớn what would be a signal change in a Hall sensor. However, the bachồng EMF is proportional khổng lồ motor tốc độ. This means that at very slow speeds (& especially at startup), the back EMF will be very low, so the motor must be started in open-loop mode until sufficient speed & baông xã EMF are generated. At that point, the controller can be switched khổng lồ back EMF sensing for commutation.
The downside of trapezoidal (six-step) commutation is that it results in torque ripple at each step of the commutation (every 60 degrees). And although in theory, the BLDC motor’s bachồng EMF is trapezoidal, in reality, the bachồng EMF wavekhung is not purely trapezoidal. Inductance in the motor smooths the baông chồng EMF into a more sinusoidal shape. And since the baông chồng EMF is more sinusoidal than trapezoidal, it is possible khổng lồ use sinusoidal commutation with BLDC motors. This requires the addition of an encoder or resolver lớn accurately track the rotor position, which adds cost to the system, but it results in significantly reduced torque ripple.
Another option is to use field oriented control (FOC). This is a method in which the current vector (magnitude and direction) is determined relative sầu to lớn the rotor, rather than on the basis of sine waves, as is done in sinusoidal commutation. While sinusoidal commutation has low efficiency at high speeds, FOC provides high efficiency across a wide speed range, as well as very smooth motion.
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While the nomenclature can be confusing, referring khổng lồ BLDC motors as trapezoidal motors serves to lớn distinguish them from BLAC motors. The ability to lớn use trapezoidal or sinusoidal commutation, or field oriented control, makes BLDC motors applicable lớn a wide range of applications with options lớn balance performance requirements with control complexity and cost.
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