The geometry of the windings in a BLDC motor give it a trapezoidal back EMF wavekhung.
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Image credit: Monolithic Power Systems
The primary distinction between brushless DC (BLDC) motors & brushless AC (BLAC) motors is their back EMF waveform. BLDC motors typically have a trapezoidal baông chồng EMF, while BLAC motors (also referred to as permanent magnet synchronous motors, or PMSM) have sầu a sinusoidal back EMF. The shape of a motor’s baông chồng EMF is important, because it determines what type of drive sầu current and commutation method should be used.
BLDC motor construction
In a BLDC motor, the rotor consists of permanent magnets, và 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 sầu current should match the back EMF wavekhung, so BLDC motors should be driven with trapezoidal waveforms (direct current). Trapezoidal drives are sometimes referred to as square-wave sầu drives, although true square waveforms are rarely used due to lớn their sharp transition between positive sầu & negative values. Instead, modified square, or quasi-square current is used.
Notice that a modified square wave spends a short amount of time at the zero crossing point, rather than abruptly changing from positive khổng lồ negative sầu.
Motor terminology can be tricky. A comtháng way lớn categorize motors is by their 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 to the next, generating an alternating (hence, AC) trapezoidal or quasi-square wavekhung.
Another way khổng lồ classify motors is by their baông chồng EMF protệp tin. BLDC motors are referred lớn as trapezoidal motors, since they produce trapezoidal baông xã EMF, & BLAC motors (aka PMSM) are referred khổng lồ as sinusoidal motors since they produce sinusoidal baông chồng EMF. But, as discussed below, the baông chồng 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 commutation in BLDC motors
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 to lớn its simple control algorithms. In most applications, the rotor position is determined by three Hall-effect sensors that are mounted on the stator, 1đôi mươi degrees apart. When the rotor passes over the sensors, they produce either a high or a low signal lớn indicate which rotor pole (N or S) is passing over. The change from high to 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 xã 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, & one is open. The open winding can be used lớn detect the zero-crossing point of the back EMF, which corresponds khổng lồ what would be a signal change in a Hall sensor. However, the back EMF is proportional khổng lồ motor tốc độ. This means that at very slow speeds (and especially at startup), the baông xã EMF will be very low, so the motor must be started in open-loop mode until sufficient tốc độ và bachồng EMF are generated. At that point, the controller can be switched to lớn bachồng EMF sensing for commutation.
Hall sensor signals, baông chồng EMF (ideally trapezoidal), torque ripple (every 60 degrees), & phase current for a BLDC motor. Image credit: Microchip Technology Inc. Other commutation methods for BLDC motors
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 baông xã EMF is trapezoidal, in reality, the baông xã EMF wavesize is not purely trapezoidal. Inductance in the motor smooths the bachồng EMF into lớn a more sinusoidal shape. And since the back EMF is more sinusoidal than trapezoidal, it is possible to use sinusoidal commutation with BLDC motors. This requires the addition of an encoder or resolver lớn accurately traông chồng the rotor position, which adds cost to lớn 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 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 khổng lồ distinguish them from BLAC motors. The ability to lớn use trapezoidal or sinusoidal commutation, or field oriented control, makes BLDC motors applicable khổng lồ a wide range of applications with options to balance performance requirements with control complexity và cost.
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