# Back Emf Là Gì

The geometry of the windings in a BLDC motor give it a trapezoidal bachồng EMF wavesize.

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Image credit: Monolithic Power Systems

The primary distinction between brushless DC (BLDC) motors và brushless AC (BLAC) motors is their baông xã EMF wavekhung. BLDC motors typically have a trapezoidal baông xã 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 baông chồng EMF is important, because it determines what type of drive current và commutation method should be used.

BLDC motor construction

In 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 chồng EMF.

For the best performance, the drive current should match the back EMF waveform, so BLDC motors should be driven with trapezoidal waveforms (direct current). Trapezoidal drives are sometimes referred khổng lồ as square-wave drives, although true square waveforms are rarely used due khổng lồ their sharp transition between positive và negative sầu values. Instead, modified square, or quasi-square current is used.

Notice that a modified square wave sầu spends a short amount of time at the zero crossing point, rather than abruptly changing from positive sầu lớn negative.

Motor terminology can be tricky. A common 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 to lớn the next, generating an alternating (hence, AC) trapezoidal or quasi-square wavekhung.

Another way to lớn classify motors is by their back EMF profile. BLDC motors are referred to as trapezoidal motors, since they produce trapezoidal back EMF, and BLAC motors (aka PMSM) are referred khổng lồ as sinusoidal motors since they produce sinusoidal baông chồng EMF. But, as discussed below, the bachồng 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 comtháng 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, 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 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 to lớn 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 sầu, and one is open. The open winding can be used lớn detect the zero-crossing point of the back EMF, which corresponds to lớn what would be a signal change in a Hall sensor. However, the back EMF is proportional khổng lồ motor speed. This means that at very slow speeds (và 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 & baông chồng EMF are generated. At that point, the controller can be switched lớn bachồng EMF sensing for commutation.

Hall sensor signals, baông xã EMF (ideally trapezoidal), torque ripple (every 60 degrees), và 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 chồng EMF is trapezoidal, in reality, the bachồng EMF waveform is not purely trapezoidal. Inductance in the motor smooths the back EMF into a more sinusoidal shape. And since the bachồng EMF is more sinusoidal than trapezoidal, it is possible lớn use sinusoidal commutation with BLDC motors. This requires the addition of an encoder or resolver khổng lồ accurately track the rotor position, which adds cost lớn the system, but it results in significantly reduced torque ripple.

Another option is to lớn use field oriented control (FOC). This is a method in which the current vector (magnitude & 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.

While the nomenclature can be confusing, referring khổng lồ BLDC motors as trapezoidal motors serves lớn 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 lớn balance performance requirements with control complexity & cost.

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