Differences between BLDC and PMSM(English ver)
BLDC stands for brushedless DC and is a DC motor without a brush.
Since it operates through friction to the rectifier side through the brush, the brush may wear out, and in severe cases, a severe spark may occur between the brush and the rectifier side. In addition, severe noise is generated and the efficiency caused by friction loss is also low, so the electric motor made to improve it is BLDC.
The above BLDC motor used an electromagnet on the outside and a permanent magnet on the inside to exclude the brush contact function. The most important part of BLDC is the inverter not shown in the motor configuration diagram. The inverter to be described is a power semiconductor device of the MOSFET and has a switching function.
There are two types of MOSFETs above. There are p-channel MOSFET that disconnects when input is given to the gate and n-channel MOSFET that enables energization when input is given to the gate. The above case is an n-channel that energizes the drain to the sorce when input is given to the gate.
There is a way to obtain rotational force by rotating the permanent magnet inside by switching using six MOSFETs
It is as shown in the picture below.
Characteristics of a BLDC motor
Let's look at the current waveform and the counter electromotive force waveform of the BLDC motor.
The BLDC motor is turned on/off by a switching element such as MOSFET, so if the switching element is turned off like in a float state, current will not flow. Therefore, as shown in Figure 1, the phase current has a quasi-square wave form.
And one of the big features of the BLDC motor is that the reverse electromotive force waveform appears in a trapezoidal form.
Figure 1 shows the phase current and reverse electromotive force waveforms on U, V, and W, respectively.
To illustrate the counter electromotive force in the trapezoidal form, let's compare the BLDC motor with the Permanent Magnet Synchronous Motor (PMSM) for a moment and move on.
The biggest distinguishing feature between the BLDC motor and the PMSM is the comparison of the waveforms of counter electromotive force.
BLDC motors have a trapezoidal counter electromotive force, while PMSM has a sinusoidal counter electromotive force.
BLDC motorPMSMBack-EMFPhase CurrentDriving methodTorque rippleCost
| Trapezoidal | Sinusoidal |
| Quasi-square waveform | Sinusoidal |
| -Two-phase excitation method -Rotor position is detected by hall sensors |
-Torque control in vector control -High resolution position sensor is needed |
| Moderate | Low |
| Low | High |
First, the shape of the reverse electromotive force varies depending on the seating orientation of the permanent magnet.
The BLDC motor is magnetized to produce a uniform magnetic flux in the form of radiation, while the PMSM is magnetized in parallel. This is related to the waveform of the phase current. In the case of BLDC motors, the phase current is a quasi-spherical wave, and in order to match a flat section in the same flat waveform, the reverse electromotive force is magnetized in a trapezoidal shape, and in the case of PMSM, the phase current is a sine wave
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