Designing a MCU-driven permanent magnet BLDC motor controller: Part 1
Phases and Poles
Phases refer to the number of separate sets of windings in the stator.
Within a phase there may be a number of poles. Poles are typically
referred to in pairs of 2, 4, 6, 8, or more (or it can be expressed in
pole pairs such as 1, 2, 3, or 4 pole pairs).
Each phase will have the same number of poles. In a 3 phase 2 pole motor going around the 360° (electrical) corresponds to the actual or mechanical 360° rotation.
As poles are added, the number of electrical degrees is multiplied to create the same mechanical or rotational degrees. For example, a 4 pole machine will take 720° (360°x2) electrical to obtain one full revolution of the rotor.
Other than this subtle difference, the number of poles does not change how the motor is driven. It only changes the actual speed of the motor with a given "electrical speed."
Back Electro-Motive Force
Take a brushless motor of any kind, stick an oscilloscope across any
two leads and spin the rotor. You will see a periodic voltage
oscillating at a frequency proportional to the motor rotational speed
(see "Phases and Poles" above).
This is the basic result of passing a winding through a magnetic field (or B-Field). The changing B-field causes a voltage to appear across the windings. This voltage is called Back Electro-Motive Force, Back EMF, or BEMF for short.
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| Figure 2. Back EMF voltages in a spinning motor (Top: Three Phases; Bottom: Three Phases Overlap) |
A motor can be wound such that the waveform you get when you spin it is either sinusoidal or more trapezoidal. Figure 2 above illustrates a more trapezoidal configuration. Driving a sinusoidal wound motor with a pure three phase sinusoidal signal provides the smooth control needed for applications like electric power steering.
However, the applications addressed here do not require such finesse. These applications just need to spin at a given speed. A little torque ripple due to not being driven optimally is not a big issue. As a result, this discussion is limited to trapezoidal wound / controlled motors.
Driving the Phases
A trapezoidally driven three phase brushless motor traverses 360
degrees electrical through a six step approach as shown in Figure 3 below. Each phase is driven
for 120 degrees electrical then tri-stated for 60 degree electrical.
At any given point, one phase is driven high and another is driven low with the third phase left floating, or tri-stated. That tri-stated portion of the waveform is where the BEMF is transitioning from one polarity to the other. The rotor position information for commutation timing is found during this transition time.
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| Figure 3. Driving a three phase brushless motor |
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