AN1905 ST Microelectronics, Inc., AN1905 Datasheet - Page 53

• Manufacturer Part Number: AN1905
• Description: St7mc Three-phase BLDC Motor Control Software Library
• Manufacturer: ST Microelectronics, Inc.

7 PMDC (PMAC) MOTOR CONSIDERATIONS
7.1 PHYSICAL CONSIDERATIONS
7.1.1 Checking the number of pair poles of the motor
In most cases, there is an easy way to check how many pair poles are present with the motor
you are working with. This applies only to motors from which you can observe any mechanical
effect (by looking at the axis for example).
The trapezoidal driving method is based on 6 steps, and each step involves a particular low /
high side driver configuration. 1 electrical cycle is then accomplished within 6 steps. The
number of pair poles gives the link between electrical frequency and mechanical frequency:
Thus, by switching from one step to another at a very low frequency (e.g. 1Hz) and simply by
controlling the mechanical effect by sight, we can determine the number of pole pairs by
counting the number of steps within 1 mechanical cycle. It is given by:
Example: assuming that 24 steps are needed to describe 1 mechanical cycle (e.g. 360 de-
grees on the axis of the motor), then the number of pole pairs is 24/6 = 4.
The software package includes a workspace containing a firmware example in order to drive
the motor step by step at a frequency of 1Hz. You can open it in the ‘Pair poles chk’ folder
and open the dedicated workspace to your toolchain. This folder contains also a s19 file (in
‘S19 file for EEPROMER’) so that you can program a MCU directly and avoid the use of an
emulator. RV1 potentiometer is used to set the PWM duty cycle. The Start/Stop push button
launches/stops the procedure.
Warning:
mechanical frequency (hertz) = electrical frequency (hertz) / number of pair poles
ST7MC THREE-PHASE BLDC MOTOR CONTROL SOFTWARE LIBRARY
number of pole pairs = total number of steps / 6
Operations are made in voltage mode with a PWM frequency of 10Khz
and duty cycle is set via RV1 potentiometer. Make sure that the duty
cycle is not too high as the winding currents can increase very quickly
in this particular mode of operation.
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