AN1904 Datasheet, PDF (45/102 Pages) STMicroelectronics – ST7MC three-phase AC induction motor control software library

AN1904
Library functions per software module
#define L6386_POLARITY ((u8)0x3F)// Positive logic level for L6386D
drivers
#define NEGATIVE_POLARITY((u8)0x00)// Negative logic level drivers
to be used for the set-up:
#define DRIVERS_POLARITY L6386_POLARITY
Phase shift
The phase shift is set-up to get 120° for three-phase AC motors, knowing that 360° are
represented by a 8-bit variable (256). We thus have Phase shift = 256 x (120/360) = 85.
#define PHASE_SHIFT ((s8)85)// (85/256) * 360 = 120 degrees
Brake
Brake parameters are divided in two categories: some of them are application dependent
and can be modified by the ST7MC control panel in the mainparam.h file: the braking
current and duration (see Section 5.4.2 on page 84), the rest are system critical parameters
directly coded in the mtc.c file.
These last are:
#define STATOR_DEMAG_TIME((u16)300)// Time in ms before applying DC
current braking
#define CURRENT_SETUP_TIME ((u8)5) // Time in ms between two
consecutive values of duty cycle during braking current increase
Figure 20 represents the current in a SELNI motor winding, with library default parameters,
to show a normal current shape during braking. It must be noted that:
● demagnetization time appears to be greater than the programmed value: this is due to
the fact that current is negligible at the beginning of the current settle time (low duty
cycles),
● using a current probe, motor standstill can be monitored by a little current spike on top
of the DC current. Beware when setting up the braking torque that this can trigger the
overcurrent protection if the DC braking current is too close from the current limitation
threshold.
The above two mentioned parameters must be set prior to the others, by empirical tests,
starting from high values (typically 500ms for demagnetization time and 5ms as the interval
between current increase steps). They can then be reduced step by step, while monitoring
the current wave form to avoid situations where the power stage can be damaged (see
Figure 21):
● if the demagnetization time is too short, a regenerative current will appear and the
energy will be transferred to the DC bus capacitor, causing the bus voltage to increase,
● if the current settle is too steep, there will be a current overshoot.
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