AN1904 Datasheet, PDF (59/102 Pages) STMicroelectronics – ST7MC three-phase AC induction motor control software library

AN1904
Library functions per software module
compile time (multiplication by 256 is used here to maintain a sufficient accuracy and to
decrease quantization effects).
Important: All variables used in this function are 16-bit. When modifying set-points, you
must verify the following:
VF_OFFSET ≤ 0xFFFF
((VF_HIGHFREQ_LIMIT × VF_COEFF) – VF_OFFSET) ≤ 0xFFFF
On the contrary, certain variables and constants may have to be declared as u32
instead of u16. (Buffer declared inside ACM_VoltageMaxAllowed, VF_OFFSET, etc.).
● Tuning the V/f characteristic
The following method can be followed to determine the three V/f curve set-points empirically
(a current probe is mandatory). This must be done in open loop, to be able to freely apply
any voltage or frequency on the motor and a speed sensor may be helpful.
● VF_LOWFREQ_LIMIT: this is the first parameter to be set, usually corresponding to the
application’s lowest operating frequency, or a value slightly above.
● V_MIN: to determine this point, slowly increase the voltage on the motor, while
maintaining the frequency to VF_LOWFREQ_LIMIT. This will result in an increasing
current in the stator winding. Stop when the current level in the motor reaches the max
value indicated by the motor manufacturer. Without indication, stop when the current
shape become distorted from a pure sine wave to a triangular waveform, indicating flux
saturation. This is the first set-point (VF_LOWFREQ_LIMIT, V_MIN).
● VF_HIGHFREQ_LIMIT: in no-load conditions, accelerate the motor by sequential
increasing the frequency and the voltage; the voltage must be increased when the
torque is not sufficient: if a speed sensor is available, this is indicated by a rapidly
growing slip. Once a sufficiently high speed has been reached (if not the application’s
highest speed), the voltage must be set to its highest value (V_MAX =255) without
saturation effects. From this point, the stator frequency must be slowly decreased,
keeping in mind that the stator frequency must never be below the rotor frequency
value: this would cause the motor to become a generator, thus injecting reactive energy
in the high voltage DC bus capacitor, causing the voltage to go above capacitor’s
maximum voltage rating. This is easy to ensure with a speed feedback; if not available,
just slowly decrease the frequency while checking the bus voltage value to monitor
regenerative current effects. While decreasing the frequency, the current will increase
in the stator winding. Stop when the current level in the motor reaches the max value
indicated by the motor manufacturer. Without indication, stop when the current shape
becomes distorted from a pure sine wave to a triangular waveform, indicating flux
saturation. This is the second set-point (VF_HIGHFREQ_LIMIT, V_MAX=255).
59/102