PROGRAMMING GUIDE
SINUS PENTA
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34.1.10. FIELD WEAKENING (VTC AND FOC ONLY)
When operating in field weakening mode, the asynchronous motor runs at a higher speed than its rated speed.
In field weaking mode, it is required to limit mechanical power not to exceed the motor ratings. In FOC and VTC mode,
the maximum motor torque is limited as detailed in the LIMITS MENU based on the external torque limit (defined in the
CONTROL METHOD MENU) and on the torque limit defined by parameters C047/C048.
Also, the field weakening mode requires limiting the maximum voltage required to the motor, mainly due to the back
electromotive force, depending on the motor flux and the electric frequency. When in field weakening mode, the motor
flux is to be properly reduced in order to limit output voltage.
Output voltage M027 must be lower than the rated motor voltage (C019) not to exceed the motor ratings—at DC bus
Vdc (M029) actually available—and not to introduce voltage harmonic distortion and current distortion. The phase-to-
phase RMS voltage actually available is Vdc/√2.
Two field weakening modes are available for the FOC and VTC control modes, that may be activated also
simultaneously:
- “static” field weakening: configured via parameter C030 (field weakening speed).
If C030 = “0: Disable”, the static field weakening is disabled.
If C030 > 0, the magnetization current is decreased in respect to the rated value set in C021 with inversely proportional
pattern to speed (1/n) at a speed higher than the rated speed (C016) scaled by C030. For example, a motor featuring
rated speed C016=1480rpm and C030=100%, with rated current C018 =100A and C021 = 25%, up to 1480rpm, the
magnetization current is C018 * C021 = 100A * 25% = 25A; at 3000rpm the magnetization current is reduced by C018 *
C021 / 3000 * (C106 * C030) = 12.3°
- “automatic” field weakening: configured via parameter C030a (field weakening time constant).
If C030a = “0: Disable”, the automatic field weakening is disabled.
If C030a > 0, the magnetization curent and flux M017 are automatically decreased so that output voltage M027 is lower
than the rated motor voltage (C019) both at DC-link voltage divided by √2 and scaled by C042. In case the motor
voltage exceeds the limits above, the automatic field weakening reduces the magnetization current until the output
voltage equals the voltage limit.
The static field weakening is promptly activated based on a fixed magnetization current/speed relation, whilst the
automatic field weakening dynamically adjusts the magnetization current with longer response time. When the operating
conditions of the motor, e.g. the load torque, vary, the motor parameters depending on temperature and the DC-link
voltage, the output voltage required to the motor and available to the motor vary accordingly. The automatic field
weakening automatically adjusts the magnetization current to fit the motor operating conditions and enhancing output
voltage.
NOTE
In VTC mode, when the motor is stalled in field weakening mode, set C030
allow for the activation of the automatic field weakening only.
The static field weakening is inactive by default, while the automatic field weakening is enabled by default.
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