Description of the Software Modules and Parameters
Parameter manual b maXX BM5800
Document No.: 5.16029.03
483
of 814
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3.7.5 Field weakening controller
At an asynchronous machine as well as at a synchronous machine regarding the operat-
ing range it is commonly distinguished between the basic speed range (typically below
the rated speed of the motor) and the field weakening range (typically above the rated
speed).
In the base speed range the flux remains constantly. In the field weakening range the flux
is reduced (ASM) or negative lsd current is applied to make greater speed possible at an
insufficient DC-link voltage
Field weakening factor
The field weakening at the voltage limit (Z142.1– bit 0 = 1) at b maXX 5000 is structured
accordant to the field weakening factor (see diagram in figure 125). The field weakening
factor represents the current level of the field weakening. The field weakening factor (pa
-
rameter Z142.2–) is a factor without units, value range is between 0 … 1.
ZFig. 126– shows that the field weakening is made via two channels. The first channel is
an open-loop controlled and the second channel is a closed-loop controlled field weaken-
ing.
Open-loop controlled field weakening
If the speed exceeds a specified speed threshold the flux is reduced after a characteristic
(see
ZFig. 126–) inversely proportional to the speed. As a speed threshold either the pa-
rameter Speed threshold field weakening (Z142.9–) acts, if bit 1 was set by the field
weakening mode Z142.1– or the rated speed (Z142.7–) if this bit was not set. The output
of the open-loop controlled field weakening is the field weakening factor after the speed
Z142.12–.
Closed-loop controlled field weakening
The second channel contains a voltage controller that evaluates the difference between
the specified limit (
Z142.23–) and the filtered actual value of the required PWM total volt-
age (Z142.7– see also bit 2 of Field weakening mode Z142.1–) and accordingly weak-
ens the flux (ASM) or applies a negative field current, so that the required voltage at the
PWM doesn't increase anymore. The time constant of the filter for the actual value of the
PWM total voltage can be parameterized with
Z142.10–.
The limit is the lower one of two values of the voltage threshold specified via the param-
eter Z142.8– and of the maximum total PWM voltage (Z142.6–), which can be adjusted
with a factor (Z142.15–). The final threshold to activate the field weakening controller is
displayed in the parameter Z142.23–. The maximum total PWM voltage is continually
calculated from the current DC-link voltage provided that there is a 100% PWM control.
The output of the closed-loop controlled field weakening is the field weakening output
Z142.13–.
Total field weakening factor
The present degree of the field weakening (Field weakening factor Z142.2–) is given by
the lowest value of two channels (minimum value between Z142.12– and Z142.13–).
This is additionally limited, so that it never falls below the specified limit of the parameter
Minimum field weak factor (
Z142.5–).
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