Description of the Software Modules and Parameters
Parameter manual b maXX BM3000
Document no.: 5.12001.07
469
of 820
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4.7.2 Current Controller
The "Current Controller" module incorporates the measurement of the phase currents
and DC link voltages, current control including prediction, dead time compensation and
PWM.
4.7.2.1 Current Prediction
The current prediction can be switched off. The voltage equations for the machine form
the starting point for the current prediction.
4.7.2.2 Dead Time Compensation
Since real semiconductor components do not behave ideally, it must be ensured that the
two transistors in a half bridge are never conducting current at the same time, i.e., are
switched on. For this reason, both IGBTs in a half bridge must be switched off for a certain
time, giving rise to dead times which are different for the individual output sections and
furthermore depend on the switching frequency of the power transistors. Due to these
dead times, a part of the theoretical nominal voltage for the PWM is lost. The dead time
effect produces a non-linear distortion in the voltage space vector. The dead time com
-
pensation Z47.50– is intended to compensate for the lost voltage.
The required voltages for the dead time compensation are determined from the sign of
the separate phase currents, the Udc actual value and current depending correction table
Z123.15–.
The table Z123.15– can be determined by autotuning (dead time measurement Z123.1–
bit 1 = 1). The measured values are valid for the parameters which are available at the
moment of the measurement: Udc actual value, PWM frequency and locking time (IGBT
dead time
Z129.9–). If the dead time measurement is not executed, the table Z123.15–
has default values defined for Udc = 540 V, PWM frequency = 8 kHz, and locking time =
4
µs.
The effective voltage of the dead time compensation is corrected according to the Udc
actual value (regardless whether the table includes default values or measured values).
Basically the correction table is only valid for the device which carried out the dead time
measurement. However the values of the table can be transferred from one device to an
-
other one with the parameter set. In all probability, the dead time compensation will oper-
ate satisfactorily if the device is of an identical type. However, this must be checked by
the user. It is recommended to measure the dead time at each device, which requires
dead time compensation.
NOTE!
The dead time measurement must be repeated, if the IGBT dead time (Z129.9–) was
changed.
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