118 - 23/May/97
At the output of the PI current controllers for phases U and V, the reference values for the
phase voltages U and V is obtained. The reference voltage for the phase W is obtained by
Uw= -Uu+ Uv. The three voltage references obtained in the prescribed manner are brought
to the PWM modulation block.
The comparators in Fig. 2 (labeled "sgn") are supplied by the triangular PWM carrier .
Comparison of the modulation signals (that is, the voltage references) with the PWM carrier
gives gating signals. These signals consist of pulses with the width determined by respective
modulation signal. The nature of this signals is digital (either 0 or 1). Their state determines
the three phase inverter switching. Finally, the average phase voltage, brought to the motor
winding, corresponds to the voltage references shown in Fig. 2.
Since the implementation of the scheme in Fig. 2 is analog, proportional and integral gains of
the current controller are determined by the value of resistors and capacitors used. For the
purpose of allowing the matching of DBM with a non standard Vickers motor, these
components are mounted on a removable plug-in modules, namely, the PWM90 hybrids.
These hybrids are fitted into the drive during the factory test. At this time, the current gain is
adjusted according to the motor that is going to be used in conjunction with the drive. Current
gains are optimized in the sense of maximizing the current loop bandwidth, while still keeping
the noise, current ripple, and overshoot at an acceptable level.
The bandwidth of the current loop lies in the range of 900-1200 Hz. In terms of the torque
response time, the motor fed from a DBM will respond to the torque command with a rise
time 200
µ
s.
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