117 - 23/May/97
5.3 Current Control Loops
Control of the magnetomotive force of the stator is implemented through the PI control of the
stator phase currents. That is, the amplitude and the spatial orientation of the vector Is is
performed by controlling its components. Equation (3) gives the references of the motor
phase currents; that is, they are bringing out waveforms that should be the stator phase
currents. It can be seen that the amplitude Is plays the role of the torque demand, or the
torque reference. Apart of the torque reference, the derivation of the current references calls
for the information about the rotor position
θ
r, which is obtained from a Resolver to Digital
converter, connected to the resolver mounted on the motor shaft.
Σ
+
-
Reference
Feedback
Kp + Ki/s
PWM carrier
Uref
sgn
Gating signals
Σ
+
-
Reference
Feedback
Kp + Ki/s
PWM carrier
Uref
sgn
Gating signals
PWM carrier
Uref
sgn
Gating signals
Σ
-
-
Phase U
Phase W
Phase V
Fig. 2
The phase currents U and V are measured by the LEM current sensors. These sensors
behave as current transformers capable of sensing both AC and DC components of the
current. The error discriminators (see Fig. 2) are determining the error, that is, the deviation
of the measured phase currents with respect to the references. The current references U
and V are obtained from the DSP, through the D/A channel, from the torque reference and
using the coordinate transformation from the rotor (d,q) to the stator (a,b,c) reference frame.
The current errors obtained are processed through the two PI current regulators.
The presence of 2, and not 3 current regulators for the 3 motor phase currents is due to the
fact that the phase currents are tied by the relation iA+iB+iC=0. Hence, only two of them are
mutually independent variables. Therefore, current control scheme possesses 2 PI
controllers; the introduction of the third (redundant) one will deteriorate the performances.
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