Environmental Engineering Reference
In-Depth Information
entered into. The second fact to notice is that because of the non-zero flux com-
mand the phase relations of the M/G currents are now displaced from what they
were when the flux command was zero. This phase shift of stator currents relative
to a rotor position is the means of building flux. The final point to note is that at the
ramp edges the M/G currents execute some continuous phase changes needed to
ensure that flux does change.
Yet another dynamic scenario to illustrate is the situation in which the M/G is
given a speed reversal command as it will experience in the power split hybrid
architecture when connected to a planetary gear set sun gear (e.g. the S/A in the
THS system). In this situation the torque command is held steady while the speed is
executing a reversal.
During a speed reversal the synchronous frame commands execute a swap in
phase. The M/G 3-phase currents then execute a sequence change from a-b-c to
a-c-b as shown in Figure 7.11(c). This is consistent with the manner in which a
3-phase ac machine executes a direction change. If any two phases are swapped the
machine rotor spins in the reverse direction, which is exactly what the FOC con-
troller has electronically commanded the M/G to do. Notice that during the phase
sequence change one of the phase currents is completely undisturbed while the
remaining two phases slew very rapidly to their new sequence.
7.3 Sensorless control
The topic of position sensorless control of ac drive systems is particularly relevant
to hybrid propulsion systems. Not only are mechanical position sensors difficult to
integrate into and package within the vehicle driveline, but they are fragile and
susceptible to EMI and signal distortion. It would be a great advantage to minimize
position sensor requirements or to eliminate the need entirely if adequate software
algorithms were available to perform the function of tracking the M/G rotor posi-
tion accurately. Not only is rotor position sensor degradation an issue in main-
taining smooth control of the hybrid M/G system, but corruption of the position
signal introduces disturbances into the voltage and current controllers that have a
tendency to unbalance the machine excitation and cause noise and vibration. An
intermittent sensor is even more insidious because the effect may come and go from
just driving over a pothole or other road disturbance.
Many investigators have tackled the problem of sensor elimination for the
various types of electric machines. Before noting what has been done to eliminate
position sensors, it should be noted that different machines require fundamentally
different types of rotor position sensing. Synchronous machines, such as permanent
magnet types, require a very accurate indication of where the rotor magnet is so that
armature current can be maintained in quadrature to the rotor flux. This requires an
absolute position sensor that resolves shaft position to typically
<
0.2
mechanical
resolution. In higher pole count electric machines the mechanical resolution of
position is even higher. To resolve to an angle of 0.176
mechanical resolution
requires an 11 bit encoder or resolver. Another complication is that not only is an
