Environmental Engineering Reference
In-Depth Information
and machine parameter variations. Switched reluctance machines are in some
respects fundamentally easier to control without mechanical position sensors
because the phases are not mutually coupled so that inactive phases can be used to
monitor inductance changes [13,14].
Still other techniques abound. A method of signal injection and detection through
heterodyning techniques was developed at the University of Wisconsin in the early
1990s for machines with some form of inherent saliency or artificial saliency. The
method of signal injection has been extended and implemented for induction
machines through the introduction of artificial saliency such as modifications to the
rotor slot opening to introduce spatial modulation of the rotor leakage inductance [15].
Figure 7.12 illustrates the technique of signal injection and detection. The accuracy of
such signal injection methods is similar to that of resolver-to-digital (R/D) converters
and mainly independent of the actual degree of rotor saliency introduced.
In Figure 7.13 the process of heterodyning is illustrated through the action of the
inverter plus electric machine with either artificial saliency (induction machine with
modified rotor) or synchronous machine with inherent saliencies (synchronous
reluctance or interior permanent magnet). With the machine rotor speed at w r and the
injected signal w c the process of heterodyning shifts these frequency components to
(2w r w c ), where the carrier frequency is about 400 Hz to as high as 2 kHz in
inverters having a switching frequency of 20 kHz. The baseband signal is the com-
manded frequency of the electric machine under velocity control. Low pass filtering
(LPF) extracts the baseband frequency for feedback control to the current regulator
(synchronous frame) and band pass filtering (BPF) extracts the now position modu-
lated carrier from the total signal for feedback to the position observer.
There has been significant research to show that rotor position information
may be gleaned by way of sensors located far from the machine under test and that
are also used for other measurements. An example of this technique can be found in
work aimed at alternator synchronous rectification control using a current sensor
located at the battery terminal. Mounting current sensors in, or on, an automotive
alternator would be prohibitive both from a durability standpoint and for cost rea-
sons. Alternator current ripple, a consequence of rectifier diode operation, contains
i qdsi
HF carrier
U do
i qds
PI
current
regulator
PWM
inverter
Σ
Σ
IM
Baseband
signal
i qds
LPF
i qds
X
i qesi
i qdsi
BPF
Recovered
HF carrier
Filter banks
Figure 7.13 Signal injection and heterodyning technique of sensorless control
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