Electricity: single generator systems – Automatic control of generator or driving means – Plural conditions
Reexamination Certificate
2001-12-10
2003-12-09
Lam, Thanh (Department: 2834)
Electricity: single generator systems
Automatic control of generator or driving means
Plural conditions
C322S010000, C322S044000, C322S029000, C322S094000
Reexamination Certificate
active
06661206
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to switched reluctance machines, and more particularly to a switched reluctance machine utilizing soft chopping to regulate the current, especially when the switched reluctance generator operates at low speed.
The continued advances in high-power switching semiconductors and control electronics have enabled the use of switched reluctance generators, which have been used extensively in motor applications in the past, to be increasingly exploited for the generation of electrical energy. The use of switched reluctance generators in such applications is highly desirable as the generators are simple and rugged due in part to the winding-free, magnet-free brushless construction of the salient pole rotor. This construction permits the use of the switched reluctance machine at high speeds and under harsh environmental conditions. Also, since the rotor lacks windings and magnets, it generally costs less than a wound or permanent magnet rotor.
A diagram of a switched reluctance machine, together with one phase winding and the associated power converter components, is shown in FIG.
1
. Each phase winding of the switched reluctance machine
10
comprises two serially-connected coils (for example, coils
12
and
14
) wound around diametrically opposed stator poles (for example poles
16
and
18
). Torque is produced in the switched reluctance machine
10
by the tendency of the nearest rotor pole pair to move to a minimum reluctance position with respect to the excited stator pole pair. The magnitude and direction of the produced torque is determined by the magnitude of the exciting phase current pulses and the placement of these pulses with respect to rotor position. Ideally, the torque generated by an unsaturated switched reluctance machine is
T
e
(
I
,&thgr;)=½
I
2
(
dL
(&thgr;)/
d
&thgr;),
where I is the phase current, L is the phase inductance and &thgr; is the rotor angle. Note that the torque direction is independent of the sign of the current so the phase current can be unidirectional. Also, the sign of the torque is determined by the placement of the phase current pulse relative to the change of phase inductance, dL(&thgr;)/d&thgr;.
FIG. 2
illustrates an idealized example of the placement of the current pulses for torque and electrical power generation in the switched reluctance machine
10
. Specifically,
FIG. 2
illustrates the idealized phase inductance variation as a function of rotor angle (&thgr;), the motoring current and the generating current, both also as a function of the rotor angle &thgr;.
Returning to
FIG. 1
, each of the rotor poles is identified by a reference character
20
. The six stator pole pairs are identified by reference characters
16
,
18
and
22
. When the stator pole
18
is not aligned with any of the rotor poles
20
, the inductance there between is at its minimum value, as shown by the horizontal segment of the
FIG. 2A
inductance curve. As the rotor angle changes, the stator pole
18
begins to overlap the rotor pole
20
, and the inductance rises and reaches a maximum value when the stator pole
18
is aligned with the rotor pole
20
. Maximum inductance is illustrated in
FIG. 2A
by the vertical line bearing reference character
30
. For motoring operation, the current is supplied to the diametrically opposed stator poles
16
and
18
via the windings
12
and
14
, respectively, during the period when the inductance is increasing and the rotor poles
20
are approaching the stator poles
16
and
18
. The motoring current is shown in FIG.
2
B. Since the inductance is increasing in this region, the torque produced acts in the direction of rotor rotation, thus producing positive torque.
To generate electrical power, current must be supplied during the period when the inductance is decreasing as the rotor poles
20
pull away from the stator poles
16
and
18
. See FIG.
2
C. Since the phase inductance is decreasing in this region, the torque opposes rotor motion. The work done by the system to pull the stator and rotor poles apart is returned as energy to the DC bus, which also supplies the motoring and the generating current. Ideally, in the generating mode, the phase current should be provided in the region where the phase inductance is decreasing, as shown in FIG.
2
C. However, given the back-EMF experienced by the switched reluctance machine
10
, the phase current should be provided several degrees before the maximum phase inductance position is reached. This assures that sufficient current is available in the phase windings
12
and
14
, for example, when the rotor poles
20
enter the region where the phase inductance begins to decrease.
Thus, the switched reluctance machine operates both as a motor and as a generator. The inductance of each phase winding (for example, the coils
12
and
14
of
FIG. 1
comprise one phase winding) varies according to the degree of overlap between the stator poles
16
and
18
and the rotor poles
20
as the latter rotate. If current is supplied while the winding inductance is increasing (i.e., the degree of overlap is increasing) then the magnetic force on the rotor poles
20
tends to increase the degree of overlap by creating a positive torque. This physical phenomena is the basis for the motoring operation of the switched reluctance machine
10
.
If current is applied to the coils
16
and
18
while the winding inductance is decreasing (i.e., the degree of overlap between the stator poles
16
and
18
and the rotor poles
20
is decreasing) then the resulting magnetic force opposes further separation of the rotor poles
20
and the stator poles
16
and
18
. This separation acting against the magnetic force demands an input of mechanical energy to the rotor, which is in turn converted by the switched reluctance machine
10
into electrical energy in the form of an increasing winding current. This current reaches its maximum value when the inductance is high and as a result the opposing magnetic force (and therefore the generated current) is large during separation between the stator poles
16
and
18
and the rotor poles
20
.
The switched reluctance machine
10
illustrated in
FIG. 1
includes the stator poles
16
and
18
plus four additional stator poles
22
. The
FIG. 1
embodiment also includes four rotor poles
20
, and is thus referred to as having a 6/4 topology (six stator poles and four rotor poles). As is recognized by one skilled in the art, a different topology can be utilized with corresponding changes in the controlling mechanism associated with the present invention (to be described herein below) without departing from the scope of the invention.
To allow rotation of the rotor poles
20
, a small air gap
24
exists between the outer periphery of the rotor poles
20
and the inner periphery of the stator poles
16
,
18
and
22
. In one embodiment, this air gap is approximately 0.25 mm, but may vary due to machining and manufacturing tolerances or by design depending on the desired characteristics of the switched reluctance machine
10
. Since a switched reluctance machine operates in accordance with the changing inductance between the rotor and stator poles, a slight change in the air gap has a significant impact on performance characteristics.
A simplified schematic of the control components associated with the phase windings
12
and
14
for providing commutation to the switched reluctance machine
10
is also illustrated in
FIG. 1. A
series connection of switch
32
and a diode
34
is connected across the DC bus
33
, with the anode terminal of the diode
34
connected to the negative voltage of the DC bus
33
. A series connection of a diode
36
and a switch
38
is also connected across the DC bus
33
, with the cathode terminal of the diode
36
connected to the positive voltage. Note that the windings
12
and
14
are serially connected between the junction of the switch
32
and the cathode terminal of the diode
34
and the junction of the switch
38
and the anode terminal of the dio
Delphi Technologies Inc.
Dobrowitsky Margaret A.
Lam Thanh
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