Electrical transmission or interconnection systems – Plural load circuit systems – Transformer connections
Reexamination Certificate
2000-07-24
2002-11-19
Fleming, Fritz (Department: 2836)
Electrical transmission or interconnection systems
Plural load circuit systems
Transformer connections
C307S104000
Reexamination Certificate
active
06483202
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to inductive power transfer; more particularly to loosely coupled systems for inductive power transfer, and in particular this invention relates to protection means for limiting the amount of current circulating in a secondary pickup coil of an inductive power transfer system.
BACKGROUND
The general structure of an inductive power transfer installation is that there is a primary conductor (or more) energised with alternating current, and one or more secondary or pickup devices which intercept the changing flux surrounding the primary conductor and convert this flux into electrical energy by means of windings. Often the pickup devices are mobile, and move alongside, or sometimes (if internal energy storage is available) away from the primary conductors.
There appears to be at least two distinct groups of inductive power transfer systems amongst the published literature. One group uses a “spread-out transformer” approach for the primary trackway, in which a series of iron laminations is used along the full length of the trackway to enhance coupling of the flux to an adjacent set of laminations comprising the flux concentrating means used to cause the collected flux to traverse the (sometimes) resonant pickup windings. The energising frequency is relatively low (from mains frequency up to about 5 kHz). Often the primary trackway is buried in a road and faces upwards to link with pickups beneath a road vehicle that face downwards. This approach provides tight coupling, and power is received essentially as if it arrives from a constant-voltage source. Examples of this type of approach are illustrated in a series of patent specifications from Bolger (e.g. U.S. Pat. No. 4007817 or FIG. 1 of U.S. Pat. No. 4800328). Klontz et al ( U.S. Pat. No. 5157319) describes an alternative tight coupling, involving a coaxial winding transformer secondary encircling a primary conductor.
Our group uses as the primary trackway an elongated loop formed from a parallel pair of conductors, without ferri/ferromagnetic material, and flux is coupled through the core (which does include ferri/ferromagnetic material) to the windings of the resonant pickup coil. This coupling is described as loose. Some versions of the track are provided with lumped resonance elements. The delivery of power is controlled by decoupling at the pickup, using a number of disclosed techniques, and because the system uses a resonant circuit as part of the pickup, the current At produces appears to come from a constant-current source. The energising frequency is relatively high (10-30 kHz), and in some examples the primary trackway is mounted upon a conveyer rail, facing sideways to couple with pickups upon self-powered conveyer units, although it is in other instances embedded within a roadway. This type of approach is illustrated in a series of patent specifications from Boys & Green, commencing with WO 92/17929.
For a comparison of these two approaches refer to
FIGS. 14 and 15
, and also a conventional transformer (FIGS.
12
and
13
), in terms of transformer equivalent circuits. (All
FIGS. 12-17
are prior art).
FIG. 12
shows an ordinary, tightly coupled transformer having a primary winding
1201
and a secondary winding
1202
.
FIG. 13
shows the transformer equivalent circuit, where winding
1301
represents the coupled flux M, winding
1302
represents the leakage flux about the primary, and
1303
represents the leakage flux about the secondary. The value of M is obtained from M=k{square root over (L)}
1
L
2
where k is typically 95% or more.
FIG. 14
shows a loosely coupled inductive power transfer pickup, with primary conductors
103
and
104
, a core
300
, and a resonant circuit comprising inductor
1401
and capacitor
1402
. Considering
FIG. 15
, the equivalent inductance
1504
(M) represents the power coupling (shared) component of the flux while
1503
is the leakage flux (such as the flux radiated from the core of the pickup while it is carrying a significant resonating current). For loosely coupled systems having a primary pathway in air, the ratio of inductors 1503:1504 is typically 0.7:0.3 whereas for the iron-cored primary with iron-cored secondary devices of Bolger and others the ratio is typically more like 0.2:0.8. In
FIGS. 15 and 17
, the trackway (constant-current source
1500
with equivalent inductors
1501
,
1502
) supplies a constant current.
FIG. 16
shows a kind of inductive-power transfer device where winding
1601
is a resonant, controllable winding and
1602
with rectifier
1605
supplies useful power—such as a constant current source for battery charging. Considering the transformer equivalent circuit of
FIG. 15
, the value of the short circuit current (if the output was to be shorted) is
I
sc
=
IM
L
1503
+
M
where M is the inductance of
1504
.
We have devised a battery charger which employs loosely coupled inductive power transfer, the subject of patent application PCT/NZ97/00053. Considering a practical circuit from the battery charger in transformer equivalent form, as shown in
FIGS. 16 and 17
; items
1705
and
1706
represent leakage inductance from the actual windings;
1705
for the large number of turns in the resonating/control winding, and
1706
for the power collection winding. The relative proportions of L in
FIG. 17
are:
1504
=30%,
1503
=65%,
1705
=c. 5%., and
1706
=c. 5impedance of the primary section as seen looking back from the inductors
1503
,
1504
(=95% of L) can then be derived by assuming that a short circuit is placed at
1708
(dashed line) and is
Z
=
(
5
%
L
+
X
c
)
95
%
L
5
%
L
+
X
c
res
+
95
%
L
+
5
%
L
=
∞
Since the denominator at resonance is zero then Z is infinite; thereby providing the basis for stating that the source acts as a current source. A Bolger type circuit is equivalent to FIG.
13
. The no-load voltage will be determined by the output impedance Z=L
1303
+L
1302
when driven from a voltage source as is done in Bolger type circuits. The output impedance is Z=L
1303
+L
1301
if the circuit were to be driven from a current source.
While the constant-current characteristic of this type of inductive power transfer system is generally an advantage, it does impose a risk should a pickup coil enter a state in which there is no control over the amount of current collected. A perfect constant-current source will have no voltage limit. An uncontrolled current resonating in a resonant secondary circuit forming part of a loosely coupled inductive power transfer system may build up to reach high levels if the circuit Q is large, whereupon a number of adverse results may occur, such as component failure, for example by overheating or breakdown of semiconductors or of dielectrics within resonating capacitors and apart from loss of function this can lead to the development of fire within the pickup device. Our usual methods for controlling secondary current rely on active control apparatus, actively causing a switching action about the resonant secondary when an over-voltage condition is detected by a voltage comparing circuit. Passive limiting, relying perhaps on the inherent bulk properties of materials should be safer than active control means. Reliance on active control can break down when several factors impinge together on a device so that active control becomes least likely to function when it is most needed. Some systems using loosely coupled (i. e. constant-current) inductive power transfer have been employed in situations where extreme reliability is a desired feature. If such systems rely solely on active control to restrict the circulating current, then in the absence of function by the active control it is likely that a catastrophic breakdown will occur.
Bolger and Ng in U.S. Pat. No. 4,800,328 (Jan. 24 1989) described the application of constant-voltage transformer principles to an inductive power transfer device by providing a saturable pickup core
Auckland Uniservices Limited
Fleming Fritz
Saliwanchik Lloyd & Saliwanchik
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