Electricity: motive power systems – Synchronous motor systems – Hysteresis or reluctance motor systems
Reexamination Certificate
2000-03-23
2002-12-17
Masih, Karen (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Hysteresis or reluctance motor systems
C318S254100, C318S434000, C318S132000
Reexamination Certificate
active
06495985
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the operation of switched reluctance machines from dual voltages, particularly those performing starting and generating functions for internal combustion engines.
2. Description of Related Art
It is common for vehicles with internal combustion engines to be equipped with separate electrical machines: one for starting the engine and one for generating electrical power to recharge the starting battery and to supply ancillary electrical loads on the vehicle. The starter is typically supplied from a storage battery carried on board the vehicle. A 12 volt battery is commonly used for private cars and small industrial vehicles, whereas a 6 volt system has been used for motorcycles and a 24 volt system is commonly used on larger industrial vehicles. While in principle there is no particular limit to which storage batteries could be made, it has been found economic to limit the choices to these noted above.
If the vehicle is, say, a private car, the electrical loads presented by auxiliary equipment (e.g., windscreen wipers, ventilation fans, seat adjusters, heaters, etc) is relatively small and consequently the generator required to supply these loads and to keep the battery in a state of charge so that the engine can be restarted is also relatively small, typically around 60% of the size of the starter motor. Normally the generator generates onto an electrical bus running around the vehicle to supply the electrical loads and provide charge for the battery.
Although electrical machines in general can operate in both motoring and generating modes, it has not normally been found to be cost effective to combine the starting and the generating duties to allow them to be carried out by one machine. This is because of the speeds and loads over which the two machines typically operate; the starter has to provide peak power at relatively low engine speeds, say up to 600 rev/min, whereas the generator has to operate over a wide speed range, say 700 to 6000 rev/min and be capable of providing full output over most of that range. The result is that the two machines tend to be very different in design.
However, with the trend towards greater electrical loads, especially on larger vehicles, generator sizes are increasing, so the resulting generator weight is an incentive to seek ways of combining the starting and generating functions into a single machine. One type of electrical machine which is favored for this dual role is the switched reluctance machine, since it is economical to produce yet is inherently rugged and can operate over a wide speed range. U.S. Pat. Nos. 5,489,810 and 5,493,195 to Ferreira and Heglund, respectively, both incorporated herein by reference, describe certain aspects of switched reluctance machines used as starter/generators for aircraft engines.
In general, a reluctance machine is an electrical machine in which torque is produced by the tendency of its movable part to move into a position where the reluctance of a magnetic circuit is minimized, i.e. where the inductance of the exciting winding is maximized. In one type of reluctance machine, the energization of the phase windings occurs at a controlled frequency. This type is generally referred to as a synchronous reluctance machine, and it may be operated as a motor or a generator. In a second type of reluctance machine, circuitry is provided for detecting the angular position of the rotor and energizing the phase windings as a function of the rotor position. This second type of reluctance machine is generally known as a switched reluctance machine and it may also be a motor or a generator. The characteristics of such switched reluctance machines are well known and are described in, for example, “The characteristics, design and application of switched reluctance motors and drives” by Stephenson and Blake, PCIM '93, Nürnberg, Jun. 21-24, 1993, incorporated herein by reference. The present invention is generally applicable to reluctance machines, particularly switched reluctance machines operating as both motors and generators.
FIG. 1
shows the principal components of a typical switched reluctance drive system. The input DC power supply
11
can be a battery or rectified and filtered AC mains for example. The DC voltage provided by the power supply
11
is switched across the phase windings
16
of the machine
12
by a power converter
13
under the control of the electronic control unit
14
. Some form of current detection
17
is typically used to provide current feedback from the phase windings to the controller. The switching must be correctly synchronized to the rotation of the rotor for proper operation of the drive. A rotor position detector
15
is typically employed to supply signals corresponding to the angular position of the rotor. The output of the rotor position detector
15
may also be used to generate a speed feedback signal.
The rotor position detector
15
may take many forms, for example it may take the form of hardware, as shown schematically in
FIG. 1
, or of a software algorithm which calculates the position from other monitored parameters of the drive system, as described in EP-A-0573198 (Ray), incorporated herein by reference. In some systems, the rotor position detector
15
can comprise a rotor position transducer that provides output signals that change state each time the rotor rotates to a position where a different switching arrangement of the devices in the power converter
13
is required.
The energization of the phase windings in a switched reluctance machine depends heavily on accurate detection of the angular position of the rotor. The importance of accurate signals from the rotor position detector
15
may be explained by reference to
FIGS. 2 and 3
, which illustrate the switching of a reluctance machine operating as a motor.
FIG. 2
generally shows a rotor pole
20
approaching a stator pole
21
according to arrow
22
. As illustrated in
FIG. 2
, a portion
23
of a complete phase winding
16
is wound around the stator pole
21
. As discussed above, when the portion of the phase winding
16
around stator pole
21
is energized, a force will be exerted on the rotor, tending to pull rotor pole
20
into alignment with stator pole
21
.
FIG. 3
generally shows typical switching circuitry in the power converter
13
that controls the energization of the phase winding
16
, including the portion
23
around stator pole
21
. When switches
31
and
32
are closed, the phase winding is coupled to the source of DC power and is energized. Many other configurations of switching circuitry are known in the art: some of these are discussed in the Stephenson & Blake paper cited above.
In general, the phase winding is energized to effect the rotation of the rotor as follows. At a first angular position of the rotor (called the “turn-on angle”, &thgr;
ON
), the controller
14
provides switching signals to turn on both switching devices
31
and
32
. When the switching devices
31
and
32
are on, the phase winding is coupled to the DC bus, causing an increasing magnetic flux to be established in the machine. The magnetic flux produces a magnetic field in the air gap which acts on the rotor poles to produce the motoring torque. The magnetic flux in the machine is supported by the magneto-motive force (mmf) which is provided by a current flowing from the DC supply through the switches
31
and
32
and the phase winding
23
. In some controllers, current feedback is employed and the magnitude of the phase current is controlled by chopping the current by rapidly switching one or both of switching devices
31
and/or
32
on and off. FIG.
4
(
a
) shows a typical current waveform in the chopping mode of operation, where the current is chopped between two fixed levels. In motoring operation, the turn-on angle &thgr;
ON
is often chosen to be the rotor position where the centerline of an inter-polar space on the rotor is aligned with the centerline of a stator pole, but may be some other angle.
In man
Mayes Peter Richard
Webster Paul Donald
Dicke, Billig & Czaja P.A.
Masih Karen
Switched Reluctance Drives Ltd.
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