Electricity: single generator systems – Generator control – With armature or primary circuit control
Reexamination Certificate
2001-11-21
2003-09-30
Ponomarenko, Nicholas (Department: 2834)
Electricity: single generator systems
Generator control
With armature or primary circuit control
C322S089000, C318S701000
Reexamination Certificate
active
06628105
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to switched reluctance drive systems. In particular, it relates to such systems operated with a limited duty cycle on a supply system with a limited capacity.
2. Description of Related Art
The characteristics and operation of switched reluctance systems are well known in the art 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.
FIG. 1
shows a typical switched reluctance drive in schematic form, where the switched reluctance motor
12
drives a load
19
. The input DC power supply
11
can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply
11
is switched across the phase windings
16
of the motor
12
by a power converter
13
under the control of the electronic control unit
14
. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. To this end, a rotor position detector
15
is typically employed to supply signals corresponding to the angular position of the rotor. The rotor position detector
15
may take many forms, including that of a software algorithm, and its output may also be used to generate a speed feedback signal.
Many different power converter topologies are known, several of which are discussed in the Stephenson paper cited above.
FIG. 2
shows one of the most common configurations for a single phase of a polyphase system in which the phase winding
16
of the machine is connected in series with two switching devices
21
and
22
across the busbars
26
and
27
. Busbars
26
and
27
are collectively described as the “DC link” of the converter. Energy recovery diodes
23
and
24
are connected to the winding to allow the winding current to flow back to the DC link when the switches
21
and
22
are opened. A capacitor
25
, known as the “DC link capacitor”, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called “ripple current”), which cannot be drawn from or returned to the supply. In practice, the capacitor
25
may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter.
FIG. 3
shows typical waveforms for an operating cycle of the circuit shown in FIG.
2
. FIG.
3
(
a
) shows the voltage being applied for the duration of the conduction angle &thgr;
c
when the switches
21
and
22
are closed. FIG.
3
(
b
) shows the current in the phase winding
16
rising to a peak and then falling slightly. At the end of the conduction period, the switches are opened and the current transfers to the diodes, placing the inverted link voltage across the winding and hence forcing down the flux and the current to zero. At zero current, the diodes cease to conduct and the circuit is inactive until the start of a subsequent conduction period. The current on the DC link reverses when the switches are opened, as shown in FIG.
3
(
c
), and the returned current represents energy being returned to the supply. This ability of a switched reluctance machine to allow energy to be returned to a supply circuit has advantages. For example, U.S. Pat. No. 5,705,918, incorporated herein by reference, discloses a generator that can transfer energy from a high-voltage bus to a low-voltage bus in order to increase generating efficiency.
The shape of the current waveform of a switched reluctance drive varies depending on the operating point of the machine and on the switching strategy adopted. As is well-known and described in, for example, the Stephenson paper cited above, low-speed operation generally involves the use of current chopping to contain the peak currents, and switching off the switches non-simultaneously gives an operating mode generally known as “freewheeling”.
Switched reluctance drives are typically driven from the mains electricity supply. Some drives, however, do not have a fixed connection to the public electricity supply because they are installed on, for example, marine or automotive equipment. In these situations, the system is typically supplied by an alternator that is driven by a fossil-fuel-fired prime mover. A storage battery is usually provided to store sufficient energy to start the prime mover and to supply loads in excess of the generator capacity. It is re-charged by the alternator when there is sufficient generated capacity above that demanded by the system load.
With the alternator/storage battery systems described above, there is inevitably a compromise between capital cost, weight and performance. While the designer wishes to have a system capable of supplying any or all loads without the voltage on the system dropping, this can only be done by increasing the capacity of the battery and/or the alternator. This increases the capital cost of the system and the weight, which in turn leads to increased running cost and/or reduced dynamic performance from the boat or vehicle. A particular problem arises when a large load is intermittently operated, especially when the system is already supplying other loads that are sensitive to voltage fluctuations. For example, vehicle or cabin lighting using incandescent filaments is a load that is well-known to be sensitive to voltage fluctuations and indeed it is common for a slight dimming to occur when another load is switched onto the same supply bus. Where the load has a duty cycle of, say several seconds on followed by some tens of seconds off, this can be irritating to the eye.
There is therefore a need for a method of intermittently operating a drive on a limited capacity bus without causing significant voltage disturbance.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a switched reluctance drive comprising: a rotor, a stator having a winding, and a controller having means for selectively connecting either of a first and/or a second voltage source to supply the winding, and an energy return path between the winding and the second voltage source to allow returned energy to be transferred from the winding to the second voltage source when the first voltage source is used to supply the winding, thereby to charge the second voltage source.
An advantage of this drive is that energy is transferred from the winding to charge up a second voltage source for intermittent use.
The second voltage source may be greater than the first voltage source. Preferably, the second voltage source is charged up to a predetermined value, for example, two or three times that of the first voltage source.
The first and second voltage sources may be connected in series or in parallel. The first and second voltage sources may each include a capacitor connected across it in parallel.
The energy return path may comprise a diode that is connected between one end of the winding and the second voltage source in such a way as to transfer energy from the winding to the second voltage source.
The means for selectively connecting either of the first and/or the second voltage sources to supply the winding may comprise a pair of switches arranged in parallel, the first switch being connected between the winding and the first voltage source and the second switch being connected between the winding and the second voltage source, so that when the first switch is opened and the second switch is closed, the second voltage source can be used to supply the winding. A third switch may be provided for connecting the winding to a common terminal of both of the first and second voltage sources.
The means for selectively connecting either of the first and/or the second voltage sources to supply the winding may comprise a change-over switch that is operable in one position to connect the first voltage source to supply the winding and in another position to connect the second voltage sourc
Dicke Billig & Czaja, PLLC
Ponomarenko Nicholas
Switched Reluctance Drives Ltd.
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