Electricity: motive power systems – Synchronous motor systems – Hysteresis or reluctance motor systems
Reexamination Certificate
2001-11-19
2003-10-28
Masih, Karen (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Hysteresis or reluctance motor systems
C318S808000, C318S807000, C318S139000, C318S254100, C318S599000
Reexamination Certificate
active
06639378
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The subject matter of this application is related to the subject matter of British Patent Application No. GB 0028733.4, priority to which is claimed under 35 U.S.C. §119 and which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to switched reluctance drive systems. In particular, it relates to such systems where the controller is able to minimize the ripple current in the DC link capacitor.
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 polyphase switched reluctance drive in schematic form, where the switched reluctance motor
12
drives a load
19
. The input DC power supply
11
can be, for example, 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. A rotor position detector
15
is typically employed to supply signals corresponding to the angular position of the rotor. 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. One of the most common configurations is shown for a single phase of a polyphase system in
FIG. 2
, 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”) that cannot be drawn from or returned to the supply. In practical terms, 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. The cost and/or size of this capacitor can be important in installations which are sensitive to drive cost and/or the space occupied by the drive, for example in aerospace or automotive applications.
The switched reluctance drive is essentially a variable-speed system and is characterized by voltages and currents in the phase windings of the machine that are quite different from those found in traditional, sinusoidally fed, types of machines. As is well known, there are two basic modes of operation of switched reluctance systems: chopping mode and single-pulse mode, both of which are described in the Stephenson paper cited above. FIGS.
3
(
a
)-
3
(
c
) illustrate single-pulse control, which is normally used for medium and high speeds in the speed range of a typical drive. FIG.
3
(
a
) shows the voltage waveform typically applied by the controller to the phase winding. At a predetermined rotor angle, the voltage is applied by switching on the switches in the power converter
13
and applying constant voltage for a given angle &thgr;
c
, the conduction angle. The current rises from zero, typically reaches a peak and falls slightly as shown in FIG.
3
(
b
). When the angle &thgr;
c
has been traversed, the switches are opened and the action of energy return diodes places a negative voltage across the winding, causing the flux in the machine, and hence the current, to decay to zero. There is then typically a period of zero current until the cycle is repeated. It will be clear that the phase is drawing energy from the supply during &thgr;
c
and returning a smaller amount to the supply thereafter. FIG.
3
(
c
) shows the current that has to be supplied to the phase winding by the power converter and the current that flows back to the converter during the period of energy return. Instead of opening both switches simultaneously, it is well known that there are advantages in opening one switch in advance of the other, allowing the current to circulate around the loop formed by the closed switch, the phase winding and one of the diodes. This is known as “freewheeling”. It is used for various reasons, including peak current limitation and acoustic noise reduction.
At zero and low speeds, however, the single-pulse mode is not suitable, due to the high peak currents that would be experienced, and the chopping mode is used to actively control the phase winding current. There are two principal variants of the chopping mode. When the current reaches a predetermined level, the simplest method of chopping is to simultaneously open the two switches associated with a phase winding, e.g. switches
21
and
22
in FIG.
2
. This causes energy to be returned from the machine to the DC link. This is sometimes known as “hard chopping”. The alternative method is to open only one of the switches and allow freewheeling to occur. This is known as “freewheel chopping” or “soft chopping”. In this mode of control, no energy is returned to the DC link from the phase winding, except at the end of the conduction period, when both switches are opened to finally extinguish the current.
With any chopping scheme, there is a choice of strategy for determining the current levels to be used. Many such strategies are known in the art. One commonly used scheme includes a hysteresis controller that enables chopping between upper and lower currents. A typical scheme is shown in FIG.
4
(
a
) for hard chopping. At a chosen switch-on angle &thgr;
on
(which is often the position at which the phase has minimum inductance, but may be some other position), the voltage is applied to the phase winding and the phase current is allowed to rise until it reaches the upper hysteresis current I
u
. At this point both switches are opened and the current falls until it reaches the lower current I
l
and the switches are closed again, repeating the chopping cycle. FIG.
5
(
a
) shows the corresponding phase current waveform for a hysteresis controller using freewheeling. The reduction in chopping frequency is immediately evident.
It should be noted that if the machine is generating rather than motoring, the phase current may rise during freewheeling. Soft chopping can still be used by alternating the power circuit states between freewheeling (with one switch open) and energy return (with both switches open). The techniques described hereafter apply equally to motoring and generating modes of operation.
The supply currents flowing in the DC link due to the phase currents in FIGS.
4
(
a
) and
5
(
a
) are shown in FIGS.
4
(
b
) and
5
(
b
) respectively. In each case, the DC link capacitor supplies a proportion of the ac component of these waveforms. (Note that these figures are idealized, since the capacitor must have zero mean current and, in practice, the behavior of the currents in the presence of supply resistance, capacitor impedance and inductance is considerably more complex.) The capacitor current in the hard chopping case has both a higher frequency and a higher root mean square (rms) value. In the freewheeling case the current is reduced in both frequency and rms magnitude. The benefits of this with respect to capacitor rating are discussed in, for example, U.S. Pat. No. 4,933,621 (MacMinn), which is incorporated herein by reference.
While the hard and soft chopping modes have been described in the context of current control, it should be noted that they can also be used in conjunction with a voltage control system, where t
Elliott Charles Richard
Turner Michael James
Dicke Billig & Czaja, PLLC
Masih Karen
Switched Reluctance Drives Ltd.
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