Electricity: motive power systems – Synchronous motor systems – Hysteresis or reluctance motor systems
Reexamination Certificate
2002-06-05
2004-07-06
Lockett, Kimberly (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Hysteresis or reluctance motor systems
C318S700000, C318S690000, C318S696000
Reexamination Certificate
active
06759826
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of reducing transient voltages in switched reluctance drive systems.
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. 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. Its output may also be used to generate a speed feedback signal. Current feedback is provided by a current transducer
17
for the or each phase winding. As discussed in the Stephenson paper cited above, reluctance machines can be operated in either a motoring or a generating mode. The input demand
18
can be torque or speed demand for motoring or a current or voltage demand for generating.
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”) which 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 is 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 which are quite different from those found in traditional types of machines fed with an alternating current. As is well known, there are two basic modes of operation of switched reluctance systems: single-pulse mode and chopping mode, both of which are described in the Stephenson paper cited above. These are briefly described here as follows.
At a predetermined rotor angle, voltage is applied to the phase winding by switching on the switches in the power converter
13
and applying constant voltage for a given angle &thgr;
c
, the conduction angle. When &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. This is shown in FIG.
3
(
a
). This mode is generally known as the single-pulse mode because there is only one pulse of voltage applied to the phase in a phase period. Single-pulse mode is normally used for the medium and high speeds in the speed range of a typical drive. Instead of opening both switches simultaneously, there are circumstances in which it is advantageous to open the second switch an angle &thgr;
f
later than the first, allowing the current to circulate around the loop formed by the closed switch, the phase winding and a diode. This technique is known as “freewheeling” and is used for various reasons, including peak current limitation and acoustic noise reduction. The inclusion of freewheeling is shown in FIG.
3
(
b
). Freewheeling can be used over a broad speed range. The timing of the initiation of freewheeling and its period are speed dependent.
At zero and low speeds, however, the single-pulse mode is not suitable, due to the high peak currents which would be experienced, and the chopping mode is used, in which the peak current is limited to some predetermined value during the overall period of conduction. As for single-pulse control, there are two principal variants of the chopping mode. The simplest method is to open simultaneously 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 and is sometimes known as “hard chopping”. 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 scheme uses a hysteresis controller which enables chopping between upper and lower current levels. 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
1
and the switches are closed again, repeating the chopping cycle. An alternative method is to open only one of the switches and allow freewheeling to occur and is known as “freewheel chopping” or “soft chopping”. FIG.
4
(
b
) shows the phase current waveform for a hysteresis controller using freewheeling or soft chopping.
U.S. Pat. No. 4,933,621 (MacMinn), incorporated herein by reference, proposes the use of freewheeling chopping to reduce the switching device losses by reducing the switching frequency, and to reduce the ripple current rating of the capacitor. U.S. Pat. No. 5,942,865 (Kim), incorporated herein by reference, describes a system for freewheeling at the end of a series of PWM pulses, the delay time being selected to reduce the radial forces on the stator and hence reduce the emitted acoustic noise. A similar approach is described in EP 700945 (Wu), incorporated herein by reference, where the freewheeling period is selected in relation to the resonant frequency of the stator, the intention being to actively cancel the vibration of the stator with a counteracting pulse of equal magnitude. Use of much longer periods of freewheeling has been proposed by WO 90/16111 (Hedlund) and U.S. Pat. No. 5,760,565 (Randall), both incorporated herein by reference, in order to reduce the peak flux and hence reduce the associated iron loss at high speeds.
None of the prior art discusses the use of freewheeling to reduce the voltage rating of the DC link capacitor. The cost of the capacitor is influenced by a number of requirements, e.g., operating temperature, life requirement, ripple current rating, internal impedance, etc., but one of the most important is the voltage rating. This rating is determined not by the nominal value of the DC link, but by the transient voltages appearing on that link and the requirement to have a safety margin above the highest expected transien
Dicke Billig & Czaja, PLLC
Lockett Kimberly
Miller Patrick
Switched Reluctance Drives Limited
LandOfFree
Control strategy for switched reluctance drive systems does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Control strategy for switched reluctance drive systems, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Control strategy for switched reluctance drive systems will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3204035