Electricity: motive power systems – Synchronous motor systems – Hysteresis or reluctance motor systems
Reexamination Certificate
2001-06-08
2003-11-11
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Hysteresis or reluctance motor systems
C318S254100, C318S162000
Reexamination Certificate
active
06646407
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to efficiency optimization and noise reduction and torque ripple reduction techniques for electric motors, and, more particularly, to an improved switched reluctance (SR) motor drive.
BACKGROUND OF THE INVENTION
Switched Reluctance (SR) motors are gaining much attention due to their low cost construction and fault tolerant operation. However, two problems that have kept SR motors away from some applications is their noisy operation and torque ripple. In vehicle propulsion applications, torque ripple can result in low-speed “cogging”, which is an undesirable characteristic, sometimes noticeable to drivers.
Techniques are available to reduce the noise and torque ripple of SR motors. Although these techniques are useful in many circumstances, there is room for improvement.
Two known approaches to SR motor design that reduce noise include: i) increasing the stator back iron thickness, and ii) increasing the air gap length. These design approaches to reduce motor noise tend to reduce motor torque density. As a consequence, the resulting SR motor can be bulky, heavy, and costly.
Known control techniques for reducing noise are generally based on modifying the phase de-excitation (turn-off) process during motor operation. The basic idea of these control techniques is to slow down the phase turn-off process by profiling the turn-off current tail to lower the noise. However, reduction of noise by the profiling of the tail current is obtained at the expense of motor efficiency.
One control technique presented by Pollock et al. employs noise cancellation by starting the phase de-excitation with zero voltage and applying the full negative voltage after one-half period of the stator natural resonance frequency. The acceleration of the stator back iron due to the negative voltage tends to cancel the initial acceleration. Thus, noise is reduced. However, this technique is not applicable for all motors, especially the high speed motors.
FIG. 1
illustrates a block diagram of a prior art SR motor control circuit
41
for a switched reluctance (SR) motor
11
. The control circuit
41
includes a current regulator (I-REG)
46
, an inverter
38
, an interpolation scheme
70
, a look-up table
72
, current sensors
74
, a position decoder
76
, and an angular velocity calculator
78
. Motor windings in the SR motor
11
are connected in series with inverter legs
40
,
42
,
45
.
In the motor controller
41
, when the speed of SR motor
11
is high, the parameters used to control SR motor
11
are phase turn-on angle, &thgr;
ON
, and phase turn-off angle, &thgr;
OFF
. At a low speed of operation of SR motor
11
, the control parameters are phase turn-on angle, &thgr;
ON
, phase turn-off angle, &thgr;
OFF
and reference current, I
REF
. Additionally, at low speeds, because the back EMF is lower than the bus voltage, V
DC
, it is necessary, in addition to controlling the phase turn-on and turn-off angles, &thgr;
ON
and &thgr;
OFF
, respectively, to limit the phase current. Current limitation is accomplished by the current regulator (I-REG)
46
regulating the reference current, I
REF
, using known techniques of chopping the current.
The two primary forms of current chopping, “hard chopping” and “soft chopping,” are often implemented in SR motor inverters, including those inverters similar to the prior art three-phase SR motor inverter
38
, as illustrated in detail in FIG.
2
. In hard chopping, both the upper and lower switches supplying a certain phase winding (illustrated in
FIG. 2
as switches
48
,
50
for the first phase winding
51
; switches
52
,
54
for the second phase winding
53
; and switches
56
,
58
for the third phase wind
55
) are turned on and off (i.e., chopped), simultaneously. In soft chopping, one switch (e.g.,
48
,
52
,
56
) is kept on at all times, while the other switch (e.g.,
50
,
54
,
58
) is chopped. As compared with soft chopping, hard chopping provides for a greater level of control of the phase current. However, with the prior art inverter
38
, hard chopping has a lower efficiency, primarily due to additional switching power losses, higher ripple current and lower power factor. Soft chopping, although it provides for higher efficiency, less ripple current, and higher power factor cannot be implemented during regenerative braking.
The reference current, I
REF
, at a lower speed of operation of SR motor
11
, takes the shape of a square wave. The leading and trailing edges of the square wave define the phase turn-on and turn-off angles, &thgr;
ON
and &thgr;
OFF
, respectively, while the amplitude is the current reference, I
REF
. In response to this reference current, I
REF
, a current regulator, I-REG, turns on with full bus voltage, V
DC
, when the leading edge (i.e., the turn-on angle, &thgr;
ON
) of the current reference, I
REF
, is encountered. The current reference, I
REF
, is then maintained with the chopping of the phase current, as described above. When the trailing edge of the reference current, I
REF
, is reached, the phase is turned off with a full negative bus voltage, −V
DC
.
At high speed, the back EMF is higher than the bus voltage, V
DC
. No current regulation chopping is used at high speeds, and the control is referred to as a “single-pulse” mode. The control parameters at high speed are, therefore, only the phase turn-on and turn-off angles, &thgr;
ON
and &thgr;
OFF,
respectively. In order to build current against a high back EMF, the phase turn-on, &thgr;
ON
, is advanced. This allows current to build before the back EMF starts to develop. The high phase inductance, of SR motor
11
holds the current for a sufficiently long time against the high back EMF, so that torque can be produced. When the turn-off angle, &thgr;
OFF,
is reached, the phase is turned off with the full negative bus voltage, −V
DC
. In this mode, there is no chopping of phase current. Both at high speed and at low speed, there exists a unique set of control parameters that can maximize certain performance indices, such as, for example, efficiency. Noise is produced both in the low speed and in the high-speed operations of SR motor
11
during the phase turn-off stage. The high di/dt (i.e., the rate of change of current) produced by the high bus voltage, V
DC
, during phase turn-off sets up vibration in the stator back iron, thus generating noise.
With respect to torque ripple, current profiling is routinely done in SR motors to reduce the torque ripple, especially at low speed operation. Several techniques have been proposed to reduce torque ripple of SR motors. All of these techniques use a high bandwidth current regulator, either hysteretic or PI type, to profile the SR motor phase current such that torque ripple is reduced. A drawback of current profiling with current regulation is that it often lowers SR motor efficiency.
Accordingly it is desirable to have an improved drive for switched reluctance motors that reduces operational noise and torque ripple without sacrificing motor efficiency.
SUMMARY OF THE INVENTION
It is an advantage of the present invention to provide an improved motor drive (control) for SR motors that reduces noise, reduces torque ripple, and increases motor efficiency. Another advantage of the present invention is that it provides a motor controller that does not require phase current sensors and current regulators, such as those required by conventional SR motor drives.
According to one aspect of the present invention, a motor control includes a DC—DC converter coupled to an inverter. The DC—DC converter can be a buck converter, a boost converter, or a buck-boost converter. A capacitor can be connected in parallel across the outputs of the DC—DC converter supplying the inverter.
This arrangement allows the control of the DC bus voltage of the SR motor inverter. The DC bus voltage is controlled optimally to increase the efficiency of the motor. An SR motor operates more efficiently when the DC bus voltage is sufficiently lowered from the motor rated voltage such that motor phas
Rahman Khwaja M.
Schulz Steven E.
DeVries Christopher
Ro Bentsu
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