Electricity: motive power systems – Induction motor systems – Power-factor control
Reexamination Certificate
1999-08-30
2001-03-13
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Induction motor systems
Power-factor control
C318S701000, C318S254100
Reexamination Certificate
active
06201368
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a switched reluctance drive system. In particular, it relates to a switched reluctance drive system that is configured to draw current at a high power factor from an electrical supply.
2. Description of Related Art
The characteristics and operation of switched reluctance machines 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, which is incorporated herein by reference.
FIG. 1
shows a typical switched reluctance drive in schematic form arranged to drive a load
19
. The drive comprises a switched reluctance motor
12
having a stator and a rotor, a power converter
13
and an electronic control unit
14
. The drive is supplied from a DC power supply
11
that can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply
11
is switched across phase windings
16
of the motor
12
by a power converter
13
under the control of the electronic control unit
14
.
FIG. 2
shows typical switching circuitry in the power converter
13
that controls the energization of the phase winding
16
. In this circuit, a switch
21
is connected between the positive terminal of a power line and one end of the winding
16
. Connected between the other end of the winding
16
and the negative terminal of the power supply is another switch
22
. Between switch
22
and the winding
16
is connected the anode of a diode
23
, the cathode of which is connected to the positive line of the power supply. Between switch
21
and the winding
16
is connected the cathode of another diode
24
, which is connected at its anode to the negative line of the power supply. Switches
21
and
22
act to couple and de-couple the phase winding
16
to the source of DC power, so that the winding
16
can be energized or de-energized.
Many other configurations of switching circuitry are known in the art, some of which are discussed in the Stephenson & Blake paper cited above.
For proper operation of the drive, the switching must be correctly synchronized to the angle of rotation of the rotor. 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 that calculates the position from other monitored parameters of the drive system, as described in e.g. European Patent Application No. 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 switched reluctance drive is essentially a variable speed system characterized by voltages and currents in the phase windings
16
that are quite different from those found in traditional machines. FIG.
3
(
a
) shows a typical voltage waveform applied by the controller to the phase winding
16
. At a predetermined rotor angle, the voltage is applied by switching on the switches
21
and
22
in the power converter
13
and applying a constant voltage for a given conduction angle &thgr;
c
. The current rises from zero, typically reaches a peak and falls slightly as shown in FIG.
3
(
b
). When &thgr;
c
has been traversed, the switches in the power converter
13
are opened and the action of the energy return diodes
23
and
24
places a negative voltage across the winding, causing the flux in the machine, and hence the current, to decay to zero. There is then a period of zero current until the cycle is repeated. It will be clear that the phase draws energy from the supply during &thgr;
c
and returns a smaller amount to the supply thereafter. It follows that the supply, shown as
11
in
FIG. 1
, needs to be a low-impedance source that is capable of receiving returned energy for part of its operating cycle. FIG.
3
(
c
) shows the current that is supplied to the phase winding
16
by the power converter
13
during the period of energy supply and the current that flows back to the converter
13
during the period of energy return.
Typically, the DC power supply
11
of
FIG. 1
is realized by rectifying the AC mains supply, as shown in
FIG. 4
where the mains supply
30
is shown as an AC voltage source
32
in series with a source impedance
34
. In most cases, the impedance
34
is mainly inductive. This inductance can be increased by adding further inductive components in series. A rectifier bridge
36
is provided having four terminals A, B, C and D, two of which, A and C, are connected to the mains supply
30
, the other two, B and D, being connected across a capacitor
38
. The rectifier bridge
36
rectifies the sinusoidal voltage of the source and the output voltage is smoothed by the capacitor
38
. Connected in parallel with the capacitor
38
and the rectifier bridge
36
is a switched reluctance drive
39
(shown schematically), typically comprising the blocks
12
,
13
and
14
of FIG.
1
.
The lines marked +V and −V in
FIG. 4
are generally known as the DC link, and capacitor
38
as the DC link capacitor.
In the absence of any load on the DC link, the capacitor
38
is charged up by successive cycles of voltage to the peak voltage of the sinusoidal supply
30
. The capacitor
38
must therefore be rated for at least the peak of the supply voltage. As resistive load is applied, and when the supply voltage is below the capacitor voltage, energy is drawn from the capacitor
38
. When the rectified supply voltage rises above the capacitor voltage, the capacitor
38
is charged up.
The size of the capacitor
38
and the amount of current drawn by the load interact. Generally, the capacitor is sized so that there is a relatively small amount of droop on the DC link voltage while the capacitor is supplying the load.
FIG. 5
shows the rectified voltage and the DC link voltage for a typically sized capacitor, from which it can be seen that the DC link voltage is held approximately constant. The shape of the current from the supply is complex, since it is dependent not only on the size of the DC link capacitor but also on the size and nature of the source impedance. If the capacitor
38
is very large (so that the voltage ripple is effectively zero) and the supply impedance is negligible, the plot of current vs time has a very large spike centered on the peak of a like plot of the rectified voltage waveform. In practice, some supply impedance is always present and has the effect of widening the width of the current pulse and hence reducing its magnitude. Nevertheless, the rectifier must be rated to carry the high peak current.
The general form of the supply current as a function of time is shown in
FIG. 5
, where it should be noted that the current is zero for a significant fraction of the overall cycle. This has an undesirable effect on the power factor of the overall circuit. Power factor is defined as the ratio of the real power supplied to the load to the apparent power (i.e. the volt-amperes) supplied to the circuit. With low supply impedance, the power factor is typically around 0.5. With inductance added to the supply it is possible to increase the width of the current pulse and hence increase the power factor, but a value of around 0.65 is generally considered to be the practical and cost-effective limit.
These low power factors can cause problems for the designers of electrical equipment, for two reasons. Firstly, the supply may have a minimum limit on the power factor that can be drawn, in which case the power factor has to be corrected by some other means. Secondly, fo
Dicke, Billig & Czaja P.A.
Leykin Rita
Nappi Robert E.
Switched Reluctance Drives Limited
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