U.S. patent application number 09/480430 was filed with the patent office on 2002-05-09 for exciter and electronic regulator for rotating machinery.
Invention is credited to GOLD, CALMAN.
Application Number | 20020053889 09/480430 |
Document ID | / |
Family ID | 23907934 |
Filed Date | 2002-05-09 |
United States Patent
Application |
20020053889 |
Kind Code |
A1 |
GOLD, CALMAN |
May 9, 2002 |
EXCITER AND ELECTRONIC REGULATOR FOR ROTATING MACHINERY
Abstract
An exciter assembly for supplying power to a superconducting
load, such as a superconducting field coil, disposed within a
cryogenic region of a rotating machine. The exciter assembly
provides an efficient and reliable approach for transferring the
electrical power energy across a rotating interface and for
controlling the ramp up and regulation of field excitation current
in the field coil. In particular, the invention provides a
controlled recirculation path for current flowing through the field
coil. The exciter assembly includes a transformer having a primary
winding and a secondary winding, a sensor which provides a control
signal indicative of the flow of field excitation current to the
superconducting load; and a current regulator which is disposed in
the rotating reference frame and, on the basis of the control
signal, regulates the field excitation current to a predetermined
set. One of the primary and secondary windings is positioned in a
rotational reference frame relative to the other of the primary and
secondary windings.
Inventors: |
GOLD, CALMAN; (Londonderry,
NH) |
Correspondence
Address: |
FRANK R OCCHIUTI
FISH & RICHARDSON PC
225 FRANKLIN STREET
BOSTON
MA
021102804
|
Family ID: |
23907934 |
Appl. No.: |
09/480430 |
Filed: |
January 11, 2000 |
Current U.S.
Class: |
318/154 |
Current CPC
Class: |
H02K 55/04 20130101;
H02K 19/36 20130101; H02H 7/06 20130101; Y02E 40/60 20130101; H02P
9/302 20130101; H02K 19/365 20130101 |
Class at
Publication: |
318/154 |
International
Class: |
H02P 005/26 |
Goverment Interests
[0001] This invention arose in part out of research pursuant to
Contract No. F336 15-99-C2970.
Claims
What is claimed is:
1. An exciter assembly for supplying power to a superconducting
load disposed within a cryogenic region of a rotating machine, the
exciter assembly comprising: a transformer having a primary winding
and a secondary winding, one of the primary and secondary windings
being positioned in a rotational reference frame relative to the
other of the primary and secondary windings; a sensor which
provides a control signal indicative of the flow of field
excitation current to the superconducting load; and a current
regulator which is disposed in the rotating reference frame and, on
the basis of the control signal, regulates the field excitation
current to a predetermined set.
2. The exciter assembly of claim 1 wherein the current regulator
includes: a first switching device in series between the secondary
winding and the superconducting load; a second switching device in
parallel with the superconducting load and between the first
switching device and superconducting load; a capacitor disposed
between the secondary winding and the first switching device and in
parallel with the second switching device; the first switching
device being closed when the second switching device is open to
provide recharging current to the superconducting load; and the
second switching device being closed when the first switching
device is open to shunt current for recirculation through the
superconducting load.
3. The exciter assembly of claim 2 wherein the first and second
switching devices are disposed within the cryogenic region.
4. The exciter assembly of claim 3 wherein the first and second
switching devices are metal oxide semiconductor devices.
5. The exciter assembly of claim 1 wherein the semiconducting load
is a superconducting coil including high temperature
superconductor.
6. The exciter assembly of claim 1 wherein the primary winding is
in the form of a stationary disk and the secondary winding is in
the form of a rotatable disk axially spaced from the stationary
disk to form a gap therebetween.
7. The exciter assembly of claim 6 wherein at least one of the
stationary disk and the rotatable disk is formed of radial
laminations.
8. The exciter assembly of claim 1 further comprising: a resistive
load; and a switch for allowing energy from the superconducting
load to flow to the resistive load in the event of a detected
fault.
9. The exciter assembly of claim 1 wherein the transformer is a
polyphase transformer driven by a polyphase AC source.
10. The exciter assembly of claim 1 wherein the current regulator
is disposed in a non-cryogenic environment.
11. The exciter assembly of claim 10 wherein the current regulator
includes: a first switching device in series between the secondary
winding and the superconducting load; a second switching device in
parallel with the superconducting load and between the first
switching device and superconducting load; a capacitor disposed in
parallel between the secondary winding and the first switching
device; the first switching device being closed when the second
switching device is open to provide recharging current to the
superconducting load; and the second switching device being closed
when the first switching device is open to shunt current for
recirculation through the superconducting load.
12. The exciter assembly of claim 11 wherein the first switching
device is an insulated gate bipolar transistor device and the
second switching device is a fast recovery rectifier diode.
13. The exciter assembly of claim 11 wherein the semiconducting
load is a superconducting coil including high temperature
superconductor.
14. The exciter assembly of claim 11 wherein the primary winding is
in the form of a stationary disk and the secondary winding is in
the form of a rotatable disk axially spaced from the stationary
disk to form a gap therebetween.
15. The exciter assembly of claim 14 wherein at least one of the
stationary disk and the rotatable disk is formed of radial
laminations.
16. The exciter assembly of claim 10 further comprising: a
resistive load; and a switch for allowing energy from the
superconducting load to flow to the resistive load in the event of
a detected fault.
17. The exciter assembly of claim 10 wherein the transformer is a
polyphase transformer drive by a polyphase AC source.
Description
TECHNICAL FIELD
[0002] This invention relates to controlling the flow of current to
windings used in rotating machinery, and more particularly to
controlling the flow of current to superconducting windings.
BACKGROUND
[0003] Superconducting windings are being used in electrical
machinery and rotating machines because of their low loss
characteristics. While the superconducting windings are maintained
at cryogenic temperatures, the power supplies used to drive the
superconducting windings are typically maintained at ambient
temperatures (300.degree. K.).
[0004] In the design of electrical machinery, incorporating high
temperature superconducting (HTS) windings (i.e., motors,
generators, magnets), the heat leak associated with the leads
carrying current from the power supply at ambient temperatures to
the cryogenically cooled windings is an overriding design factor
which dictates the cost and thermal capacity of closed-cycle
cryogenic cooling apparatus. These losses increase as the
temperature difference between ambient and coil temperature
increases. A number of approaches have been suggested to minimize
the impact of heat leaks in such systems especially those in which
the leads carry currents approaching 1 KA. Unfortunately, where
vapor cooling of leads is not an option, these approaches introduce
high voltages into the system or do not eliminate the need for a
high current lead pair entering the cryogenic environment with
attendant heat leaks. In cases where the superconducting coil is
rotating with respect to a warm stator coil, the problem of heat
leaks into the cryogenic environment becomes more critical due to
the design constraints imposed by the thermal path impedance of a
stationary cryocooler coupled indirectly to a rotating heat load or
constraints on the size, weight, and thermal capacity of a rotating
cryocooler.
[0005] There exist a number of large scale commercial and defense
applications of HTS coils (e.g., magnet systems, generators and
synchronous motor field windings) which require relatively constant
magnetic fields, and in which ample time is available to ramp the
coil current up to its initial desired value prior to regulated
operation. In electrical machine systems incorporating HTS
windings, the current in the HTS coil is subject to flux creep due
to the finite losses in the HTS conductor. The dissipation due to
this finite albeit small resistive loss requires that the current
be restored periodically, i.e., "pumped" via regulating circuitry
back to its desired level. The energy input requirement is only
that required to make up for the flux creep. Electronic circuits
and mechanisms, which perform these functions, are referred to as
"flux pumps".
SUMMARY
[0006] The invention features an exciter assembly and approach for
supplying power to a superconducting load, such as a
superconducting field coil, disposed within a cryogenic region of a
rotating machine. The exciter assembly provides an efficient and
reliable approach for transferring the electrical power energy
across a rotating interface and for controlling the ramp up and
regulation of field excitation current in the field coil. In
particular, the invention provides a controlled recirculation path
for current flowing through the field coil.
[0007] In one aspect of the invention, the exciter assembly
includes a transformer having a primary winding and a secondary
winding, a sensor which provides a control signal indicative of the
flow of field excitation current to the superconducting load; and a
current regulator which is disposed in the rotating reference frame
and, on the basis of the control signal, regulates the field
excitation current to a predetermined value. The secondary winding
is positioned in a rotational reference frame relative to the
primary winding.
[0008] In essence, the current regulator provides a controlled
recirculation path for current flowing through the superconducting
load. By monitoring the flow of excitation of current in the load,
once the desired level of current is provided in an initial charge
up period, current to the load need only be provided relatively
infrequently and for very short durations. The persistence
characteristic of the coil current achieved in the power electronic
control permits the exciter primary side source of AC signal to be
turned off during the persistence phase. This reduces both core and
winding losses and thus permits a considerably reduced winding
rating in the exciter transformer. Moreover, by intelligently
controlling the flow of current, the size, weight, and voltage
rating of associated components for providing power (e.g., exciter
transformer) can be significantly reduced, thereby increasing the
overall efficiency and decreasing the cost of the system. This
approach for supplying power to superconducting loads is
particularly well suited for HTS superconducting rotating machines,
such as those described in co-pending applications, Ser. No.
09/415,626, entitled "Superconducting Rotating Machines", filed
Oct. 12, 1999, and Ser. No. ______ entitled "HTS Superconducting
Rotating Machine", filed Jan. 11, 2000, both of which are
incorporated by reference.
[0009] Embodiments of this aspect of the invention may include one
or more of the following features.
[0010] The current regulator includes a first switching device in
series between the secondary winding and the superconducting load,
a second switching device in parallel with the superconducting load
and between the first switching device and superconducting load,
and a capacitor disposed in between the secondary winding and the
first switching device and in parallel with the second switching
device. The first switching device is closed when the second
switching device is open to provide recharging current to the
superconducting load, and the second switching device is closed
when the first switching device is open to shunt current for
recirculation through the superconducting load.
[0011] In one embodiment, the first and second switching devices
are disposed within the cryogenic region, for example, the same
region within which the superconducting load is disposed. In this
case, the first and second switching devices are preferably metal
oxide semiconductor devices. Cryogenic cooling of metal oxide
semiconductor devices has been shown to decrease their
on-resistance characteristics, thereby further reducing losses in
the recirculation loop.
[0012] In an alternative embodiment, the current regulator is
disposed in a non-cryogenic environment. Thus, cryogenic cooling is
limited solely to the superconducting load. Such an arrangement
allows the use of higher voltage semiconductor devices including an
insulated gate bipolar transistor and a fast recovery rectifier for
the first and second switching devices, respectively. Complexity of
the assembly and associated drive electronics is reduced because
large power blocks can be used instead of array of MOSFETs.
Although more power is dissipated in the higher voltage,
non-cryogenically cooled devices, the power is dissipated outside
of the cryogenic environment and sufficient mass is available to
cool the devices without complex thermal management. Moreover, in
the event of failure of the switching devices or associated
electronics, repair and maintenance is facilitated since there is
no need to open the cryostat to gain access to the switching
devices.
[0013] The load is a superconducting coil including high
temperature superconductor. The primary winding is in the form of a
stationary disk and the secondary winding is in the form of a
rotatable disk axially spaced from the stationary disk to form a
gap therebetween. In essence, the rotating disk and stationary disk
provide a transformer for inducing AC voltage and current in the
superconducting load. In one embodiment, the stationary disk and
the rotatable disk are formed of radial laminations.
[0014] In all of the embodiments described above, the exciter
assembly can further include a resistive load and a switch for
allowing energy from the superconducting load to flow to the
resistive load in the event of a detected fault.
[0015] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a schematic representation of a brushless exciter
and regulating circuit in accordance with the invention.
[0017] FIG. 2 is a diagrammatic representation of a transformer of
the brushless exciter assembly of FIG. 1.
[0018] FIG. 3 is a schematic representation of an alternative
embodiment of a brushless exciter assembly.
[0019] FIG. 3A is a schematic representation of another alternative
embodiment of a brushless exciter assembly.
[0020] FIG. 4 is a diagrammatic representation of a polyphase
transformer used in a polyphase embodiment of the invention.
[0021] FIG. 5 is a diagrammatic representation of a alternative
embodiment of a polyphase transformer.
[0022] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0023] Referring to FIG. 1, a brushless exciter assembly 100
includes a regulating circuit 101 for use with superconducting
rotating machinery is shown. In general, the brushless exciter
assembly 100 provides the necessary electrical energy to one or
more superconducting field windings, for example, windings of a
rotor assembly. The superconducting field windings are represented
here by an HTS field coil 102 and may be in any of a wide variety
of configurations including layer-wound or pancake coils. Field
coil 102 is maintained within a cryogenic chamber (e.g., cryostat)
of the rotor assembly. The cryogenic chamber (not shown) maintains
field coil 102 at temperatures conducive to superconduction (e.g.,
20.degree.-110.degree. K.). One example, of a superconducting field
winding having a configuration well suited for use in a rotating
machine is described in the above referenced application Ser. No.
09/415,626, entitled "Superconducting Rotating Machines" filed Oct.
12, 1999. More particularly, the brushless exciter assembly
provides the energy across a rotating reference interface to field
coil 102 of the rotor assembly.
[0024] As shown in FIG. 1, brushless exciter assembly 100 includes
a transformer 106 having a primary winding 108 for receiving
current from an AC power source 110 and a secondary winding 112. AC
power source 110 is preferably a high frequency excitation source
(e.g., 400 Hz to 2 KHz) to permit reduction of the core
cross-section and mass of transformer 106. In one embodiment,
transformer 106 includes a core 107 constructed of segmented
sections of distributed air-gap pressed powder core material, such
as powdered iron U-cores. In this case the core segments are
separated by air gaps and the core segments are disposed in a
radial direction and lie in an angular relationship with respect to
one another to form "circular disk" of the transformer core.
Brushless exciter assembly 100 also includes a full-wave bridge
rectifier 114 connected to secondary winding 112 for rectifying the
AC current and providing the DC current required by field coil 102
and a storage capacitor 116. In essence, brushless exciter assembly
100 serves as a "flux pump" for transferring power across the
cryogenic barrier in the absence of conductive leads or joints.
[0025] Referring to FIG. 2, primary winding 212 and secondary
winding 208 (structural equivalents of primary winding 108 and
secondary winding 112 in FIG. 1) include a stationary core 202 and
a rotating core 204, respectively. Stationary core 202 is spaced
from rotating core 204 by an air gap 205 (e.g., 1-4 mm) and may be
mounted, for example, to the stator assembly of the rotating
machine. Rotating core 204 is formed of a high permeability
material (e.g., iron) and includes a groove 206 within which a coil
winding 208 is disposed. Stationary core 202 is similarly formed of
a high permeability material and includes a groove 210 within which
a coil winding 212 is disposed. To reduce eddy currents, rotating
core 204 and stationary core 202 are formed as laminations
separated by varnish or oxide.
[0026] Cores 202, 204 are positioned such that winding 208 is
positioned directly across from winding 212. Each of the coil
windings is wound from a continuous insulated copper wire.
[0027] Thus, the disk arrangement provides transformer 106 having
secondary winding 112 rotating relative to primary winding 108 of
the transformer (or vice versa). An important feature of this
particular arrangement is that the flux linkage generated by
stationary core 202 and rotating core 204, when stationary, is the
same as when the rotating core rotates. In other words, transformer
106 has a design having a rotation invariant coupling between the
primary winding and the secondary winding. That is, the induced
voltage is independent of rotational speed and instantaneous
position of primary windings relative to secondary windings. This
feature advantageously allows field coil 102 to be charged prior to
rotating core 204 rotating (i.e., before rotating machine
operates).
[0028] Referring again to FIG. 1, regulating circuit 101 includes a
pair of MOSFET switching devices 120, 122 arranged in a half bridge
arrangement similar to a synchronous stepdown ("buck") DC-DC
converter topology. In this embodiment, however, the conventional
LC output filter has been replaced with field coil 102 with the
converter used to regulate the average field excitation
current.
[0029] As in any superconducting magnet coil, the HTS field coil
must be monitored for incipient quench and a provision must be made
to dump the energy stored in the coil in sufficient time to prevent
catastrophic failure. Thus, to address this concern, the
persistence of an additional switching device 138 in series with
the coil. Thus, switch 138 is normally closed and shunts out a dump
resistor 140 used to dissipate energy from field coil 102 in the
event of a quench. That is, in normal operation, switching device
138 is normally conducting to allow current to bypass dump resistor
140, which is preferably positioned outside the cold space of the
rotor assembly.
[0030] Further, in this embodiment, MOSFET devices 120, 122 are
n-channel enhancement mode devices exhibiting low on-resistance and
supporting the maximum reverse voltage in the voltage regulator
without breakdown. Moreover, MOSFET devices are cryogenically
cooled to advantageously decrease its on-resistance, but at the
expense of reducing its operating voltage rating (e.g., as much as
20%). In certain embodiments, to further decrease the on-resistance
of the MOSFET switching devices, several MOSFET devices are
connected in parallel. Indeed, many MOSFET devices can be combined
in a large parallel array.
[0031] Field current controller 130 contains a pair of gate driver
circuits for controlling the respective gates of MOSFET devices
120, 122. The gate drivers are responsive to logical control
signals, one being the inversion of the other. In embodiments in
which MOSFET devices 120, 122 are cryogenically cooled it is
generally preferable to cryogenically cool field current controller
130 as well.
[0032] In operation, alternating current supplied from AC power
source 110 induces a changing magnetic field in primary winding 108
which, in turn, induces alternating current flow in secondary
winding 112. Rectifier 114 converts the alternating current induced
in secondary winding 112 into direct current. The direct current is
selectively transferred to field coil 102 by MOSFET devices 120,
122. In particular, MOSFET devices are controlled by controller 130
to alternately (1) provide charging current to field coil 102
("ramping" mode) and (2) recirculating current in the field coil
("persistence" mode).
[0033] In ramping mode, MOSFET device 122 is closed, while MOSFET
device 120 is open. As a result, DC current from rectifier 114 (and
capacitor 116) is applied to field coil 102, thereby ramping up the
current flow. In ramping mode, the DC current is supplied until the
desired current levels within field coil 102 are achieved, at which
time the respective states of MOSFET devices 120, 122 are changed
to place voltage regulator into persistence mode.
[0034] In persistence mode, MOSFET device 122 is open, while MOSFET
device 120 is closed to shunt current flowing through field coil
102. At cryogenic temperatures, field coil 102 is superconducting
and has a relatively low loss. Thus, aside from the low loss of the
field coil, a substantial portion of loss in the persistence mode
is attributable to MOSFET device 120. MOSFET device 120 is designed
for minimum voltage drop since it carries current for a majority of
the time (the field coil 102 presumably reasonably persistent).
Further details of the operation of a switching circuit operated in
accordance with alternating ramp up and persistence modes is
described in U.S. Pat. No. 5,965,959, assigned to the assignee of
the present invention, and incorporated herein by reference.
[0035] It is appreciated that MOSFET switching devices 120, 122 are
not active simultaneously. Indeed, simultaneous control is
prevented by switching controller 130 and, in accordance with
standard practice, a short time interval or "deadtime" is permitted
to elapse between the activation of MOSFET devices 120, 122.
Furthermore, because operation of MOSFET switching devices 120, 122
is mutually exclusive, in certain embodiments, only one logic
signal is required to operate switching control. That is, the gate
drive signals provided by switch controller 130 are essentially
complementary logic signals.
[0036] As discussed above, direct current from rectifier 114 (and
capacitor 116) is applied to field coil 102, to ramp up to a final
excitation current level in field coil 102 in a relatively short
time (e.g., several seconds). At this point, MOSFET switches 120,
122 are switched to place the regulator in persistence mode. Once
in the persistence mode the input current flowing through primary
winding 108 drops essentially to zero. Switch controller 130 uses a
sensing circuit 132 to monitor the current level at field coil 102.
Due to the low loss characteristic of the field coil, the field
excitation current decays slowly. However, when sensing circuit 130
detects a drop below a predetermined threshold value (e.g., 1% of
the final excitation current), MOSFET switch 122 is activated for a
very short period (e.g., <10 msecs) to pump the current in field
coil 102 back up to the desired final excitation current level. In
particular, capacitor 16 discharges for that short period
(generally less than 1/4 of a sine wave) to pump up field coil 102,
at which point MOSFET devices 120, 122 are switched to place
regulator back into persistence mode. The field current is
periodically restored by controlling the duration of operation the
pump switch as required. It will be appreciated that the transfer
of energy from the capacitor to the field coil is a resonant
discharge of the capacitor. The capacitor cannot be allowed to
discharge beyond zero volts or the coil will recharge it in the
opposite polarity due to the resonant behavior. Thus, the maximum
duration of the restorative operation is one-quarter of the
resonant period of the capacitor with the coil inductance. This
would also correspond to the maximum increment of current increase
in the inductor at the end of the restorative cycle. The controller
is programmed to adjust or regulate the incremental change in coil
current per pump cycle to be less than the maximum increment
available by adjusting the duration of switch operation.
[0037] In essence, capacitor 116 serves to provide a "trickle"
charge to field coil 102 at relatively infrequent times and for a
relatively short duration. For example, field coil 102 may only
require tens of milliseconds to pump up with several minutes
passing between periods of recirculation. An important advantage of
controlling regulating circuit 101 in this way relates to
transformer 112. Specifically, because current flows through
secondary winding 108 for only very short periods of time, a
transformer having a secondary winding with a much lower rating can
be used. Indeed, the size and rating of the secondary winding can
be selected such that it is allowed to operate above its normal
rating for the short period of time needed to charge field coil 102
to its desired value during the initial ramp up period.
[0038] A data logging and master motor controller 134 is provided
to receive and store data from field current controller 130.
Bidirectional communication between field current controller 130 on
the rotating side and the data logging and master motor controller
134 on the stationary side of the motor is required. Setpoint
commands for controlling the field current must be issued and acted
upon by the control electronics and rotating power electronics.
While the rotating control is autonomous for any setpoint currently
issued, the rotating control must acquire and monitor coil voltage
and current for both current regulation and coil protection.
Controller 134 must receive telemetry indicating status on the
rotating side. This status may include temperatures and other data
indicating the condition of the field coil. Field coil controller
130 includes a microcontroller with A/D conversion and digital I/O
to locally control the switching devices for regulation of coil
current, sense coil conditions, and telemeter data and status to
the stationary side.
[0039] The bi-directional communication uses pulse code modulated
carrier infrared optical transmission and reception. The interface
protocol for communicating with controller 134 may thus be made
standard RS-232 or RS-485, thus permitting any serial port equipped
computer to communicate with the motor. In one embodiment, 56.8 kHz
carrier modulated at 1200 baud serial data rate is used. The
transmitter and receiver are closely coupled, and the received
signal is made rotation invariant, as was the case above with the
primary and secondary windings of transformer 106. Rotation
invariance is achieved through the use of several synchronously
drive 940 nm IR emitters 136 to form an area (ring) light source.
The rotating side microcontroller converts acquired signals and
transmits digitally encoded data strings to the stationary side on
a regularly determined time schedule. Commands from the stationary
side are sent to the rotating side as required to change the
setpoint for the field current. The communication is half-duplex,
thus one wavelength can be used for both transmission and reception
across the rotating boundary. It is appreciated that somewhat
higher or lower modulation frequencies are useable and that other
IR wavelengths are also acceptable.
[0040] Referring to FIG. 3, in an alternative embodiment, the
regulating circuit is removed from the cold space leaving only the
field coil within the cryogenic environment. By positioning the
electronics outside the cold space allows the use of higher voltage
semiconductor devices including IGBTs for the first and second
switching devices. For example, as shown in FIG. 3, high power IGBT
devices 142, 144 are substituted for the MOSFET devices 120, 138 of
the cold embodiment of FIG. 1, respectively. A fast recovery
rectifier diode 146 is substituted for MOSFET device 122. One
advantage of this embodiment is that complexity of the assembly and
associated drive electronics is reduced because large power blocks
can be used instead of array of MOSFETs. Although more power is
dissipated in the higher voltage, non-cryogenically cooled IGBT
devices, the power is dissipated outside of the cryogenic
environment and sufficient mass and is available to cool the
devices without complex thermal management. Moreover, in the event
of failure of the switching devices or associated electronics,
repair and maintenance is facilitated since there is no need to
open the cryostat to gain access to the switching devices.
[0041] In general, the semiconductor switching devices described
above have a voltage rating commensurate with the operational
voltages of the system. However, it is appreciated that overvoltage
protection, such as snubber or clamping circuitry can be
incorporated in accordance with well-accepted, standard practices
to address transient levels of voltages that may exceed the
switching devices rating. Such transients are particularly
problematic during the startup in an induction mode of a
synchronous motor.
[0042] Referring to FIG. 3A, an alternative embodiment of a
regulating circuit having an additional fast recovery diode 150 is
shown. Diode is 150 generally has a voltage rating identical to
that of diode 146. In essence, the addition of diode 150 forms a
full-bridge rectifier arrangement which, as will be described
below, provides additional advantages. IGBT devices 142 and 144 are
also shown to have diodes 142a, 144a, which are typically
co-packaged with the IGBT devices. In the MOSFET arrangement of
FIG. 1, such diodes are generally in the form of intrinsic
drain-to-source diodes.
[0043] In operation, when the end of field coil 102 connected to
IGBT device 140 is positive relative to the end of the field coil
connected to IGBT device 142, the co-packaged diodes 140a, 142a
would conduct through the filed coil and charge up capacitor 116
during a first half cycle. When the polarity of field coil 102 is
reversed in the opposite half cycle diodes 146 and 150 conduct.
Thus, as AC current is induced in field coil 102, unipolar DC is
produced n capacitor 116 as if secondary winding 112 of transformer
106 was being bridge rectified into capacitor 116. In essence,
field coil 102 acts as a source for charging capacitor instead of
transformer 106.
[0044] This approach is particularly attractive in applications
wherein a synchronous superconducting motor is started as an
induction motor and then once the shaft speed of the motor reaches
a certain threshold level (e.g., measured by a tachometer), the
motor is switched into a synchronous mode and ramp up of the field
coil is started. Thus, energy can be provided to capacitor 116 (in
essence, "precharged") without energy being provided from secondary
winding 112. In certain applications, a substantial amount of
energy can be stored in capacitor 116 prior to activating source
110.
[0045] Referring to FIG. 4, a three-phase exciter transformer 200
is shown to include three primary structures 202a, 204a, 206a and
corresponding secondary structures 202b, 204b, 2026b. Primary
structures 202a, 204a and 206a are concentrically disposed around a
longitudinal axis 210 of the transformer and are radially spaced
from each other. Secondary structures 202b, 204b and 206b are
similarly positioned around axis 210 and are axially spaced from
corresponding primary windings by a gap 214 (e.g., 1-4 mm). Each
primary structure includes U-shaped core members 216, 218, 220
formed of a relatively high permeability material for supporting
primary windings 216a, 218a, 220a. The primary windings are
positioned in opposing relationship with secondary windings 216b,
218b and 220b supported in U-shaped core members 216', 218', and
220'. The U-shaped core members of the primary structures and
secondary structures serve to provide isolation between adjacent
windings. Each of primary structures 202a, 204a and 206a are driven
by a corresponding phase of a three-phase AC source (not shown).
Secondary structures 202b, 204b, and 206b are connected to a
conventional solid-state polyphase rectifier.
[0046] In an alternative embodiment, a three-phase transformer 300
includes three secondary 302a, 304a, 306a surrounded by three
concentrically mounted primary structures 302b, 304b, 306b along an
axis 310. As was the case described above, windings of the primary
structures are supported by U-shaped core members 302, 304, 306 and
windings of the secondary structures are supported within
corresponding U-shaped core members 302', 304', and 306'. U-shaped
core members 302, 304, 306 are spaced from corresponding U-shaped
core members 302', 304', and 306' by a gap 314.
[0047] Other embodiments are within the scope of the claims.
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