Note: Descriptions are shown in the official language in which they were submitted.
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SYSTEMS, APPARATUS, AND METHODS FOR SOFT STARTING
LARGE AC MOTORS WITH A VARIABLE FREQUENCY DRIVE
RELATED APPLICATION
[0001] This claims priority to U.S. Provisional Patent Application
No.
61/659,133, filed on June 13, 2012, entitled "METHOD FOR SOFT STARTING
OF LARGE MOTORS".
FIELD
[0002] The invention relates generally to AC (alternating current)
motors,
and more particularly to starting large AC motors.
BACKGROUND
[0003] One or more problems may result when starting large (e.g.,
"medium voltage") AC motors via direct connection to a utility power source.
For
example, a large AC motor may draw four to six times its rated current (known
as
inrush current) at a low power factor upon startup. This may cause sig
nificant
transient voltage drops in the network of the utility power source, which may
adversely affect other equipment and systems connected thereto. Also, the AC
motor may undergo severe thermal and mechanical stress during a direct on-line
start, which may shorten the life of the motor and/or limit the number of
starts in a
given period. Furthermore, during acceleration of a large AC motor, large
torque
pulsations may occur that can excite system torsional resonances that have
been
known on at least one occasion to cause a broken motor shaft. To overcome the
aforementioned problems, large AC motors may be "soft started" with a variable
frequency drive (VFD). A VFD may controllably increase the magnitud e and
frequency of voltage applied to an AC motor during start-up. The volta ge
magnitude and frequency may start at very low values and may then increase to
the rated voltage of the AC motor and to the frequency of the utility power
source
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(e.g., 60 hertz) as the AC motor reaches its rated speed. However, VFDs used
to
start large AC motors are typically very large and very expensive, often
exceeding the cost of the AC motor. Therefore, a need exists to provide less
costly systems, apparatus, and methods for starting large AC motors.
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SUMMARY
[0004] According to one aspect, a system for starting an alternating
current
(AC) motor is provided. The system includes a variable frequency drive
configured to be coupled to a utility power source and to provide an output
voltage having a variable frequency and a variable peak magnitude; an AC motor
having a rated voltage, at least one lead winding connection, and a least one
tapped winding connection; a first contactor coupled in series between the
variable frequency drive and the at least one tapped winding connection, the
first
contactor controlled by the variable frequency drive to selectively connect
and
disconnect the output voltage to and from the at least one tapped winding
connection; and a second contactor coupled in series to the at least one lead
winding connection, the second contactor controlled by the variable frequency
drive to selectively connect and disconnect power received from the utility
power
source to and from the at least one lead winding connection; wherein the
output
voltage has a maximum peak magnitude less than the rated voltage of the AC
motor.
[0005] According to another aspect, an alternating current (AC) motor is
provided. The AC motor includes a plurality of lead winding connections
configured to receive a rated voltage of the AC motor from a utility power
source,
and a plurality of tapped winding connections configured to receive a maximum
voltage less than the rated voltage, wherein the number of tapped winding
connections equals the number of lead winding connections.
[0006] According to yet another aspect, a method of starting an alternating
current (AC) motor is provided. The method includes providing an AC motor
having a rated voltage, a plurality of lead winding connections, and a
plurality of
tapped winding connections, the plurality of lead winding connections
configured
to receive power from a utility power source; providing a variable frequency
drive
having a voltage rating less than the rated voltage of the AC motor; coupling
a
first contactor in series between the variable frequency drive and the
plurality of
tapped winding connections, the first contactor controlled by the variable
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frequency drive to selectively connect and disconnect an output voltage of the
variable frequency drive to and from the plurality of tapped winding
connections; and
coupling a second contactor in series to the plurality of lead winding
connections, the
second contactor controlled by the variable frequency drive to selectively
connect and
disconnect utility power received from the utility power source to and from
the
plurality of lead winding connections.
[0006a] According to one aspect of the present invention, there is
provided a
system for starting an alternating current (AC) motor, comprising: a variable
frequency drive configured to be coupled to a utility power source and to
provide an
output voltage having a variable frequency and a variable peak magnitude,
wherein
the utility power source provides 3-phase power; an AC motor having a rated
voltage,
a plurality of lead winding connections, and a plurality of tapped winding
connections,
wherein the plurality of tapped winding connections include a first tapped
winding
connection within a first winding of the AC motor, a second tapped winding
connection within a second winding of the AC motor, and a third tapped winding
connection within a third winding of the AC motor, wherein the first, second,
and third
windings are in a star or Y-connection configuration; a first contactor
coupled in
series between the variable frequency drive and the plurality of tapped
winding
connections, the first contactor controlled by the variable frequency drive to
selectively connect and disconnect the output voltage to and from the
plurality of
tapped winding connections; and a second contactor coupled in series to the
plurality
of lead winding connections, the second contactor controlled by the variable
frequency drive to selectively connect and disconnect power received from the
utility
power source to and from the plurality of lead winding connections, wherein
the
output voltage has a maximum peak magnitude in a range of about 33% to about
67% of the rated voltage of the AC motor, wherein while the second contactor
has
disconnected utility power from the plurality of lead winding connections, the
variable
frequency drive is configured to cause the first contactor to connect the
output
voltage of the variable frequency drive to the plurality of tapped winding
connections,
which causes the AC motor to increase in speed, wherein after the output
voltage
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and frequency of power provided to the AC motor from the variable frequency
drive
has caused an increase in speed of the AC motor, the variable frequency drive
is
configured to cause the first contactor to disconnect the output voltage of
the variable
frequency drive from the plurality of tapped windings and cause the second
contactor
to connect utility power received from the utility power source to the
plurality of lead
winding connections.
[0006b]
According to another aspect of the present invention, there is provided
an alternating current (AC) motor system comprising: an AC motor including: a
plurality of lead winding connections configured to receive a rated voltage of
the AC
motor from a utility power source; a plurality of tapped winding connections
configured to receive a maximum voltage less than the rated voltage, wherein
the
number of tapped winding connections equals the number of lead winding
connections, wherein the plurality of tapped winding connections comprises a
respective plurality of voltage taps in a range of about 33% to about 67% of
the rated
voltage; wherein the plurality of lead winding connections comprises three
voltage
phase connections coupled to respective leads of first, second, and third
windings of
the AC motor; wherein the plurality of tapped winding connections comprises a
first
tapped winding connection within the first winding, a second tapped winding
connection within the second winding, and a third tapped winding connection
within
the third winding; and wherein the first, second, and third windings are in a
star or
Y-connection configuration, and a variable frequency drive configured to be
coupled
to a utility power source and to provide an output voltage having a variable
frequency
and a variable peak magnitude, the output voltage having a maximum peak
magnitude in a range of about 33% to about 67% of the rated voltage of the AC
motor, wherein the variable frequency drive is configured to selectively cause
the
utility power source to be connected to and disconnected from the plurality of
lead
winding connections, wherein the variable frequency drive is configured to
selectively
cause the voltage output of the variable frequency drive to be connected to
and
disconnected from the plurality of tap windings connections, wherein while the
utility
power source is disconnected from the plurality of lead winding connections,
the
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variable frequency drive is configured to cause the output voltage of the
variable
frequency drive to be connected to the plurality of tapped winding
connections, which
causes the AC motor to increase in speed, wherein after the output voltage and
frequency of power provided to the AC motor from the variable frequency drive
has
caused an increase in speed of the AC motor, the variable frequency drive is
configured to cause the output voltage of the variable frequency drive to be
disconnected from the plurality of tapped windings and is configured to cause
the
utility power source to be connected to the plurality of lead winding
connections.
[0006c] According to another aspect of the present invention, there is
provided
a method of starting an alternating current (AC) motor, comprising: providing:
an AC
motor having a rated voltage, a plurality of lead winding connections, and a
plurality
of tapped winding connections, wherein the plurality of tapped winding
connections
includes a first tapped winding connection within a first winding of the AC
motor, a
second tapped winding connection within a second winding of the AC motor, and
a
third tapped winding connection within a third winding of the AC motor,
wherein the
first, second, and third windings are in a star or Y-connection configuration;
a variable
frequency drive having a voltage rating less than the rated voltage of the AC
motor,
wherein the variable frequency drive is connected to a utility power source; a
first
contactor coupled in series between the variable frequency drive and the
plurality of
tapped winding connections, the first contactor controlled by the variable
frequency
drive to selectively connect and disconnect an output voltage of the variable
frequency drive to and from the plurality of tapped winding connections; and a
second
contactor coupled in series between the utility power source and the plurality
of lead
winding connections, the second contactor controlled by the variable frequency
drive
to selectively connect and disconnect utility power received from the utility
power
source to and from the plurality of lead winding connections, wherein the
variable
frequency drive has a voltage rating in a range of about 33% to about 67% of
the
rated voltage of the AC motor, and starting the AC motor including: while the
second
contactor has disconnected utility power from the plurality of lead winding
connections, the variable frequency drive causing the first contactor to
connect the
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output voltage of the variable frequency drive to the plurality of tapped
winding
connections, which causes the AC motor to increase in speed; after the output
voltage and frequency of power provided to the AC motor from the variable
frequency
drive has caused an increase in speed of the AC motor, the variable frequency
drive
causing the first contactor to disconnect the output voltage of the variable
frequency
drive from the plurality of tapped windings and causing the second contactor
to
connect utility power received from the utility power source to the plurality
of lead
winding connections, wherein the utility power source provides 3-phase power.
[0007] Still other aspects, features, and advantages of the invention may
be
readily apparent from the following detailed description wherein a number of
example
embodiments and implementations are described and illustrated, including the
best
mode contemplated for carrying out the invention. The invention may also be
capable
of other and different embodiments, and its several details may be modified in
various
respects, all without departing from the scope of the invention. Accordingly,
the
drawings and descriptions are to be regarded as illustrative in nature, and
not as
restrictive. The invention covers all modifications, equivalents, and
alternatives falling
within the scope of the invention.
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BRIEF DESCRIPTION OF DRAWINGS
[0008] The drawings, described below, are for illustrative purposes only
and are not necessarily drawn to scale. The drawings are not intended to limit
the scope of the invention in any way.
[0009] FIG. 1 illustrates a schematic diagram of a system for starting
large
AC (alternating current) motors with a variable frequency drive (VFD)
according
to the prior art.
[0010] FIG. 2 illustrates a schematic diagram of a VFD according to the
prior art.
[0011] FIG. 3 illustrates a schematic diagram of a system for starting
large
AC motors with a VFD according to embodiments.
[0012] FIG. 4 illustrates a flowchart of a method of starting a large AC
motor with a VFD according to embodiments.
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DETAILED DESCRIPTION
[0013] Reference will now be made in detail to the example
embodiments
of this disclosure, which are illustrated in the accompanying drawings.
Wherever
possible, the same reference numbers will be used throughout the drawings to
refer to the same or like parts.
[0014] Large AC (alternating current) motors may include medium
voltage
AC motors, which may have a rated voltage ranging from about 600 V (volts) AC
to about 15,000 V (or 15 kV) AC. The "rated voltage" of a motor is a
standardized term established by the National Electrical Manufacturers
Association (NEMA) that generally refers to a motor's operating voltage
usually
+1- 10%. Large AC motors may also include high voltage AC motors and, in
some cases, other types of AC motors that may have a rated voltage below the
above voltage range for medium voltage AC motors.
[0015] The aforementioned problems of starting a large AC motor
may be
overcome by "soft starting" the AC motor with a variable frequency drive
(VFD).
A VFD may initially apply to an AC motor at startup a low or near-zero voltage
having a low or near-zero frequency. As the AC motor speed reaches its rated
speed, the VFD may controllably increase both the voltage magnitude and
frequency to the AC motor's rated voltage and a utility power source's
frequency.
At about that point, power supplied to the AC motor may be switched from the
VFD directly to the utility power source. However, using a VFD to controllably
increase the voltage magnitude up to the AC motor's rated voltage, as may be
known, may require a very large and expensive VFD having a voltage rating the
same or substantially the same as the rated voltage of the AC motor.
[0016] In one aspect, a VFD having a voltage rating much lower
than the
rated voltage of an AC motor may be used to soft start the AC motor. Instead
of
connecting the VFD to an AC motor's lead winding connections, as may be done
conventionally, the VFD may be connected to tapped winding connections
additionally provided by the AC motor. By connecting the VFD to the tapped
winding connections, and by temporarily reducing the AC motor's load (e.g.
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compressors, pumps, and/or fans) during startup, a smaller and less expensive
VFD having a voltage rating much less than the AC motor's rated voltage may be
used to smoothly start the AC motor. In other aspects, methods of starting AC
motors are provided, as will be explained in greater detail below in
connection
with FIGs. 1-4.
[0017] FIG. 1 illustrates an example of a known system 100 for starting a
large AC motor 102 in accordance with the prior art. AC motor 102 may be a 3-
phase medium voltage AC motor having a first winding 104, a second winding
106, and a third winding 108 arranged in a star or Y.-connection
configuration.
First winding 104 may have a first lead winding connection 105. Second winding
106 may have a second lead winding connection 107, and third winding108 may
have a third lead winding connection 109. AC motor 102 may be coupled to a
load (not shown), which may be, e.g., one or more compressors, pumps, fans,
and/or other suitable equipment.
[0018] System 100 may also include a variable frequency drive (VFD) 110
and a reactor 118. VFD 110 may have a voltage rating that is the same or
substantially the same as the rated voltage of AC motor 102. VFD 110 may be
coupled to receive 3-phase power via conductors 111, 112, and 113 (one
conductor per phase) from a utility power source 114. VFD 110 may be
configured to output 3-phase power having a variable peak voltage magnitude
and a variable frequency via conductors 115, 116, and 117 (one conductor per
phase). Reactor 118, which may be a 3-phase reactor, may be coupled in series
to VFD 110 via conductors 115, 116, and 117. Reactor 118 may provide
inductance (which may add impedance) to the 3-phase output of VFD 110.
[0019] System 100 may further include a first contactor 122 and a second
contactor 130. First contactor 122 may include a first control switch 123, a
second control switch 124, and a third control switch 125 each coupled in
series
to reactor 118 via respective conductors 119, 120, and 121. First control
switch
123 may also be coupled in series to first lead winding connection 105 via
conductor 126. Second control switch 124 may also be coupled in series to
second lead winding connection 107 via conductor 127, and third control switch
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125 may also be coupled in series to third lead winding connection 109 via
conductor 128.
[0020] Second contactor 130 may include a first control switch 131, a
second control switch 132, and a third control switch 133 each coupled in
series
to utility power source 114 via respective conductors 134, 135, and 136 (one
conductor per phase). First control switch 131 may be coupled in series to
first
lead winding connection 105 via conductor 137. Second control switch 132 may
be coupled in series to second lead winding connection 107 via conductor 138,
and third control switch 133 may be coupled in series to third lead winding
connection 109 via conductor 139.
[0021] First contactor 122 and second contactor 130 may be controlled by
VFD 110. That is, VFD 110 may control the opening and closing of first,
second,
and third control switches 123, 124, and 125 to connect and disconnect the
output voltage of VFD 110 to and from AC motor 102. Similarly, VFD 110 may
control the opening and closing of first, second, and third control switches
131,
132, and 133 to connect and disconnect utility power of utility power source
114
to and from AC motor 102.
[0022] Each of conductors 111-113, 115-117, 119-121, 126-128, 134-136,
and 137-139 may be an electrical wire or cable of suitable gauge and/or size.
[0023] To start up AC motor 102, system 100 may operate as follows:
Upon or prior to startup, VFD 110 may cause first contactor 122 to connect the
variable voltage output of VFD 110 (via reactor 118) to AC motor 102, while
VFD
110 may cause second contactor 130 to disconnect utility power (received from
utility power source 114) from AC motor 102. That is, first, second, and third
control switches 123, 124, and 125 of first contactor 122 may be closed by VFD
110, while first, second, and third control switches 131, 132, and 133 of
second
contactor 130 may be opened by VFD 110. VFD 110, which may receive 3-
phase power from utility power source 114, may then initially provide a low or
near-zero voltage having a low or near-zero frequency to each of first,
second,
and third lead winding connections 105, 107, and 109 (separated by appropriate
phase angles) via respective conductors 126, 127, and 128. The application of
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voltage to AC motor 102 may cause the rotor (not shown) of AC motor 102 to
begin rotating (in other words, the speed of AC motor 102 begins to increase
from zero). The speed of AC motor 102 may be monitored by VFD 110 via
feedback of, e.g., motor voltage and motor current. As the initial speed of AC
motor 102 is sensed, VFD 110 may gradually and controllably increase both the
output voltage peak magnitude and frequency. As the speed of AC motor 102
continues to increase, so too does the output voltage peak magnitude and
frequency provided by VFD 110.
[0024] As the motor speed reaches the rated speed of AC motor 102, the
voltage peak magnitude and frequency provided by VFD 110 may be at or near
VFD 110's voltage rating (i.e., at or near the rated voltage of AC motor 102)
and
the frequency of utility power source 114 (which may be, e.g., 60 hertz). At
about
this point, power provided to AC motor 102 may be switched from VFD 110 to
utility power source 114. VFD 110 may cause second contactor 130 to connect
utility power (received from utility power source 114) to AC motor 102, while
VFD
110 may cause first contactor 122 to disconnect the variable output voltage of
VFD 110 (via reactor 118) from AC motor 102. That is, first, second, and third
control switches 123, 124, and 125 of first contactor 122 may be opened by VFD
110, while first, second, and third control switches 131, 132, and 133 of
second
contactor 130 may be closed by VFD 110. In some cases, AC motor 102 may be
momentarily coupled to both VFD 110 and utility power source 114. Reactor 118
may limit current exchanged between VFD 110 and utility power source 114 in
this situation. To ensure that VFD 110 may be able to startup and drive AC
motor 102 to its rated speed, VFD 110 may have a voltage rating that is the
same
or substantially the same as the rated voltage of AC motor 102. For example,
if
the rated voltage of AC motor 102 is 6.9 kV AC, the voltage rating of VFD 110
may be about 6.9 kV AC.
[0025] In some embodiments, VFD 110 may have a configuration similar
or identical to a VFD 210 of FIG. 2. VFD 210 may output a voltage having a
magnitude and frequency that may vary. The frequency may vary, e.g., from 0
up to the frequency of the AC input line which, as shown, may be from a 3-
phase
power source and may be, e.g., 60 hertz. The voltage magnitude may vary, e.g.,
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from 0 up to about the voltage rating of VFD 210. VFD 210 may include a
controller 240 and a power circuit 242. Controller 240 may control the
operation
of power circuit 242 and may be coupled to motor voltage feedback line 244 and
motor current feedback line 246. Voltage feedback line 244 and current
feedback
line 246 may be coupled to AC motor 202. Controller 240 may monitor voltage
feedback line 244 and current feedback line 246 to determine the speed of AC
motor 202 and consequently determine whether to adjust the output voltage
magnitude and frequency in accordance with programming (e.g., a motor model)
stored in and/or executing on controller 240. In some embodiments, controller
240 may include a microprocessor or other suitable CPU (central processing
unit)
and a memory for storing software routines to determine motor speed and the
criteria for varying the output voltage magnitude and frequency.
Alternatively,
controller 240 may transmit feedback information to another component (not
shown) and receive commands from that component regarding adjustments to
the output voltage magnitude and frequency. In some embodiments, power
circuit 242 may convert received AC line voltage to a DC voltage and then
invert
the DC voltage back to a pulsed DC voltage whose RMS (root mean square)
value simulates an AC voltage. In some embodiments, power circuit 242 may
include a rectifier, an inverter, and/or PWM (pulse width modulation)
circuitry
configured to vary the output voltage of VFD 210.
[0026] FIG. 3
illustrates a system 300 for starting a large AC motor 302 in
accordance with one or more embodiments. AC motor 302 may be a 3-phase
medium voltage AC motor having a rated voltage in a range of 600 volts AC to
15,000 volts AC. In some embodiments, AC motor 320 may be an induction (or
asynchronous) AC motor or a synchronous AC motor. AC motor 302 may
alternatively be another suitable type of AC motor that may benefit from soft
starting. AC motor 302 may have a first winding 304, a second winding 306, and
a third winding 308 arranged in a star or Y-connection configuration. First
winding 304 may have a first lead winding connection 305. Second winding 306
may have a second lead winding connection 307, and third winding 308 may
have a third lead winding connection 309. AC motor 302 may also have a first
tapped winding connection 355 within first winding 304, a second tapped
winding
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connection 357 within second winding 306, and a third tapped winding
connection 359 within third winding 308. In some embodiment, the first,
second,
and third tapped winding connections 355, 357, and 359 are configured such
that
the tap voltages may be about 50% of the rated voltage of AC motor 302. In
some embodiments, the first, second, and third tapped winding connections 355,
357, and 359 may be configured as voltage taps in a range of about 33% to
about 67% of the rated voltage of AC motor 302. AC motor 302 may be coupled
to a load (not shown), which may be, e.g., one or more compressors, pumps,
fans, and/or other suitable equipment.
[0027] System 300 may also include a variable frequency drive (VFD) 310,
which may be coupled to receive 3-phase power via conductors 311, 312, and
313 (one conductor per phase) from a utility power source 314. VFD 310 may be
configured to output 3-phase power having a variable peak voltage magnitude
and a variable frequency via conductors 315, 316, and 317 (one conductor per
phase). The maximum peak magnitude of the output voltage provided by VFD
310 may be equal to about the voltage rating of VFD 310. VFD 310 may have a
voltage rating that is much less than the rated voltage of AC motor 302. In
some
embodiments, the voltage rating of VFD 310 may be about 50% of the rated
voltage of AC motor 302. For example, if AC motor 302 has a rated voltage of
13.8 kV AC and first, second, and third tapped winding connections 355, 357,
and 359 are configured as about 50% voltage taps, a VFD 310 having a voltage
rating of about 6.9 kV AC may be used. In some embodiments, the voltage rating
of VFD 310 may be in a range of about 33% to about 67% of the rated voltage of
AC motor 302 in accordance with similar or identical voltage tap
configurations of
tapped winding connections 355, 357, and 359 of AC motor 302. In some
embodiments, VFD 310 may be configured or programmed with appropriate data
(e.g., rated speed, desired startup time, etc.) pertaining to AC motor 302. In
some embodiments, VFD 310 may be configured similarly or identically as VFD
210.
[0028] System 300 may further include a first contactor 322 and a second
contactor 330. First contactor 322 may be coupled in series between VFD 310
and the first, second, and third tapped winding connections 355, 357, and 359
of
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AC motor 302, First contactor 322 may include a first control switch 323, a
second control switch 324, and a third control switch 325 each coupled in
series
to VFD 310 via respective conductors 315, 316, and 317. Unlike system 100,
system 300 may not need and, thus, may not include a reactor, such as reactor
118, coupled in series to the outputs of VFD 310, because the amount of AC
current flowing into AC motor 302 during startup may not need to be reduced by
a reactor. First control switch 323 may also be coupled in series to first
tapped
winding connection 355 via conductor 326. Second control switch 324 may also
be coupled in series to second tapped winding connection 357 via conductor
327,
and third control switch 325 may also be coupled in series to third tapped
winding
connection 359 via conductor 328.
[0029] Second contactor 330 may be coupled in series between utility
power source 314 and the first, second, and third lead winding connections
305,
307, and 309 of AC motor 302. Second contactor 330 may include a first control
switch 331, a second control switch 332, and a third control switch 333 each
coupled in series to 3-phase utility power source 314 via respective
conductors
334, 335, and 336 (one conductor per phase). First control switch 331 may also
be coupled in series to first lead winding connection 305 via conductor 337.
Second control switch 332 may also be coupled in series to second lead winding
connection 307 via conductor 338, and third control switch 333 may be coupled
in
series to third lead winding connection 309 via conductor 339.
[0030] First contactor 322 and second contactor 330 may be controlled by
VFD 310. That is, VFD 310 may control the opening and closing of first,
second,
and third control switches 323, 324, and 325 to connect and disconnect the
output voltage of VFD 310 to and from AC motor 302. Similarly, VFD 310 may
control the opening and closing of first, second, and third control switches
331,
332, and 333 to connect and disconnect utility power of utility power source
314
to and from AC motor 302. In some embodiments, contactors 322 and/or 330
may have more than one control switch per phase line and/or may be of other
configurations suitable for connecting and disconnecting power between VFD
310 and AC motor 302 and between utility power source 314 and AC motor 302,
respectively.
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[0031] Each of conductors 311-313, 315-317, 326-328, 334-336, and 337-
339 may be an electrical wire or cable of suitable gauge and/or size.
[0032] To start up AC motor 302, system 300 may operate as follows:
Upon or prior to startup, VFD 310 may cause first contactor 322 to connect the
variable voltage output of VFD 310 to AC motor 302, while VFD 310 may cause
second contactor 330 to disconnect utility power (received from utility power
source 314) from AC motor 302. That is, first, second, and third control
switches
323, 324, and 325 of first contactor 322 may be closed by VFD 310, while
first,
second, and third control switches 331, 332, and 333 of second contactor 330
may be opened by VFD 310. VFD 310, which may receive 3-phase power from
utility power source 314, may then initially provide a low or near-zero
voltage
having a low or near-zero frequency to each of first, second, and third tapped
winding connections 355, 357, and 359 (separated by appropriate phase angles)
via respective conductors 326, 327, and 328. In some embodiments, the load
(not shown) of AC motor 302 may be reduced temporarily during startup. The
application of voltage to AC motor 302 may cause the rotor (not shown) of AC
motor 302 to begin rotating (in other words, the speed of AC motor 302 begins
to
increase from zero). The speed of AC motor 302 is monitored by VFD 310 via
feedback of, e.g., motor voltage and motor current. As the initial speed of AC
motor 302 is sensed, VFD 310 may gradually and controllably increase both the
output voltage peak magnitude and frequency. As the speed of AC motor 302
continues to increase, so too does the output voltage peak magnitude and
frequency provided by VFD 310.
[0033] As the motor speed reaches the rated speed of AC motor 302, the
frequency provided by VFD 310 may be at or near the frequency of utility power
source 314 (which may be, e.g., at or about 60 hertz) and the voltage peak
magnitude provided by VFD 310 may be at or about VFD's voltage rating, which
may be the percent of the rated voltage of AC motor 302 determined in
accordance with the tapped winding connections 355, 357, and 359, as described
above. At about this point, power provided to AC motor 302 may be switched
from VFD 310 (applied to first, second, and third tapped winding connections
355, 357, and 359) to utility power source 314 (applied to first, second, and
third
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lead winding connections 305, 307, and 309). VFD 310 may cause second
contactor 330 to connect utility power (received from utility power source
314) to
AC motor 302, while VFD 310 may cause first contactor 322 to disconnect the
variable voltage output of VFD 310 (via reactor 318) from AC motor 302. That
is,
first, second, and third control switches 323, 324, and 325 of first contactor
322
may be opened by VFD 310, while first, second, and third control switches 331,
332, and 333 of second contactor 330 may be closed by VFD 310. In some
cases, AC motor 302 may be momentarily coupled to both VFD 310 and utility
power source 314.
[0034] A VFD 310 suitable for soft starting AC motor 302 having a rated
voltage of 13.8 kV AC with first, second, and third tapped winding connections
355, 357, and 359 configured as about 50% voltage taps may be, e.g., a Perfect
Harmony drive, 6.9kV output as manufactured by Siemens LD, New Kensington,
PA, USA.
[0035] FIG. 4 illustrates a method 400 of starting an AC motor in
accordance with one or more embodiments. At process block 402, method 400
may include providing an AC motor having a rated voltage, a plurality of lead
winding connections, and a plurality of tapped winding connections, the
plurality
of lead winding connections configured to receive power from a utility power
source. In some embodiments, the AC motor may be a 3-phase medium voltage
motor such as, e.g., AC motor 302 of FIG. 3, and the plurality of lead winding
connections may be, e.g., first, second, and third lead winding connections
305,
307, and 309, respectively. The plurality of tapped winding connections may
be,
e.g., first, second, and third tapped winding connections 355, 357, and 359,
respectively. The tapped winding connections may be configured as 50% voltage
taps and/or may be configured as voltage taps in a range of about 33% to about
67% of the rated voltage of the AC motor. The AC motor may alternatively be
any suitable type of AC motor having a plurality of lead winding connections
and
a plurality of tapped winding connections as described herein.
[0036] At process block 404, method 400 may include providing a variable
frequency drive (VFD) having a voltage rating less than the rated voltage of
the
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AC motor. For example, the VFD may be, e.g., VFD 310 of FIG. 3, and may
have in some embodiments a voltage rating that is about 50% of the rated
voltage of the AC motor. In some embodiments, the VFD may have a voltage
rating in a range of 33% to 67% of the rated voltage of the AC motor. In some
embodiments, the VFD may be configured similarly or identically as, e.g., VFD
210 of FIG. 2.
[0037] At process block 406, coupling a first contactor in series between
the VFD and the plurality of tapped winding connections may be performed. The
first contactor may be controlled by the VFD to selectively connect and
disconnect an output voltage of the VFD to and from the plurality of tapped
winding connections. For example, the first contactor may be first contactor
322
of FIG. 3, which may be controlled by VFD 310. In some embodiments, the first
contactor may have a plurality of control switches, one switch per power
phase,
such as, e.g., first, second, and third control switches 323, 324, and 325 of
first
contactor 322, wherein the VFD may control the opening and closing of the
plurality of control switches to selectively connect and disconnect the output
voltage of the VFD to and from the plurality of tapped winding connections
[0038] At process block 408, method 400 may include coupling a second
contactor in series to the plurality of lead winding connections. The second
contactor may be controlled by the VFD to selectively connect and disconnect
utility power (from a utility power source) to and from the plurality of lead
winding
connections. For example, the second contactor may be second contactor 330 of
Fig. 3, which may be controlled by VFD 310. In some embodiments, the second
contactor may have a plurality of control switches, one switch per power
phase,
such as, e.g., first, second, and third control switches 331, 332, and 333,
wherein
the VFD may control the opening and closing of plurality of control switches
to
selectively connect and disconnect utility power to and from the plurality of
lead
winding connections.
[0039] The above process blocks of method 400 may be executed or
performed in an order or sequence not limited to the order and sequence shown
and described. For example, in some embodiments, process block 402 may be
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performed after or in parallel with process block 404. Similarly, process
block
406 may be performed after or in parallel with process block 408.
[0040] Persons skilled in the art should readily appreciate that the
invention described herein is susceptible of broad utility and application.
Many
embodiments and adaptations of the invention other than those described
herein,
as well as many variations, modifications, and equivalent arrangements, will
be
apparent from, or reasonably suggested by, the invention and the foregoing
description thereof, without departing from the substance or scope of the
invention. For example, although described in connection with medium voltage
3-phase AC motors, one or more embodiments of the invention may be used with
other types of AC motors where soft starting of the AC motor is desired.
Accordingly, while the invention has been described herein in detail in
relation to
specific embodiments, it should be understood that this disclosure is only
illustrative and presents examples of the invention and is made merely for
purposes of providing a full and enabling disclosure of the invention. This
disclosure is not intended to limit the invention to the particular apparatus,
devices, assemblies, systems or methods disclosed, but, to the contrary, the
intention is to cover all modifications, equivalents, and alternatives falling
within
the scope of the invention.
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