Note: Descriptions are shown in the official language in which they were submitted.
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CURRENT MODULATION MOTOR CONTROLLER
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates generally to electrical
motors and associated methods of speed control. More
particularly, the present invention relates to a controller
that employs current modulation which operates to optimize the
performance of an induction motor at variable speeds.
Description of the Prior Art
Variable speed electrical motors and controllers are
available and well known in the art . Variable speed electrical
motors often employ controllers that employ such methods as
mechanical gearing, inverter or voltage control.
The prior art mechanical gearing methods typically employ
gearing systems that reduce the output speed of a motor shaft.
Often such mechanical gearing systems are adjustable to allow
the motor to be operated at various speeds . The mechanical
speed controls are often not used due to large size, high cost
and difficult control methods.
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The prior art inverter methods are well known and utilized
in the art. The inverter method employs an inverter that
converts a DC input into a six step current waveform that is
applied to the motor. This method is used primarily in high
end applications.
Prior art voltage control methods vary the speed of
electric motors by varying the AC input. The speed variation
is accomplished either by varying the voltage or the phase
angle of the AC input that drives the motors. Employing the
voltage control method is relatively low in cost, but requires
the motor to rotate at a high rate of speed. This makes these
types of motors noisy and having a lower life expectancy due
to brush wear on the commutator shaft of the motor. The
voltage control method is exemplified by U.S. Patent No.
4,099,108 to Okawa et al., entitled VARIABLE SPEED CONTROL
APPARATUS FOR INDUCTION MOTOR, issued on July 4, 1978 and U.S.
Patent No. 5,111,374 to Lai et al., entitled HIGH FREQUENCY
QUAS-RESONANT DC VOLTAGE NOTCHING SCHEME OF A PWM VOLTAGE FED
INVERTER FOR AC MOTOR DRIVES, issued on May 5, 1992.
One common type of prior art variable speed motor that
employs the voltage control method, is a split phase capacitor
motor, which is known in the art as a PCS motor. PCS motors
are typically employed in small load applications which do not
require high start/running torque.
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A drawback with PCS motors is that they require components
such as capacitors and centrifugal switches for generating the
necessary torque to drive such motors. These components make
these motors more costly and difficult to design. Also,
another drawback with the PCS motors is that they can only be
operated at a limited range of speeds. This is caused by PCS
motors having a limited "slip range". The "slip factor" of a
motor is the quantity that expresses how far below the
synchronous speed a motor can be driven . The synchronous speed
is the full rotational speed of the motor, which is
proportional to the AC frequency input.
An object of the present inventian is to provide an
improved induction motor speed controller.
A further object of the present invention is to provide
an induction motor speed controller that does not utilize
capacitors and centrifugal switches.
A further object of the present invention is to provide
a controller that enables an induction motor to have a wider
"slip range", thereby allowing the motor to be operated at a
wider range of speeds.
A further object of the present invention is to provide
an improved method for controlling the speed of an induction
motor.
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SUMMARY OF THE INVENTION
According to the invention, there is provided an apparatus
for controlling the speed of an induction motor, the motor
having a first winding and a second winding which are adapted
to be coupled to an AC source for supplying an AC input signal,
the apparatus comprising: a first switching device coupled to
the first winding and a second switching device coupled to the
second winding, each of the switching devices being operative
in a low impedance state thereby enabling current to flow
through the associated winding of each switching device and a
high impedance state thereby preventing significant current
flow through the associated winding of each switching device;
and a controller means for switching each switching device
between its high and low impedance states in a sequence for
inducing a phase shift between voltage signals of the first
winding and the second winding.
In an embodiment of the invention, there is provided an
apparatus for controlling the speed of an induction motor, the
motor having a ffirst winding and a second winding which are
adapted to be coupled to an AC source for supplying an AC input
signal, the apparatus comprising: a first switching device
coupled to the first winding and a second switching device
coupled to the second winding, each of the switching devices
being operative in a low impedance state thereby enabling
current to flow through the associated winding of each
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switching device and a high impedance state thereby preventing
significant current flow through the associated winding of each
switching device; and a controller means for switching each
switching device between its high and low impedance states in
a sequence for inducing a phase shift between voltage signals
of the first winding and the second winding; wherein the first
and second switching devices comprise first and second
respective triacs, and wherein the first triac is connected in
series with the first winding and has a first gate input
connected to the controller means, and the second triac is
connected in series with the second winding and has a second
gate input connected to the controller means.
According to the invention, there is further provided a
method for controlling the speed of an induction motor, the
motor having a f first winding and a second winding which are
adapted to be coupled to an AC source for supplying an AC input
signal, the method comprising the steps of: detecting a first
zero crossing point of the AC input signal; generating first
and second delays measured from the first zero crossing point
of the AC input signal; after the first delay has occurred,
switching on the first winding enabling current to flow through
the first winding; after the second delay has occurred,
switching on the second winding enabling current to flow
through the second winding; detecting a second zero crossing
point of the AC input signal; and switching off the first and
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the second windings thereby preventing significant current flow
through the first and second windings.
According to the invention, there is further provided a
method for controlling the speed of an induction motor, the
motor having a first winding and a second winding wh~.ch are
adapted to be coupled to an AC source for supplying an AC input
signal, the method comprising the steps of:
I. a) i. detecting a first zero crossing point of
the AC input signal;
ii. generating a first delay; and
iii. after the first delay has occurred,
switching on the first winding enabling
current to flow through the first winding;
and simultaneously
b) i. detecting a first zero crossing point of
the voltage signal across the second
winding;
ii. generating a second delay; and
iii. switching on the second winding enabling
current to flow through the second
winding; and then
II. a) i. detecting a second zero crossing point of
the AC input signal; and
ii. switching off the first winding thereby
preventing significant current flow
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through the first winding; and
simultaneously
b) i. detecting a second zero crossing point of
the voltage signal across the second
winding; and
ii. switching off the second winding thereby
preventing significant current flow
through the second winding.
Other advantages, objects and features of the present
invention will be readily apparent to those skilled in the art
from a review of the following detailed description of the
preferred embodiment in conjunction with the accompanying
drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The embodiments of the invention will now be described
with reference to the accompanying drawings, in which:
Figure 1 is a schematic of the prior art PCS motor;
Figure 2 is a schematic of the prior art PCS motor with
dual capacitors;
Figure 3 is a schematic of one preferred embodiment of the
current modulation motor controller of the present invention;
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Figure 4 is a preferred method for coupling the gate
inputs of the triacs to the controller of the present
invention;
Figure 5 is a flow chart illustrating the starting mode
sequence of the controller of the present invention;
Figure 6 is a wave form diagram of the voltage signal
across the motor windings when in the starting mode of the
present invention;
Figure 7 is a flow chart illustrating the full speed mode
sequence of the controller of the present invention;
Figure 8 is a wave form diagram of the voltage signal
across the motor windings when in the full speed mode of the
present invention;
Figure 9 is a flow chart illustrating a variable speed
mode sequence of the controller of the present invention;
Figure 10 is a wave form diagram of the voltage signal
across the motor windings when in the variable speed mode of
the present invention; and
Figure 11 is a flow chart illustrating a further variable
speed mode sequence of the controller of the present invention.
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DETAILED DESCRIPTION OF THE INVENTION
Referring to Figure 1, there is shown an embodiment of a
prior art PCS motor 12. The motor 12 includes two stator
windings 14, l6 and a rotor 18. Both windings 14, 16 may be
of equal inductance and resistance, which makes them both
"running" windings. One side of each of the windings 14, 16
are coupled to one side of an AC source 10, while a capacitor
20 is coupled across the other side of the windings 14, 16.
A switch 22 is also included in the assembly and includes two
output terminals 24, 26 and an input terminal 28. Each of the
two output terminals 24, 26 is coupled to one side of the
capacitor 20, while the input terminal 28 is coupled to the
other side of the AC source 10.
The switch 22 being placed in the position as shown
applies the AC source 10 to the motor 12 and also places the
capacitor 20 in series with winding 16. This causes the AC
source 10 to be applied across both of the windings 14, 16.
The capacitor 20 induces a phase shift between the voltages
across each of the windings 14, 16. The phase shift creates
a pseudo two phase voltage across the windings 14, 16. The two
phase voltage generates a rotating magnetic field within the
motor 12 that provides the necessary torque to start turning
the rotor 18 in the specified direction. If capacitor 20 was
taken out of the circuit, the motor 12 would not start because
there would be no rotating magnetic field to generate the
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necessary torque to start turning the rotor 18. The switch 22
being placed in the position where input terminal 28 is coupled
to output terminal 26, places the capacitor 20 in series with
winding 14. This position of the switch 22 enables the
direction of the motor 12 to be reversed.
The value of the capacitor 20 is selected to provide a
large enough "start current" to start the rotor 18 rotating.
The capacitor 20 must also be of a value sufficient to supply
an optimum "running current", which ensures the motor 12 runs
efficiently at full speed. This can be a problem because very
often no one capacitor can supply a current suitable for both
starting and running situations. The amount of "start current"
required very often is large, which potentially can damage the
motor 12 when running at full speed. In order to overcome this
problem, a second capacitor 24 can be placed in parallel with
capacitor 20 as shown in Figure 2.
Referring to Figure 2, there is shown a second embodiment
of a prior art PCS motor 12. In this embodiment, the motor 12
includes a running capacitor 20 coupled across a series coupled
centrifugal switch 32 and start capacitor 30. When the motor
12 is starting, the switch 32 is closed placing the running
capacitor 20 and start capacitor 30 in parallel. Such a
configuration allows the motor 12 to receive a high starting
current and torque when the AC input 10 is applied. Once the
motor 12 nears operational speed, the centrifugal switch 32
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opens removing the start capacitor 30. At this time the
running capacitor 20 is left in the circuit providing an
optimum "running current" that wold not damage the motor 12.
The use of capacitors and switches is a drawback with the
prior art PCS motors shown in Figure 1 and Figure 2. Adding
such components increases the cost and weight of the motors.
Also, the capacitors make the design of a speed controller for
the motors more difficult because the designer must work within
the limits imposed by the capacitors or the centrifugal switch.
The prior art PCS motors can be made to run at variable
speeds by varying the phase" angle applied to the two motor
windings. The speed of the motors can be reduced below
synchronous speed due to the increased "slip factor". The
increased "slip factor" can be accomplished by utilizing
selected taps of an external auto transformer, internal to the
windings or by phase angle delay of the applied signal.
Another drawback with PCS motors is that they have a
limited "slip range", which limits the range of speeds the
motor can be operated. The "slip range" is limited because the
phase angle difference is achieved by varying the phase of one
of the windings with respect to the AC input. As will be
discussed later, a wider phase angle difference can be
achieved, by varying the phase of both of the windings with
respect to the AC input, thus providing a wider "slip range".
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A wider "slip range" enables such motors to run at a wider
range of speeds.
Now referring to Figure 3, there is shown an embodiment
of the current modulation motor controller 110. An advantage
of the controller 110 is that it does nat require the use of
capacitors to induce a phase shift between the motor winding
voltages. As discussed earlier, a phase shift is necessary in
order to develop the necessary torque to start and drive such
motors . The present invention induces the phase shift by a
current modulating technique, which is implemented by a
controller 130, which is preferably a microcontroller.
The current modulation motor controller 110 includes an
induction motor 112 that has a run winding 114, a start winding
116 and a rotor 118. One side of both the run winding 114 and
start winding 116 is coupled to an AC source 144. The AC
source 144 provides an AC input signal that actually drives the
motor 112. At full speed the motor 112 turns at a rate
proportional to the frequency of the AC input signal, which is
known in the art as the synchronous speed.
A run triac 122 is coupled between the other side of the
run winding 114 and the other side of the AC source 144.
Similarly a start triac 126 is coupled between the other side
of the start winding 116 and the other side of the AC source
144. Each of the triacs 122, 126 has a gate input that enables
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them to be individually switched from a state of high impedance
to a state of low impedance. The triacs 122, 126 are utilized
for controlling the current that flows through the windings
114, 116 by being switched between the state of high and low
impedance. In the state of high impedance no significant
amount of current will flow, while in the state of low
impedance current will flow in the windings 114, 116.
Utilizing triacs 122, 126 is a preferred method for controlling
the current through the windings 114, 116. Alternatively, this
function can be accomplished by utilizing SCR's, FET
transistors or other types of switching devices.
A controller 130 is coupled to each of the triacs 122,
126. The controller 130 has a first output 132 that is
connected to the gate input of the start triac 126 and a second
output 134 that is similarly connected to the gate input of the
run triac 122. The controller 130 develops signals at the
first output 132 and the second output 134 for switching the
triacs 122, 126 between the states of high and low impedance.
By selectively switching the run triac 122 and start triac 126,
the controller 130 can modulate the currents within the
windings 114, 116.
The controller 130 also includes a first input 136 and a
second input 138. The first input 136 is connected to the AC
input 144 through a buffer 137. The first input 136 enables
the controller 130 to monitor the AC input 144. The second
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input 138 is connected across the starting winding 116 through
a buffer 139. The second input 138 allows the controller 130
to monitor the voltage signal across the start winding 116.
The two buffers 137, 139 further isolate the controller 130
from any affects of the motor winding inductance.
The controller 130 has a user input 140, which is
connected to a user I/O 128. The user I/O 128, which is
usually embodied as a panel of switches, enables a user to
interface with the controller 130. The controller 130 has
another input that is coupled to a logic power supply 142,
which in turn is coupled across the AC source 144. The logic
power supply 142 converts the AC power from the AC source into
a DC voltage that can be utilized as a logic level by the
controller 130 and other DC devices.
During operation, the controller 130 controls the motor
112 by modulating the currents within both windings 114, 116.
The controller 130 modulates the currents by developing signals
at both of the outputs 132, 134, which switches the triacs 122,
126 at specified times. The timing of the signals developed
at both outputs 132, 134 will differ depending on the mode of
operation of the motor 112, which includes a starting, a full
speed and a variable speed mode.
Referring to Figure 4, there is shown a preferred
embodiment for coupling the gate inputs of the triacs 122, 126
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to the controller 130. Figure 4 replaces the boxed in section
120 of Figure 3. In the preferred embodiment the gates of the
start triac 126 and the run triac 122 are each connected to the
controller 130 through an optical coupler 124. The two optical
couplers 124 serve to isolate the controller from any
potentially damaging effects of the motor winding inductance.
Referring to Figure 5, a flow chart of a starting mode
sequence 146 of the controller 130 is shown. The starting mode
sequence 146 consists of a loop executed every half cycle of
the AC input provided by the AC source 144. The starting mode
sequence 146 begins in a TURN OFF START AND RUN TRIAC step 148,
which causes the controller 130 to send signals to the gate
inputs of the triacs 122, 126 ensuring that they are in the off
state. Ensuring both triacs 122, 126 are in the off state
enables the starting mode sequence 146 to be initiated at any
synchronous time with respect to the AC input. After the
triacs are shut off, a WAIT FOR ZERO CROSSING OF AC INPUT step
150 will be executed.
In the WAIT FOR ZERO CROSSING OF AC INPUT step 150, the
controller 130 will monitor the AC input signal. The
controller 130 will not advance to the next step until a zero
crossing of the AC input is detected. Thus, the run triac 122
is only turned on at the beginning of each half cycle of the
AC input 144, which is at the zero crossing. Turning the run
triac 122 on at this time ensures that maximum power will be
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applied to the run winding 114 because it will conduct for the
full half cycle of the AC input 144. After the zero crossing
is detected, a TURN ON RUN TRIAC step 152 will be executed.
A signal will be generated at the second output 134 that will
be felt at the gate input of the run triac 122, thereby turning
it on. This will energize the run winding 114 generating a
magnetic field within the motor 112. The magnetic field
generated at this time is not rotating, which cannot generate
sufficient torque to start the motor 112. After the run triac
122 is turned on, an INITIATE TIME DELAY ROUTINE step 154 will
be executed.
In the INITIATE TIME DELAY ROUTINE step 154, a Delay X
Value will be generated that will correspond to the amount of
electrical degrees before the start triac 126 is turned on.
This delay is necessary in order to induce a phase shift
between the start winding 116 and the run winding 114 voltages,
which will supply the necessary torque to start the motor 112.
As discussed earlier, the prior art PCS motors generate the
phase shift by components such as capacitors. An advantage of
the present invention is that the use of such components is
eliminated.
The Delay X Value generated can be a fixed or variable.
The value of the delay will correspond to the level of starting
torque the motor 112 will have. For the proper amount of
torque to be generated, the delay should be no less than ninety
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degrees. The delay can be greater than ninety degrees, but
this will reduce the starting torque in a motor from the
maximum at ninety degrees. A delay value of ninety degrees
will generate the maximum starting torque because the maximum
amount of current will flow in the start winding 116 at ninety
degrees, which is the peak value of the AC input.
After the delay value is generated, a TURN ON START TRIAC
step 156 is executed. A signal will be generated at the first
output 132 that will be felt at the gate input of the start
triac 126, thereby turning it on. This will energize the start
winding 116 creating a rotating magnetic field within the motor
112 sufficient to start it rotating.
After the start triac 126 is turned on, a MOTOR AT FULL
ROTATIONAL SPEED step 158 is executed. In this step, the
controller 130 will see if the motor 112 has reached its full
speed. The rotational speed of the motor 112 can be determined
by a fixed time or an external input. If full rotational speed
has not been reached, the controller 130 will loop back to the
beginning of the sequence 146. The start mode sequence 146
will be repeated until full rotational speed is attained. When
full rotational speed is attained, the controller 130 will
leave the sequence 146.
Referring to Figure 6, there is shown a voltage signal
generated across each of the windings in the start mode. As
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you can see the run winding is energized for the full half
cycle of the AC input, while the start winding is only
energized after delaying X Degrees. The value of Delay X will
correspond to the number of degrees the phase angle of the
start winding will be out of phase with the run winding.
Referring to Figure 7, there is shown a flow diagram of
a full speed sequence of the controller 130. The sequence 160
consists of a loop executed every half cycle of the AC input.
The full speed mode sequence 160 assumes that the start mode
sequence 146 has been completed. The full speed mode sequence
160 starts in a WAIT FOR ZERO CROSSING OF AC INPUT step 162.
In this step, the controller 130 will monitor the AC input
signal at its first input 136 (Figure 3). The controller 130
will not advance to the next step until a zero crossing of the
AC input 144 is detected. When the zero point crossing of the
AC input 144 is detected, the controller 130 will then advance
to the next step. At this point the sequence 160 splits into
a left and right branch as shown. The left branch can be
executed by itself or simultaneously with the right branch.
When the left branch is executed by itself, a TURN ON RUN
WINDING TRIAC step 164 is executed next . In this step, the run
winding 114 is energized generating a magnetic field within the
motor 112. Because the motor 112 is already rotating, the
magnetic field generated from just the run winding will be
sufficient to continue to drive the motor 112. After the run
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triac 122 is turned on, a WAIT FOR AT LEAST 160 DEGREE DELAY
TO OCCUR step 166 is executed. In this step, the controller
130 will wait at least 160 degrees or near half cycle of the
AC input before advancing to the next step. This ensures that
maximum power is applied to the run winding 114 due to it being
energized for the full or near full half cycle. The run
winding 114 may still be energized after the run triac 122 is
shut off due to transients developed therein. This is the
reason why the run triac 122 is shut off between 160 and 180
degrees. After the delay, the controller 130 will then execute
a TURN OFF RUN WINDING TRIAL step 168 where the run triac 122
will be turned off.
As discussed above, energizing just the run winding 114
is sufficient to drive the motor 112 in the full speed mode,
but some applications might require a larger running torque.
If this is the case, then the right branch can be executed
simultaneously with the left branch in a time shared mode.
When both branches are executing simultaneously a WAIT FOR
START WINDING ZERO CROSS SIGNAL step 170 will also be executed
at the zero crossing of the AC input signal. In this step, the
controller 130 will monitor the signal across the start winding
116, which is out of phase with respect to the voltage of the
AC input 144.
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At this point even though the start winding 116 is not
energized, there will be a voltage across the start winding
116. When the signal across the start winding 116 reaches the
zero crossing, the controller 130 will then execute a DELAY 128
ELECTRICAL DEGREES step 172. In this step, the controller 130
will wait for 128 degrees of the half cycle before advancing
to the next step.
After the 128 degree delay, the controller 130 will then
execute a TURN ON START WINDING TRIAC step 174. In this step,
the start winding 116 will be energized along with the already
energized run winding 114. This will provide a larger torque
for driving the motor 112 in the full speed mode. The larger
torque is generated because energizing the start winding 116
after the delay induces a phase shift between the two windings,
which generates the larger magnetic field within the motor 112.
This delay serves the same function as a running capacitor of
the PCS motor of Figure 2. The value of the delay is 128
degrees because in a given motor, optimum running conditions
were achieved with this value.
After the start triac 126 is turned on, a DELAY 52
ELECTRICAL DEGREES step 176 will be executed. In this step,
the controller 130 will wait fifty-two degrees before turning
off the start triac 126. After the fifty-two degree delay, a
TURN OFF START WINDING TRIAC step 178 will be executed where
the start triac 126 will be turned off.
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After the start triac 126 is turned off, the controller
130 will loop back to the beginning of the sequence 160 if the
motor continues to run in the full speed mode. If during
operation it is desired to reduce the speed of the motor 112,
then the controller 130 will initiate the variable speed
sequence.
Referring to Figure 8, there is shown a voltage signal
generated across each of the windings in the full speed mode.
As you can see the run winding is energized for the full half
cycle of the AC input, while the start winding is only
energized after a delay of 128 degrees. The 128 degree delay
will cause the voltage across the start winding to be out of
phase with the run winding.
Referring to Figure 9, there is shown a flow diagram of
a variable speed mode sequence 180 of the controller 130. The
sequence 180 consists of a loop executed every half cycle of
the AC input . The sequence 180 assumes the full speed mode
sequence 160 is currently executing. The variable speed mode
sequence 180 starts in a WAIT FOR ZERO CROSSING OF AC INPUT
step 184. In this step, the controller 130 will monitor the
AC input signal at its first input 136 (see Figure 3) . The
controller 130 will not advance to the next step until a zero
crossing of the AC input 144 is detected. When the zero
crossing point of the AC input 144 is detected, the controller
130 will then advance to the next step. At this point the
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sequence 180 splits into left and right branches as shown. The
left branch will be executed simultaneously with the right
branch in a time shared mode.
Referring to the left branch, a DELAY Y DEGREES REQUIRED
FOR DESIRED MOTOR SPEED step 186 is executed next. In this
step, the run winding 114 is left de-energized for Y degrees
from AC mains zero crossing. Next, a TURN ON RUN WINDING TRIAC
step 188 is executed. This delay in energizing the run winding
will weaken the magnetic field within the motor 112. After the
run triac 122 is turned on, a WAIT FOR 180-Y DEGREES DELAY TO
OCCUR step 189 is executed. After the delay, the controller
130 will then execute a TURN OFF RUN WINDING TRIAC step 190.
The run winding 114 may still be energized after the run triac
122 is shut off due to transients developed therein.
Referring to the right branch, a WAIT X DEGREES FOR START
WINDING ZERO-CROSS SIGNAL step 192 will be executed at the zero
crossing of the second input 138, i.e. the start winding
reference input. In this step, the controller 130 will monitor
the voltage signal across the start winding 116, which is out
of phase with respect to the voltage of the AC input 144. At
this point, even though the start winding 116 is not energized,
there will be a voltage across the start winding 116. When the
signal across the start winding 116 reaches the zero crossing,
the controller 130 will then execute a DELAY 128 ELECTRICAL
DEGREES step 194. In this step, the controller 130 will wait
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for 128 degrees of the half cycle before advancing to the next
step.
After the 128 degree delay, the controller 130 will then
execute a TURN ON START WINDING TRIAC step 196. In this step,
the start winding 116 will be energized along with the already
energized run winding 114.
After the start triac 126 is turned on, a DELAY 180-X-128
ELECTRICAL DEGREES step 198 will be executed. In this step,
the controller 130 will wait 180-X-128 electrical degrees
before turning off the start triac 126. After the 180-X-128
delay, a TURN OFF START WINDING TRIAC step 200 will be executed
where the start triac 126 will be turned off. Note that those
skilled in the art will realize that the value of X must be
less than or equal to fifty-two degrees in this particular
embodiment.
After the start triac 126 is turned off, the controller
130 will loop back to the beginning of the sequence 180 if the
motor continues to run in the variable speed mode. If the
motor 112 is to be maintained at the same speed, the same value
of DELAY Y will be generated when re-executing the loop. If
the motor speed is again changed, then the controller will
generate a new value of DELAY Y. If during operation it is
desired to operate the motor 112 at full speed, then the
controller 130 will initiate the full speed sequence 160.
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Stopping the motor is accomplished by turning off both the
start triac 126 and the run triac 122 simultaneously.
The above steps alter the magnetic field within the motor,
by modulating the currents within the windings so that the
phase of each winding has a different phase angle with respect
to the AC input signal. This will generate a phase difference
between the run winding 122 and start winding 126 that will
enhance the motor slip, which will allow the motor 112 to be
operated at a wider range of speeds. As discussed earlier, the
prior art PCS motors reduce motor speed by varying the applied
voltage equally to the windings which greatly limits the range
of slip. Additionally, the above steps enable the motor slip
to be further enhanced by varying the phase difference of the
start winding 116 with respect to the AC input signal, which
cannot be accomplished by using a capacitor.
Now referring to Figure 10, there is shown a voltage
signal generated across each of the windings in the variable
speed mode. As you can see, both the run windings and start
windings are turned on at their respective delay times. These
delay times will provide the proper phase shifts within each
of the windings to increase the slip range of the motor.
Figure 11 shows a flow chart of another variable speed
sequence of the controller 130. The sequence 210 consists of
a loop executed every half cycle of the AC input . The variable
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CA 02267035 1999-03-26
speed sequence 210 assumes the full speed mode sequence 160 is
currently executing. The sequence 210 starts in a WAIT FOR
ZERO CROSSING OF AC INPUT step 220. In this step, the
controller 130 will monitor the AC input signal at its first
input 136 (see Figure 3). The controller will not advance to
the next step until a zero crossing of the AC input 144 is
detected. When the zero crossing point of the AC input 144 is
detected, the controller 130 will then advance to the next
step. At this point the sequence 210 splits into left and
right branches as shown. The left branch will be executed
simultaneously with the right branch in a time shared mode.
Referring to the left branch, a DELAY Y DEGREES REQUIRED
FOR DESIRED MOTOR SPEED step 230 is executed. In this step,
the run winding 114 is left de-energized for Y degrees from AC
mains zero crossing. Next, a TURN ON RUN WINDING TRIAL step
240 is executed. This delay in energizing the run winding will
weaken the magnetic field within the motor 112. After the run
triac 122 is turned on, a WAIT FOR 180-Y DEGREES DELAY TO OCCUR
step 250 is executed. After the delay, the controller 130 will
then execute a TURN OFF RUN WINDING TRIAL step 260.
Referring to the right branch, a DELAY Z DEGREES step 270
will be executed when the zero crossing point of the AC input
144 is detected. At this step, the controller 130 will wait
for Z degrees of the half cycle before advancing to the next
step. After the delay, the controller 130 will then execute
CA 02267035 1999-03-26
a TURN ON START WINDING TRIAL step 280. In this step, the
start winding will be energized along with the already
energized run winding 114. After the start triac 126 is turned
on, a WAIT FOR 180-Z DEGREES DELAY TO OCCUR step 290 will be
executed. After a 180-Z degree delay, a TURN OFF START WINDING
TRIAL step 300 will be executed where the start triac 126 will
be turned off. After the start triac 126 is turned off, the
controller 130 will loop back to the beginning of the sequence
210.
Numerous modifications, variations and adaptations may be
made to the particular embodiments of the invention described
above without departing from the scope of the invention, which
is defined in the claims.
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