Note: Descriptions are shown in the official language in which they were submitted.
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APPLICATION FOR UNITED STATES PATENT
SPECIFICATION
LOAD AND SPEED SENSITIVE MOTOR STARTING
CIRCUIT AND METHOD
FIELD OF THE INVENTION
The present invention relates generally to alternating current (AC)
motors and to disconnect switches and circuits for use with AC motors. More
specifically, the present invention relates to a circuit and method used with
the
start, or auxiliary, winding of an AC motor wherein the auxiliary winding is
energized when starting the motor from rest and then disconnected at a given
motor speed. The present invention also relates to such a circuit and method
used with both split-phase and capacitor-start motors.
BACKGROUND OF THE INVENTION
It is well known that a single-phase AC motor produces an
alternating magnetic field, one pulling first in one direction, then in the
opposite
direction as the polarity of the magnetic field changes. This is because the
single-phase AC motor is energized by a single alternating current source. The
major distinction between the different types of single-phase AC motors is how
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they go about starting the motor in a particular direction. Motor start is
usually
accomplished by some device or circuit that introduces a phase-shifted
magnetic
field on one side of the motor shaft, or rotor.
Split-phase motors achieve their starting capability by having two
separate windings wound in the motor stator. The two windings are separated
such that one winding is used only for starting. The stat-ting, or auxiliary,
winding is wound with a smaller wire size having higher electrical resistance
than the main windings. Both windings are energized when the motor is started.
The starting winding produces a field that appears to rotate. This rotation
causes the motor to start. A centrifugal switch then disconnects the starting
winding when the motor reaches a predetermined speed.
The winding and centrifugal switch arrangement of a capacitor-
start motor is similar to that used in a split-phase motor. In the capacitor-
start
motor, a capacitor is used in series with the starting winding to produce a
phase
shift and the appearance of a rotating field. Here again, when the motor
approaches a predetermined running speed, the starting switch opens thereby
disconnecting the starting winding and the motor continues to run. One
starting
circuit for use with a motor of this type is disclosed in U.S. Pat. No.
4,622,506.
The method and apparatus of the present invention is an improvement of that
starting circuit.
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Various types of switches, and controls therefor, are also well
known in the electrical arts. This includes the mechanical switch and the
centrifugal actuator mounted on the motor rotor, as alluded to previously.
Mechanical switches of the centrifugal type are subject to problems such as
limited life, fatigue, friction, vibration, mounting position, contact wear,
among
others. Also, the centrifugal switch includes a radial member that blocks
axial
airflow through the motor, which may impair cooling. This radial member also
requires additional room in the motor housing, which may be objectionable in
various applications.
In another known start winding disconnect system, Hall effect
sensors or pick-up coils are used to detect motor speed to actuate a
disconnect
switch. This approach may be objectionable because of the requirement of
adding an extra element such as a magnet on the motor shaft, and the pick-up
coil to sense speed. These extra parts and the assembly required may be cost
objectionable.
In another known disconnect system, a timer is started upon initial
energization of the motor. When the timer times out, the disconnect switch is
actuated to disconnect the auxiliary winding. This approach is not load or
speed
sensitive, but rather disconnects the auxiliary winding only after a
preselected
time regardless of motor speed and regardless of load. This approach is
limited
to dedicated applications where the load on the motor is known beforehand, and
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the delay time set accordingly. If the load on the motor is increased, the
motor
speed will not be up to the desired threshold at the noted cutout time. On the
other hand, if the load on the motor is decreased, the motor will accelerate
faster, and full voltage will be applied across the capacitor for a longer
time than
is desired, which in turn may damage the motor and/or the capacitor. Capacitor
burn-out is a significant problem when reducing the loading of the motor in
timed disconnect systems.
Another known approach is to sense current through the main
winding and then actuate the disconnect switch at a designated condition. This
requires a current sensor in series with the main winding and the start or
auxiliary winding, which is objectionable to many manufacturers because of the
cost of the extra components and the assembly cost of modifying the circuit
and
inserting such components in series in the circuit. This approach may also be
objectionable due to the extra wattage and heat because current is still
flowing
through the sensor in the run mode after starting.
SUMMARY OF THE INVENTION
The present invention addresses and solves the above noted and
other problems in a particularly simple and effective electronic control
system
for an auxiliary winding disconnect switch. The invention is load and speed
sensitive, and is AC line voltage fluctuation insensitive. The invention
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eliminates the need for extra components on the motor shaft, around the shaft,
or in series in the motor circuit. There is no need for physical modification
of
the motor components or the windings.
The present invention provides a new and useful circuit and
method for measuring the voltage at the main motor winding and detecting the
points in the electromagnetic wave cycle at which this voltage "crosses" zero.
In other words, it detects the points at which the main motor winding voltage
switches instantaneously from positive to negative and vice versa. The method
and circuit also measures the voltage at the auxiliary motor winding. The
voltages measured in the main winding and in the auxiliary winding are each
compared by the circuit as a means for starting and restarting the auxiliary
winding. The circuit and method also detects the points in the electromagnetic
wave cycle where the current in the auxiliary winding "crosses" zero and
compares the phase of these current zero crossing points with a window pulse
that is generated when the main voltage crosses zero. When the zero current
crossing points fall within the window pulse, this means that the auxiliary
winding is up to proper operating speed and the auxiliary winding is
disconnected by the starting circuit. If the load on the main motor winding
increases thereby causing the motor rotor speed to decrease below a certain
predetermined speed, the auxiliary winding will again be switched back into
the
circuit to boost the speed of the main motor winding.
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As previously alluded to, the present invention provides an
improvement over the circuit disclosed and claimed in U.S. Pat. No. 4,622,506.
In the circuit of that embodiment, one was always measuring the voltage across
the auxiliary winding. In the circuit of the present invention, one is always
measuring the current zero crossing points in the auxiliary winding. More
specifically, when the switch to the auxiliary winding is closed, the voltage
of
the auxiliary winding has no meaning because it contains no information as to
motor speed, or RPM. This is why the auxiliary current is measured at this
condition. When the switch is open, no current is passing through the
auxiliary
winding and there is no current information that is available. This is why the
auxiliary winding voltage is measured at this condition, based on RPM
information available.
Other aspects and advantages of the new and useful circuit and
method will be apparent to those having skill in the art upon review of the
attached drawings and the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 schematically shows a typical environment in which the
preferred embodiment of the invention is used.
Fig. 2 is a schematic block circuit diagram illustrating a motor
starting circuit in accordance with the invention.
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Fig. 3 is a detailed circuit diagram of the circuitry of Fig. 2.
Fig. 4 is a timing diagram illustrating operation of the circuitry of
Figs. 2 and 3.
Fig. 5 is another timing diagram illustrating operation of the
circuitry of Figs. 2 and 3 as it relates to RPM information.
DETAILED DESCRIPTION
Referring now to the drawings in detail wherein life numbered
elements represent like elements throughout, Fig. 1 shows a main winding 1 and
1o auxiliary winding 2 of an AC motor that are each connected to an AC power
source 3. When the motor reaches a given threshold speed, a switch 5 that is
connected in series with the auxiliary winding 2 is opened to disconnect the
auxiliary winding 2 from the power source 3. A current detection means 6 is
also provided for detecting and measuring the current flow through the
auxiliary
winding 2 when it is energized. The current detection means 6 provides the
circuit of the present invention with the capability of detecting the points
in the
sinusoidal AC current waveform at which the current of the auxiliary winding 2
"crosses" the zero point. This will also be called the "current zero crossing"
of
the auxiliary winding 2. The significance of this will become more apparent
later in this detailed description.
Fig. 2 shows a control circuit, generally identified 10, including
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main voltage detecting means 7 for sensing the magnitude of voltage across the
main winding 1, and auxiliary voltage detector means 8 for sensing the
magnitude of voltage across the auxiliary winding 2. A voltage comparator
means 13 is provided which is responsive to the main and auxiliary voltage
detectors 7, 8, respectively, and responds to a given relation between the
magnitudes of the main and auxiliary winding voltages. A main voltage zero
crossing means 9 is provided which senses the sinusoidal AC voltage waveform
of the main winding voltage and the points at which the main winding voltage
"crosses" zero or switches polarity. Window pulse means 11 responds to the
main voltage zero crossing means 9 and generates a window pulse. The width
of the window pulse that is generated is not a limitation of the present
invention.
An auxiliary winding current zero crossing means 12 is provided which senses
the cunent flow through the auxiliary winding 2 and the points at which the
auxiliary winding current also "crosses" zero or switches flow direction, as
was
mentioned previously. A phase comparison means 14 is provided which senses
the auxiliary current zero crossing points in relation to the main voltage
window
pulse that has been generated. The voltage comparison means 13 generates a
pulse shape 15 and the phase comparison means 14 generates a pulse shape 16
as well. Logic means 17 is provided to respond to the pulse shapes 15, 16 to
turn "off ' or to turn "on" the switch 5. This can be done, for example, by
means of a triac driver 18 to disconnect or reconnect, respectively, the
auxiliary
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winding 2 from the AC power source 3. In effect, the triac driver 18 is
switched
on and off by negative voltage, which will become apparent later in this
detailed
description.
Fig. 3 shows the detailed circuitry for the schematic shown in Fig.
2 and like reference numerals are used to facilitate clarity. There are
various
portions of the detailed circuitry that correspond to the various functions
illustrated in Fig. 2. For example, the final logic 17 portion of the
schematic
shown in Fig. 2 corresponds to the NAND gate 86 shown in Fig. 3. The gate 86
is used with other circuit components to trigger the triac 90 and to turn the
auxiliary winding 2 on and off. A pair of windings 91, 92 are transformer
windings that effectively provide current sensing means for the auxiliary
winding 2.
Again referring to Fig. 3, a power supply portion of the circuit
includes a pair of transistors 55, 56 that are connected to other components
including a pair of zener diodes 78, 79, a resistor 44 and a capacitor 67. The
voltage across the main winding 1 is the same as the potential as across the
resistor 43, the capacitor 69 and the diode 77. The main voltage sensing 7
portion of the circuit includes other resistors 36, 37, 38, capacitor 64, and
a
diode 73. The auxiliary voltage sensing 8 portion of the circuit includes the
resistors 39, 40, 41, 42, the capacitor 65, the diode 74 and the transistor
53.
Voltage comparison 13 is accomplished by use of a comparator 81. In actuality,
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the comparator 81 is one-fourth of a quad comparator chip, or other integrated
circuit, which provides access to other comparators 80, 82, 83. Specifically,
the
input pin 6 of the comparator 81 senses the main winding voltage 7 and the
input pin 7 of the same comparator 81 senses the auxiliary winding voltage 8.
The output pin 1 of the comparator 81 feeds into the resistor 45 and the input
pin 6 of the NAND gate 85. Depending upon the input at the input pin 5 of the
gate 85, a pulse shape 15 is generated at the output pin 4 of the gate 85. The
significance of this will be discussed later in this detailed description.
Referring now to the main voltage zero crossing 9 and the auxiliary
to current zero crossing 12 portions of the circuit 10, it will be seen that
the
voltage 7 across the main motor winding 1 is at the same potential as that
across
the resistors 20, 21 of the circuit shown in Fig. 3. Also included in the main
voltage zero crossing 9 portion of the circuit are two transistors 50, 51, a
resistor
22 and a capacitor 60. A delay portion of the circuit is also provided by the
resistors 23, 24, 25, 26, the diode 70, the transistor 52 and the capacitor
61. As
previously discussed, another comparator 80 is provided, the output pin 2 of
which feeds into a pulse shape generator 11 portion of the circuit. The pulse
shape generator 11 portion of the circuit includes the resistors 27, 46 and
the
capacitor 68.
The auxiliary winding current zero crossing 12 portion of the
circuit includes the remaining pair of comparators 82, 83, a number of
resistors
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28, 29, 30, 31, 32, 33, 34, 35, a pair of capacitors 62, 63 and a pair of
diodes 71,
72. As shown, the output pins 13, 14 of the comparators 82, 83, respectively,
feed into one of the input pins 2 of a phase comparator gate 84. The output
pin
3 of the phase comparator gate 84 generates a pulse 16 on the "stop" side of
the
circuit by means of the resistor 47, the capacitor 66 and the diode 75. This
output is fed into the input gate 8 of the final logic gate 86. In this
fashion, the
pulse shapes 1 S, 16 are used to turn the triac driver 18 "on" and "off" as
required. The triac switching circuit is provided by virtue of the resistors
48, 49
and the transistor 57 at the output pin 10 of the logic gate 86. By use of
this
configuration, the triac driver 18 is switched on and off by negative voltage.
See Fig. 2.
The initial starting of the motor on application of line voltage 3 is
activated by means of the voltage comparison 13. The voltage comparison 13
senses the low auxiliary winding voltage relative to the main winding voltage
and initiates the triac turn on through logic 17. Immediately after the first
turn
on, the auxiliary winding 2 is leept energized by the phase comparison 14. The
voltage comparison 13 cannot be used to maintain the starting condition
because, after the first energization of the auxiliary winding 2, the voltage
on
the auxiliary winding 2 is that of the line voltage source 3, and does not
represent the motor speed by means of induced voltage from the main winding 1
through the rotor. The maintenance of the auxiliary energization by means of
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winding current phase controls is explained in following paragraphs.
Refernng now to Fig. 4, it will be demonstrated how the main
voltage detecting means 7 in the circuit of the present invention views the
magnitude of the main winding voltage, shown in the top graph as a generally
sinusoidal waveform. The graph below it shows the points V1, V2, V3 at which
the main winding voltage crosses zero. That is, the points at which voltage
polarity is instantaneously reversed. The next graph illustrates that, at each
of
these voltage zero crossing points Vl, V2, V3, a window pulse P1, P2, P3,
respectively, is generated by the window pulse means 11. As the speed of the
motor "ramps up" or increases, the auxiliary winding current zero crossing
means 12 senses the current flow through the auxiliary winding 2 and the
points
I1, I2, I3 at which the auxiliary winding current "crosses" zero or switches
flow
direction. Phase comparison means 14 is provided to sense the auxiliary
winding current zero crossing points Il, I2, I3 in relation to the main
voltage
window pulses P 1, P2, P3. As the auxiliary current zero crossing point I3,
for
example, falls within the window pulse P3, the phase comparator 14 generates a
pulse shape 16 to be received by the logic control 17 and turn off the triac
driver
1 ~. If, because of load increase causing a reduction of motor speed, the
auxiliary winding 2 needs to be reenergized, the voltage comparison means 13
is activated to the restart condition, which is the same as the initial motor
start
condition. The starting sequence is thereby reactivated to connect the
auxiliary
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winding 2. The sequence is that voltage comparison 13 is activated by the
sensing of a low auxiliary winding voltage 2 relative to the main winding
voltage 1. This sensing is used for initial turn on the triac 18 after which
the
phase comparison means 14 is used to maintain the auxiliary winding 2
reenergization until the motor speed increases to the desird auxiliary winding
denergization speed.
Refernng now to Figs. 1 and 5, it will be seen that function of the
circuit is derived from the accessibility of RPM information of the motor. For
example, when the switch 5 is open, the auxiliary voltage provides RPM
information and there is no auxiliary current available. That is, the
auxiliary
winding voltage is really a function of the voltage across the main winding 1
and the motor RPM. The voltage across the auxiliary winding 2 has a direct
relationship to the motor RPM. When the switch 5 is closed, the auxiliary
voltage is the same as the line voltage and it contains no RPM information. On
the other hand, the auxiliary current zero crossing does have RPM information.
This is translated as shown in the uppermost figure showing the switch 5 "on"
and "ofd' positions. As the switch 5 is initially closed, the speed of the
motor
"ramps up" to the point that the auxiliary winding 2 can be "cut out" of the
circuit and the switch 5 opened up. During this time, auxiliary current zero
crossing is determined by the circuit. After the switch 5 opens up, there is
no
current flowing through the auxiliary winding 2 and the voltage across the
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auxiliary winding 2 is a function of the voltage across the main winding 1 and
the motor RPM. As the motor continues and RPM decreases due to load
imposed on the motor, the switch 5 again needs to be closed to increase the
motor torque to try and regain speed. This switch 5 closure "boosts" the motor
so that the RPM increases to the point that the auxiliary winding 1 no longer
needs to be energized. This continues throughout the operation cycle of the
motor.
Accordingly, it will be apparent that there has been provided a new
and useful circuit and method for measuring the voltage at the main motor
winding and detecting points in the electromagnetic wave cycle where the main
winding voltage "crosses" zero. The method and circuit also measures the
voltage at the auxiliary motor winding. The voltages measured in the main
winding and the auxiliary winding are each compared by the circuit as a means
for starting and restarting the auxiliary winding. The circuit and method also
detects the points in the electromagnetic wave cycle at which the current in
the
auxiliary winding "crosses" zero and compares the phase of these current zero
crossing points with a window pulse that is generated when the main winding
voltage crosses zero. When the zero current crossing point falls within the
window pulse, the auxiliary winding is up to proper operating speed and the
auxiliary winding is disconnected by the switching circuit. If the load on the
motor increases causing the motor speed to decrease below a certain level, the
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auxiliary winding will again be switched back into the circuit to boost the
speed
of the main motor winding.
The scope of this invention is to include deenergization of the
auxiliary winding based on cunent phase changes with motor speed of the main
winding and/or the auxiliary winding current. With the auxiliary winding
energized, both the main winding and auxiliary winding current phases change
with motor speed during the motor starting or slowing. The disclosed circuit
functions on the phase shift of the auxiliary winding relative to the line
voltage
phase. The main winding current phase shift with motor speed change is in the
opposite direction as found for the auxiliary current. A change in logic and
the
circuit location of the current sensor from the auxiliary winding to the main
winding could have been made to activate the deenergization of the auxiliary
winding on the phase shift of the main winding current relative to the line
voltage. Further, with two current sensors, the deactivation could have been
activated by comparison of the phase of the auxiliary winding relative to the
main winding. The circuit previously described in detail is an embodiment
based on an attempt to minimize the cost of achieving the desired control. it
is
to be understood that the scope of the disclosure and appended claims are not
limited to the specific embodiments described and depicted herein.
