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Description  |
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BACKGROUND OF THE INVENTION
The present invention generally relates to methods and means for
controlling motors. More specifically, the invention relates to methods
and means for controlling operation of permanent split capacitor motors.
Shredders are devices, as the nomenclature implies, that are used to shred
items, for example, by slicing. To that end, shredders that slice include
a plurality of blades driven by a motor, usually a permanent split
capacitor motor. Typically they are used to shred paper-based products,
but in some installations they are also used to shred a diversity of
items, such as garbage.
Paper shredders are often used to shred sensitive documents so that the
documents cannot be easily reassembled and thus the matter therein
recognized or read. Typically, when more sheets of paper are fed into the
shredder than the shredder can slice at one time, the shredder will jam.
In my commonly assigned allowed U.S. patent application, Ser. No.
07/970,070, which issued Oct. 12, 1993 as U.S. Pat. No. 5,252,904,
provides an arrangement wherein the slicing blade motor is automatically
reversed to eject the paper, thereby relieving the jamming. There are
other arrangements wherein the :motor can be coupled to the blades by a
slip clutch or the like and be permitted to slip.
It can be appreciated that it is advantageous to increase the capacity of a
shredder so that the throughput, including the number of items of papers
that can be shredded at one time, can be increased. Typically, the
throughput is increased by increasing the size of the motor used to run
the blades that affect the shredding. However, simply increasing the size
of the motor is not always a practical option inasmuch as there may be
space limitations and/or cost limitations involved.
SUMMARY OF THE INVENTION
The present invention provides a circuit for, at least temporarily,
increasing the output of a motor as a loading thereof increases so that
the motor can continue working, at least for a short duration.
Advantageously, the circuit is employed in a shredder so that as more than
a typical load is placed on the shredder motor, the motor can, at least
temporarily, accommodate the excessive load.
In an embodiment, the invention provides a circuit for boosting power
supplied to a motor, comprising:
a sensing stage configured to sense loading on the motor;
a comparator stage coupled to the sensing stage and configured to detect
loading on the motor in excess of a preselected threshold; and
a power boost stage coupled to the comparator stage and configured to at
least temporarily increase current through the motor in response to
detection of excessive loading of the motor by the comparator stage so as
to increase power output of the motor.
In an embodiment of the invention, the motor is a permanent split capacitor
motor having run and auxiliary windings and a capacitor coupled across the
windings, and the sensor stage is coupled to sense voltages at a circuit
node between the capacitor and the auxiliary winding.
In an embodiment of the invention, the sensor stage is a voltage divider.
In an embodiment of the invention, the comparator stage is a Zener diode,
and the Zener diode is reverse biased coupled to the voltage divider.
In an embodiment of the invention, the power boost stage is a switched
capacitance coupled across the windings and a relay associated therewith
to selectively temporarily switch the capacitance across the windings
whenever the loading on the motor becomes excessive.
In an embodiment, the invention provides a circuit for boosting the power
output of a permanent split capacitor motor having run and auxiliary
windings and a capacitor coupled across the windings, comprising:
a sensing stage coupled to a circuit node between the capacitor and the
auxiliary winding;
a voltage comparison stage coupled to the sensing stage;
a switch activator stage coupled to the voltage comparison stage;
a switched capacitor, comprising a series coupled controllable switch and
capacitor, which is coupled across the windings and the switch of which is
coupled to the switch activator stage.
In an embodiment, the invention provides a shredder with a set of blades, a
motor coupled to drive the blades, and a circuit configured to control the
shredder, the circuit including a subcircuit for at least temporarily
increasing the power output of the motor upon excessive loading of the
motor.
These and other features of the invention will become clear with reference
to the following detailed description of the presently preferred
embodiments and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a schematic diagram of a circuit for increasing the
power output of a permanent split capacitor motor when the load exceeds a
preselected amount.
FIG. 2 illustrates a shredder in which the circuit of FIG. 1 can be
implemented.
FIG. 3 illustrates a schematic diagram of a circuit for controlling the
operation of the shredder of FIG. 2 incorporating the circuit of FIG. 1.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
As described above, the present invention provides the circuit for
increasing the power output of a motor, as the load on the motor exceeds a
preselected threshold, thereby providing a compensation for the excessive
loading. Specifically, the circuit can be configured to increase the power
output of a permanent split capacitor motor when the load on the motor
exceeds the threshold.
In FIG. 1, there is an exemplary embodiment of the invention in the form of
a circuit 10. In FIG. 1, there is illustrated a reversing permanent slip
capacitor (PSC) motor 12 which is appropriately coupled between a suitable
alternating current (AC) power supply 14 (e.g., 115 VAC) and a reversing
switch 16. As is known, in a permanent split capacitor motor, such as the
motor 12, two windings 18 and 20 are provided and configured such that one
is the run/forward winding and the other is the auxiliary/reverse winding,
depending on the direction in which the motor is run. In FIG. 1, the motor
12 is illustrated in its forward mode or state, i.e., winding 18 is
employed as the run or forward winding and winding 20 is the auxiliary or
reverse winding. Accordingly, for the motor 12 to be run in its reverse
direction, the switch 16 must be switched so as to connect the winding 20
between the power supply 14 and electrical ground.
A capacitor 22 is coupled across both windings 18 and 20. As is known, a
capacitor such as the capacitor 22 determines the amount of phase shift
between the current and voltage phases in the power drawn by the motor 12.
Also coupled across the windings 18 and 20 is a series circuit comprising a
10 ohm, 10 watt resistor 24 and a 10 .mu.F capacitor 26 and a relay
activated switch 28. In a manner that will be further described
hereinbelow, the switch 28 is selectively closed or thrown so as to couple
the resistor 24 and capacitor 26 across the windings 18 and 20 to further
increase the capacitance thereacross. By increasing the capacitance across
the windings 18 and 20, the output of the motor 12 can be increased, at
least temporarily, to accommodate or compensate for an excessive loading
on the motor 12. In effect, the phase shift is altered to increase the
amount of current drawn by the motor 12. Also, in effect, this arrangement
provides a switched capacitance across the windings 18 and 20.
To accomplish the foregoing, there is provided a circuit 30 coupled to a
circuit node 32 situated between the auxiliary winding 20 and the
capacitor 22, which circuit 30 in effect measures the loading on the motor
12 and then causes the switch 28 to close when the loading exceeds a
preselected threshold.
In the circuit 30 there is provided a voltage divider 34 comprising a 46.4K
ohm, 1% resistor 36 and a 2.55K ohm, 1% resistor 38 coupled in series
between the node 32 and electrical ground. A diode 40 is coupled between
the voltage divider 34 and the node 32. As can be appreciated, the diode
40 allows only for flow of current in one direction, thereby providing a
half-wave rectified signal to the voltage divider 34. The voltage divider
34 is configured such that the reduced amplitude voltage signal resulting
at a node 42 between the resistors 34 and 36, is approximately 12 volts or
greater, under normal loading conditions.
In essence, the voltage divider 34 acts as a sensing or sensor stage.
The circuit 30 further includes an NPN transistor 50 configured in open
collector or common emitter mode by virtue of its emitter being coupled to
ground and its collector being coupled to a power source, namely node 32,
albeit via a 1.5K ohm, 10 watt current limiting resistor 52 and a diode 54
coupled in series. The diode 54 is disposed to provide a half-wave
rectified signal to the collector of the transistor 50, which signal has
the same polarity as that supplied to the voltage divider 34.
As also illustrated, the base of the transistor 50 is coupled to the
circuit node 42 of the voltage divider 34 via a Zener diode 60. The Zener
diode 60 has a known breakdown or breakover voltage, preferably of about
12 volts. It can be appreciated that as long as the voltage of the signal
generated by the voltage divider 34 at the circuit note 42 exceeds the
breakover voltage of 12 volts of the Zener diode 60, the transistor 50
will be turned on and in an "on" state. Otherwise, the transistor 50 will
be placed in an "off" state.
In essence, the Zener diode 60 acts as a comparator stage.
Also coupled between the collector of the transistor 50 and electrical
ground are a relay 70 and a 100 .mu.F storage capacitor 72. It can be
appreciated that while the transistor 50 is turned on, the storage
capacitor 72 is drained and the relay 70 is deenergized inasmuch as the
transistor 50 will direct current from its collector to electrical ground.
However, should the transistor 50 be turned off, the storage capacitor 72
will quickly change and the coil of the relay 70 will be energized.
As illustrated, the switch 28 is associated with the relay 70 and is
activated by same. Therefore, the switch 28 will be caused to open and
close, depending on the energization of the relay 70. When the relay is
energized, the switch 28 will be caused to closed thereby placing the
resistor 24 and capacitor 26 across the windings 18 and 20. When the relay
is de-energized, the resistor 24 and capacitor 66 are decoupled from
across the windings 18 and 20.
As can be appreciated, as loading on the motor 12 increases, the voltage
present at circuit node 32 between the reverse winding 20 and the PSC
capacitor 22 decreases. In turn, the voltage signal generated at circuit
node 42 by the voltage divider 34 decreases. When the voltages at node 32
and 42 decrease sufficiently, i.e., enough to cause the voltage of the
signal at node 42 to fall below the breakover voltage of the Zener diode
60, the transistor 50 will turn off and the relay 70 will be permitted to
rapidly energize thereby causing the switch 28 to quickly close. Once the
loading is eased, the converse will occur.
To prevent chatter or excessive oscillation between turn on and turn off of
the transistor 50, i.e., to provide sufficient hysteresis, in the above
described arrangement, in the illustrated configuration there is coupled
across the collector of the transistor 50 and the circuit node 42 of the
voltage divider 34, a circuit 80 comprising a 1.0 .mu.F capacitor 82, an
NPN transistor 84, a Zener diode 86, and a 5.6K ohm resistor 88. The
capacitor 82 is coupled between the node 42 of the voltage divider network
34 and the collector of the transistor 84. The emitter of transistor 84 is
coupled to ground. Coupled between the base of the transistor 84 and the
collector of the transistor 50 (and the capacitor 72) is the Zener diode
86 and the resistor 88.
As long as the transistor 84 is off, the capacitor 82 will remain charged
to the potential or voltage level of the signal at node 42. However, it
can be appreciated that when a voltage at the collector of the transistor
50 (i.e., of the storage capacitor 72) exceeds the breakover voltage of
the Zener diode 86 (i.e., upon occurrence of an excessive loading), the
transistor 84 is turned on. When the transistor 84 is turned on, the
capacitor 82 is allowed to discharge to ground over a calculated time
constant. The trick is to select the value of the capacitor 72 to be such
that it discharges more slowly than the capacitor 82 so that capacitor 82
can fully discharge before capacitor 72 does. This will allow the switch
28 to remain closed for a determinable short time period and then prevent
closure of the switch 28 for another determinable short time period. As a
result, the circuit 80 introduces some hysteresis into the circuit 10.
It can be appreciated that due to the foregoing configuration, the circuit
10 of FIG. 1 automatically steps up the power output of the permanent
split capacitor motor 12 when additional power is needed. This is done by
constantly sensing the voltage at the node 34 and comparing a voltage
related thereto to the breakover voltage of the Zener diode 60. As the
load increases, the voltage at node 32 decreases. When the voltage at node
32 falls below the reference voltage of the Zener diode 60, the output of
the transistor 50 switches to a high impedance, allowing the relay 70 to
quickly energize and cause switch 28 to close. As a result, the capacitor
26 is placed across the windings 18 and 20 thereby increasing the power
output of the motor 12. However, such increase in the output of the motor
should only be made temporary as the motor will overheat and the power
transferred thereto will become inefficient.
With the exception of the Zener diodes 60 and 86, the diodes preferably are
of the type generally designated IN4006. The Zener diodes 60 and 86
preferably are of the type generally designated IN4742. The relay 70
preferably is of the type designated JTIE-DC24V sold by Aromat. The
transistors 50 and 84 preferably are of the type generally designated
TIP122.
In FIG. 2, there is illustrated a paper shredder 90, in this instance,
Model 1036/1246 manufactured by the assignee of this invention, the
General Binding Corporation. Such a shredder includes an input tray 92
into which papers are fed for shredding. The shredded papers fall from
beneath an underside 94 of the shredder 90 where they are collected for
disposal.
In FIG. 3, there is illustrated a circuit 100 for implementation of the
invention in a shredder such as the shredder 90 of FIG. 2. In that regard,
there is illustrated in FIG. 3 a circuit 100 including a basic circuit 102
for operation of the PSC motor 104 of a 1036/1246 shredder as adapted to
incorporate therein a circuit 103 embodying the principles of the
invention. The basic circuit 102 provides for operation of the motor 104
and shredder in three modes, namely, ON, AUTO-REVERSE and OFF/REVERSE.
This basic circuit 102 is the subject of allowed and commonly assigned
application Ser. No. 07/970,070 filed Nov. 2, 1992, and fully incorporated
herein by reference.
Like the motor of FIG. 1, the PSC motor 104 of FIG. 3 includes run and
auxiliary windings 106 and 108, respectively, and a capacitor 110 coupled
thereacross. The windings 106 and 108 are commonly connected to a suitable
AC power source 112, preferably a 115 volt AC source. The motor 104 is
suitable coupled to a set of shredder blades 111.
In the ON mode, the motor runs in its forward direction to drive the set of
blades 111 coupled thereto for shredding action. In the AUTO-REVERSE mode,
the motor will automatically reverse when the blades are jammed, thereby
to release the material causing the jam. In the REVERSE mode, an operator
manually directs the motor 104 and blades 111 to operate in the reverse
direction to release any material in the blades 111.
The windings 106 and 108 are coupled to a relay actuated reversing switch
114 that selectively couples the run winding 106 or the reverse winding
108 to ground, depending on the direction in which the motor 104 is to be
run.
There is also included a series circuit comprising a 10 ohm, 10 watt
resistor 124, a 10 .mu.F capacitor 126 and relay activated switch 128
similar to the resistor 24, capacitor 26 and switch 28 of the embodiment
of FIG. 1. And, there is disposed between the reversing switch 114 and the
winding 108 a normally closed contact 130 of a control switch 132, the
interaction of which with the remainder of the circuit 100 is described
below. This switch 132 is used to place the circuit 100 in the OFF/REVERSE
mode.
Another switch 134 has one end coupled to a circuit node 136 located
between the power supply 112 and the windings 106 and 108 and another end
coupled to another portion of the basic circuit 102 via a 680 ohm current
limiting resistor 138. The resistor 138 carries current transmitted
through the switch 136 to a pair of branching diodes 140 and 142, the
diode 140 being further connected and biased to direct half wave rectified
current to a coil 144 and 100 .mu.F storage capacitor 146 of a first relay
and the diode 142 being further connected and biased to direct half wave
rectified current to a coil 148 and a 470 .mu.F storage capacitor 150 of a
second relay.
The first relay is constructed to selectively actuate the reversing switch
114. The second relay is constructed to actuate another switch 160
disposed to selectively connect the winding 108 to the switch 114.
It can be appreciated that when the switch 134 is closed, the capacitor 146
will rapidly charge. When the capacitor 146 is sufficiently charged, the
coil 144 of the first relay will be energized and the switch 114 will be
actuated to connect winding 108 to ground thereby causing the motor 104 to
run in its forward direction. This places the shredder in its ON mode,
with the shredder blade 111 rotating for shredding action.
At the same time that the capacitor 146 is so charged and the coil 144 of
the first relay is so energized and the switch 114 is so actuated, the
capacitor 150 is similarly charged, the coil 148 of the second relay is
similarly energized and the switch 160 is similarly actuated. Essentially,
the winding 106 is coupled to the basic circuit 102. In that regard, the
coil 106 is coupled to one side of a normally open contact 162 associated
with switch 132 and two branching resistors, 6K ohm resistor 164 and 3K
ohm variable resistor 166. The other side of the normally open contact 162
of the switch 132 is coupled to the circuit 103 via circuit node 128,
which circuit node 128 is located between winding 106 and capacitor 110
and which provides the node at which motor loading is sensed.
The first branching resistor 164 is further coupled to a diode 170 biased
and coupled to direct half wave rectified current to the capacitor 150 and
coil 148 of the second relay. The second branching variable resistor 166
is further coupled to a 5K ohm resistor 172 and a diode 174 biased and
coupled to direct half wave rectified current to the capacitor 146 and
coil 144 of the first relay.
The circuit 103 in relevant part is identical to the circuit 30 of FIG. 1
and thus a description thereof is not further provided herein except where
necessary. Like reference numerals have been provided in FIG. 3 for the
like circuit components of FIG. 1.
In addition to the foregoing, however, there is provided a 1K ohm, I watt
resistor 180 and diode 182 coupled between node 42 of the circuit 103 and
node 136 of circuit 102. As set forth above, 139 is selectively coupled to
node 136 via resistor 138 and normally open switch 134. The resistor 180
and diode 182 serves to provide half wave rectified current to the
capacitor 82 when the motor 104 is first caused to be turned on by closure
of the switch 134 so that the Zener diode 60 and transistor 50 initially
recognize a load on the motor 104 within the acceptable range. It can be
appreciated that once the switch 134 is released, the resistor 180 and
diode 182 are decoupled from the circuit node 136 so that the circuit 103
operates in the manner set forth above for the circuit 30 of FIG. 1.
The relays 144 and 148, like the relay 70, preferably are of the type
JTIE-DC24V sold by Aromat. The diodes 140, 142, 170 and 174, like the
non-Zener diodes 40 and 54, preferably are of the type generally
designated as IN4006.
In operation, the motor is turned on by momentary closing of the switch
134. This supplies current to the coils 144 and 148 of the first and
second relays, respectively, as set forth above. This, in turn, causes the
first relay to move the switch 114 to the normally open position, which
couples the winding 108 between the AC power supply 112 and ground and
starts the motor 104 rotating in the forward direction. At the same time,
the second relay causes the switch 160 to connect the winding 106 to the
branching resistors 164 and 166. Just enough current is transmitted
through the two branching resistors 164 and 166 to keep the first and
second relays energized. The variable resistor 166 sets a load threshold
on the shredder motor 104 at which a reversing operation is triggered.
It can be appreciated that the reversing threshold is set to be higher than
the threshold at which circuit 103 triggers application of the capacitor
126 across the windings 106 and 108. Thus, the shredder will first attempt
to increase the power to the motor 104, and if the load is not too
excessive (e.g., not more than a few pages above design), the motor 104
will continue to operate in forward setting. Otherwise, if the load is too
excessive, e.g, a few pages above design, then the motor 104 will be
caused to reverse in the manner established by the circuit 102.
To that end, when the load level or threshold set by the variable resistor
166 is exceeded, the circuit 102 senses the excessive load and
automatically reverses the motor 104 to remove the load. In particular,
any load on the motor causes a reduction in the voltage at the circuit
node 113. The voltage drop causes a corresponding drop in the current
through the relay windings 144 which holds the switch 114 closed. An
excessively heavy load or a stalling of the motor 104 drops the voltage
enough to release the relay contact 114 so that it returns to the normally
open position, as shown in FIG. 3. The opening of the switch 114 connects
the winding 106 to ground through switch 160, which has not opened. When
the winding 106 is so connected, winding 108 is effectively phase shifted
relative to the winding 106 and the motor 104 is, thus, caused to run in
the reverse direction.
Reverse operation of the motor 104 continues so long as the capacitor 150
remains charged at a voltage sufficient to supply holding current to the
second relay winding 148. The capacitor 150 discharges sufficiently in,
preferably, one to two seconds to permit the switch 160 to open. Opening
of the switch 160 then turns the motor 104 off.
The motor 104 also may be turned off during normal, or forward, operation
when desired by manual momentary operation of the switch 132. By
temporarily actuating the switch 132 and then releasing the switch 132,
the normally closed contacts 130 open to break the current path between
the winding 108 and ground. At that time, the capacitor 146 discharges and
the switch 114 opens and the motor 104 stops.
The motor 104 also may be manually caused to operate in its reverse
direction. To that end, if the switch 132 is held in the operated
position, the normally closed contact 130 will open thereby beating the
current path through the run winding 108. At the same time, the normally
open contact 162 closes to provide a current path to couple the auxiliary
winding 106 to ground through closed switch 160 and this causes the motor
104 to operate in the reverse direction for so long as the switch 132 is
held in the operated position. Releasing the switch 132 then stops the
motor 104. It is thus possible to manually clear a jam by holding the
switch 132 in the operated position until the jam is clear.
Although modifications and changes may be suggested by those skilled in the
art, it is the intention of the inventor to embody within the patent
warranted hereon all changes and modifications as reasonably and properly
come within the scope of the contribution to the art.
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