Solid material comminution or disintegration – Processes – Miscellaneous
Reexamination Certificate
2001-08-13
2003-05-13
Rosenbaum, Mark (Department: 3725)
Solid material comminution or disintegration
Processes
Miscellaneous
C241S036000
Reexamination Certificate
active
06561444
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a shredder provided with a shredding mechanism driven for shredding wastepaper by a motor and, more particularly, to a driving and controlling system for driving and controlling a shredding mechanism included in a shredder and capable of simultaneously shredding a small to a large number of paper sheets.
BACKGROUND ART
A generally known shredder comprises an induction motor, i.e., ac motor, driven by power supplied from a commercial power supply system of single-phase 100 V ac and 50 or 60 Hz, a reduction gear connected to the output of the motor and capable of reducing an input speed at a predetermined reduction ratio to a lower output speed and of increasing an input torque to a higher output torque, and a shredding mechanism, such as a rotary cutting mechanism, connected to the output side of the reduction gear.
FIG. 6
shows the speed-torque characteristic of the induction motor. The induction motor operates stably in a speed range between a point P
6
corresponding to synchronous speed and a point P
7
corresponding to stall torque. Therefore, the motor is operated in operating conditions represented by a line between the points P
6
and P
7
. When load on the motor increases, the slip of the motor increases, current supplied to the motor increases and, consequently, a high torque is produced for shredding.
When the number of superposed paper sheets to be shredded simultaneously is increased and load on the induction motor, i.e., ac motor, employed in the known shredder is increased, the operating condition of the motor changes from the side of the point P
6
corresponding to the synchronous speed toward the side of the point P
7
corresponding to the stall torque and the current increases exponentially. Since power is supplied from a commercial power supply system to the motor, the voltage applied to the motor is unchangeable. Therefore, input power to the motor increases sharply in proportion to the current. To provide for such a condition, a lead-in cable connecting the shredder to the commercial power supply system must have a capacity large enough to withstand the high current.
If the shredder is overloaded and nothing is done to rectify the undesirable condition, the power supplied to the shredder will increase beyond the rated input power of the shredder or the operating speed of the motor decreases below the speed corresponding to the point P
7
corresponding to the stall torque and stalls and the shredder is unable to exercise its function. Moreover, a current exceeding a current specified by the electrical appliance regulation law will flow through the service outlet and a circuit breaker will open the corresponding circuit to protect electrical appliances other than the shredder from overcurrents.
Generally, to avoid such a condition, the shredder is provided with means for stopping the motor before the torque of the motor reaches the stall torque at the point P
7
and reversing paper sheets taken into the shredding mechanism to return the paper sheets to the feed side. If the shredder is thus reversed, shreds and scraps of the paper sheets subjected to shredding scatter in the shredding mechanism and around a feed unit, necessitating cleaning work.
Since a condition where the shredder is unable to function normally occurs if an excessively large number of paper sheets are fed simultaneously in a pile into the shredder, a reduced number of paper sheets are fed in a pile. However, if the shredder is used generally by unspecified people who do not necessarily have sufficient knowledge of the functions of the shredder, the interruption of shredding operation will occur very often, making the operation of the shredder very complicated. Those problems are attributable to a fact that it is difficult to control the operating speed and the torque of the induction motor optionally.
Furthermore, the conventional shredder employing an induction motor, i.e., an ac motor, has the following problems.
FIG. 9
shows the operating characteristic of a shredder provided with an induction motor as a driving means.
FIG. 9
shows the variation of the operating speed of the motor with load loaded on the motor by wastepaper. In a nonloaded state P
1
where any wastepaper is not fed to the shredder, the motor operates at an operating speed N
1
. In a fully loaded state P
2
the motor operates at an operating speed N
2
. The slip of the induction motor increases when wastepaper is fed to the shredder and an actual load increases, and the operating speed decreases from N
1
to N
2
. As obvious from
FIG. 9
, the induction motor has three states, i.e., a waiting state after the start of the motor in which wastepaper is not fed yet, a shredding state where the motor is operating in conditions between the states P
1
and P
2
on the characteristic curve and a stopped state where power is not supplied to the motor. The motor is operating at high operating speeds in most of the time whether or not wastepaper is fed to the shredder.
The induction motor is designed so as to operate at a high efficiency in a high-load region to operate under a maximum shredding load by using a current not exceeding a limit current of the commercial power supply system. Therefore, the efficiency of the induction motor is low when the induction motor operates in a low-load region.
FIG. 10
shows the load-torque characteristic and the load-current characteristic of an induction of such a design. Suppose that the torque of the motor is T
3
and a current I
3
is supplied to the motor when the motor is in a state corresponding to a low-load point P
3
, and the torque of the motor is T
4
and a current I
4
is supplied to the motor when the motor is in a state corresponding to a high-load point P
4
. Then,
(
T
3
/
I
3
)<(
T
4
/
I
4
) (1)
When the motor is in the state corresponding to the high-load point P
4
, a voltage drop across the winding due to the resistance of the winding and the current I
4
necessary for producing the torque T
4
is large. The design of the winding of the motor is determined in anticipation of such a large voltage drop. However, although the intensity of the current I
3
necessary for producing the torque T
3
is low and the voltage drop across the winding is small when the motor is in the state corresponding to the low-load point P
3
, unnecessary power is supplied to the motor because the voltage applied to the motor is fixed and hence the efficiency is low. In
FIG. 10
, a curve A indicates motor current, a curve B indicates torque, and a curve C indicates excitation current.
FIG. 11
is a graph showing the variation of starting current required by an induction motor at start with time. A high starting current flows in a period between motor start time t
8
and time t
9
when the operating speed stabilizes.
The conventional shredder employing the induction motor, i.e., ac motor, has the nonloaded state P
1
where any wastepaper is not fed to the shredder, the loaded state P
1
-P
2
where the shredder is loaded with wastepaper and the operating speed of the motor decreases from N
1
to N
2
, and the fully loaded state P
2
where the shredder is operating under a full load condition as shown in FIG.
9
. However, the motor is operating at high operating speeds in most of the time whether or not wastepaper is fed to the shredder. Therefore, the motor operates at substantially fixed high operating speeds regardless of load, so that the motor and the shredding mechanism generate noise and vibrations, which deteriorates environmental conditions significantly.
Since importance is attached to the high-load state when designing the induction motor for the shredder, the induction motor operates at the highest efficiency under a high load. Therefore, in the nonloaded state or the low-load state where the shredder is not operating for shredding, the motor operates at a low efficiency as shown in FIG.
10
and consumes much power wastefully. As is obvious from the relation between the motor current indicated by the curve A and the excit
Abe Tadashi
Hori Takashi
Takahashi Tomofumi
Yokomine Shunichi
Yoneyama Hiroaki
Kabushiki Kaisha Meiko Shokai
Ladas & Parry
Rosenbaum Mark
LandOfFree
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