Electricity: electrical systems and devices – Safety and protection of systems and devices – Motor protective condition responsive circuits
Reexamination Certificate
1999-11-04
2001-06-26
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Motor protective condition responsive circuits
C361S024000, C361S029000
Reexamination Certificate
active
06252752
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an overload protection apparatus for protecting a motor and a driving mechanism driven by the motor from an overload, and concerns a speed reducer provided with such an apparatus.
FIG. 1
is a circuit diagram that shows a feeder circuit for a three-phase motor and a conventional overload protection apparatus for protecting a driving mechanism driven by the three-phase motor from an overload, and respective lines from a three-phase power source of R, S and T phases, not shown, are connected to three lines of U, V and W phases respectively connected to three terminals of a motor M with a molded case circuit breaker (CB)
91
interpolated in between. The respective lines of U, V and W phases are connected to the three-phase motor M for driving, for example, a conveyor belt, with a electromagnetic contactor (MC)
92
interpolated in between. The lines of U and W phases, which connect the electromagnetic contactor
92
and the three-phase motor M, are provided with thermal relays (OCR)
93
, and the line of W phase is further provided with a current transformer
94
. The output of the current transformer
94
is inputted to the input terminals k and l of an overload protection apparatus
90
. The overload protection apparatus
90
is interpolated between the U-phase line and the W-phase line, and between the b terminal of the overload protection apparatus
90
and the W-phase line are interpolated a push button switch for stopping
95
, a parallel circuit between a push button switch for starting
96
and an electromagnetic contactor
92
, and a series circuit consisting of the electromagnetic coil of the electromagnetic contactor
92
and a normally-closed contact of a thermal relay
97
.
The following description will discuss the operation of such an overload protection apparatus
90
. When the push button switch for starting
96
is closed with the molded case circuit breaker
91
being closed, the electromagnetic coil of the electromagnetic contactor
92
is excited, the electromagnetic contactor
92
is closed, and three-phase currents are allowed to flow through the three-phase motor M so that the motor M is started. Here, the current transformer
94
detects the motor currents, and the resulting output is inputted to the overload protection apparatus
90
, with the result that the overload protection apparatus
90
obtains an output voltage V that is proportional to the motor driving current I. Then, the overload protection apparatus
90
compares the levels of the output voltage V and a starting detection reference voltage E
1
, and when V>E
1
, detects the starting of the three-phase motor M.
After a predetermined time has elapsed from the detection of the starting, the output voltage V obtained by the overload protection apparatus
90
and an overload detection reference voltage E
2
are compared in their levels, and when V>E
2
, time counting is started, and after a predetermined time has been counted, the electromagnetic coil is demagnetized, with the result that the electromagnetic contactor
92
is opened, and the motor driving current I is cut off so that the motor M is stopped; thus, the motor and the driving mechanism are protected from the overload.
However, in the conventional overload protection apparatus of this type, the voltage V that is proportional to the motor driving current I is detected, and this is compared with the overload detection reference voltage E
2
that has been preliminarily set, and if the voltage V exceeds the reference voltage E
2
, it is judged as an overload so that a trip signal is outputted; thus, the electromagnetic contactor
92
for opening and closing the power supply line to the motor M is allowed to open by the trip signal so that the motor driving current is cut off.
However, the following problems arise in the conventional arrangement as described above.
(1) Since the motor driving current is cut off by disconnecting the power supply line by using the electromagnetic contactor
92
, a complex wiring job is required upon installing the overload protection apparatus
90
.
(2) It is difficult to install the overload protection apparatus in a motor driving system without the electromagnetic contactor
92
, such as a motor driving system using a knife switch or a push button switch.
(3) In the driving mechanism driven by a motor, in the case of a low ambient temperature, an increased motor output torque is required for driving due to high viscosity in the grease, etc., and an increased motor driving current is consequently required. Therefore, in the case of a lower ambient temperature as compared with the temperature at the time of the setting of the overload detection reference value of the overload protection apparatus, there is a possibility that the overload protection apparatus might trip unnecessarily.
BRIEF SUMMARY OF THE INVENTION
One of the objects of the present invention is to provide an overload protection apparatus with a semiconductor switch for cutoff of the power supply which allows a cutoff of a motor driving current even in a power-connecting state, makes it possible to simplify the wiring process, and is easily installed even in the case of the application of a knife switch or a push button switch, and also to provide a speed reducer provided with such an overload protection apparatus.
Another object of the present invention is to provide an overload protection apparatus which can detect an overload accurately, and readily address the overload by compensating for variations in the overload detection reference value due to temperature variations, and also to provide a speed reducer having such an overload protection apparatus.
The overload protection apparatus of the present invention is provided with: a semiconductor switch installed in a power supply line for a motor; an overload detection circuit having a current detector, installed in the power supply line, for detecting a motor driving current, an overload detection reference value setting circuit for setting an overload detection reference value and a first operational amplifier for comparing a voltage that is proportional to the motor driving current detected by the current detector with the overload detection reference value set by the overload detection reference value setting circuit, and for outputting a signal corresponding to the result of the comparison; a starting circuit having a first comparator for comparing a voltage corresponding to the motor driving current upon starting the motor with a first reference value that is a reference voltage at the time of starting the motor, and for outputting a signal corresponding to the result of the comparison; a trip circuit having a second operational amplifier for comparing a value of integration obtained by integrating an output of the first operational amplifier with a second reference value, and for outputting a signal corresponding to the result of the comparison, and a second comparator for comparing an output of the second operational amplifier with the first reference value, and for outputting a signal corresponding to the result of the comparison; and a switch operation circuit, installed between a constant-voltage source and an output end of the second comparator and/or an output end of the first comparator, for outputting a signal in response to the outputs of the first and second comparators so as to open and close the semiconductor switch.
In the overload protection apparatus of the present invention, the motor driving current is cut off by the semiconductor switch at the time of an overload so that the motor driving current can be cut off at a higher speed as compared with the conventional electromagnetic contactor; thus, it is possible to reduce damage in the event of an overload trip.
Moreover, the overload detection reference value setting circuit is provided with a temperature-sensitive resistor and a variable resistor that are series-connected to a constant-voltage source. Therefore, the overload detection reference value
Helfgott & Karas P.C.
Sherry Michael J.
Tsubakimoto Chain Co.
LandOfFree
Overload protection apparatus and a speed reducer having... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Overload protection apparatus and a speed reducer having..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Overload protection apparatus and a speed reducer having... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2544751