Blown fuse sensor

Each arm forming a three-phase full wave rectifier bridge includes a semiconductor diode, a current transformer and a fuse. The current transformer is connected to a pulse extinction indicator circuit through a pulse transformer. When no current flows through the arm for a predetermined time or more, the pulse extinction detection circuit produces a DC signal indicating the blowing of the fuse. The rectifier bridge, the current transformers, the fuses and the pulse extinction detection circuit are all disposed on the rotor of a brushless synchronous machine.

BACKGROUND OF THE INVENTION 
This invention relates to a device for sensing the blowing of a fuse 
connected in series with a rectifier element forming a rectifier device, 
and more particularly to a blown fuse sensor device for use with a rotary 
rectifier to sense externally and automatically the blowing of a fuse due 
to a shortcircuit of the rectifier element involved. 
Brushless synchronous machines, for example, brushless AC generators, are 
provided with a rotary rectifier for rectifying the output from an AC 
exciter employed and for energizing the field of an associated main 
generator. The rotary rectifier normally has a three-phase bridge 
configuration and includes a protective fuse serially connected with each 
of at least two rectifier elements disposed on each rectifier arm of the 
bridge. When a shortcircuit occurs in any one of the rectifier elements, 
the associated fuse is blown thereby protecting the remaining sound 
rectifier elements from any overcurrent. 
In order to sense the blowing of the protective fuse in the conventional 
type of rotary rectifiers such as described above, it is already known to 
monitor the behavior of the fuse by means of a stroboscope either by 
causing a visible change such as the firing of an indication lamp due to 
the blowing of the fuse or by imparting a mechanical indication function 
to the fuse itself. Such measures are premised on the monitoring of rotary 
rectifiers through visual observation and the reliability thereof depends 
on whether or not it is certain that, upon the occurrence of a fault on 
any rectifier element, the behavior indicator can be visually monitored. 
This requires a great deal of labor. 
It is also previously known to employ special fuses having a contact 
mechanism attached thereto in order to sense the blowing of the fuses. 
This measure causes the possible fear that a malfunction may be caused by 
the bad engagement of the contacts involved and is disadvantageous in 
that, with such a fuse required to be disposed on the rotor of a brushless 
synchronous machine, there are fears that the rotor may be rotated in an 
unbalanced state because the fuse has a mechanically moving member, the 
design and construction of rotary rectifiers lacks versatility and so on. 
Accordingly, it is an object of the present invention to provide a new and 
improved blown fuse sensor device for surely sensing the blowing of a fuse 
without the necessity of using mechanical contacts or the like. 
It is another object of the present invention to provide a new and improved 
blown fuse sensor device formed of a contactless circuit to prevent 
malfunction caused from the bad engagement of contacts and not including 
any mechanically moving member. 
It is still another object of the present invention to provide a new and 
improved blown fuse sensor device capable of being installed on the rotor 
of a brushless synchronous machine to prevent the rotor from rotating in 
an unbalanced state and also to alleviate structural limitations. 
SUMMARY OF THE INVENTION 
The present invention provides a blown fuse sensor device comprising a 
series circuit including a fuse, a current detector means connected in the 
series circuit to detect the current flowing through the series circuit, a 
converter means responsive to an output from the current detector means to 
produce a pulse signal, and a signal generator means for generating a 
signal indicating the blowing of the fuse when a pulse signal from the 
converter means is not applied thereto for at least a predetermined time 
interval.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 of the drawings, there is illustrated a 
conventional blown fuse sensor device for use with a rectifier device. The 
arrangement illustrated comprises a three-phase full-wave rectifier 
circuit arranged in a three-phase bridge configuration including a 
plurality of branches or arms each formed of a series circuit of a 
rectifier element 10 such as a semiconductor diode and a fuse 12. A set of 
contacts 16 is operatively coupled to each of the fuses 12 to be closed in 
response to the blowing of the associated fuse 12 to thereby produce an 
electrical signal. The rectifier circuit is connected to a three-phase 
load 14. 
In operation three-phase AC power from a three-phase electric source (not 
shown) is rectified by the respective rectifier elements 10 and is then 
supplied, as DC power, to the load 14 through the associated fuses 12. It 
is assumed that a fault such as a shortcircuit occurs in any of the 
rectifier elements 10 causing an excessive current to flow through that 
series circuit including the failed rectifier element 10. Under the 
assumed conditions, that fuse 12 connected to the failed rectifier element 
10 is fused or blown to prevent the continuation of an abnormal state due 
to excessive current. At the same time, that contact set 16 operatively 
coupled to the blown fuse 12 is closed to produce an electrical signal. 
This signal is applied to a suitable processing circuit (not shown) and is 
used as an alarm indicating the blowing of a fuse or a command for 
suspension of the operation. 
In the arrangement of FIG. 1, however, it is necessary to employ special 
fuses having the contact mechanism annexed thereto and, in addition, there 
has been a fear that a malfunction may occur due to the bad engagement of 
the contacts because the contact sets are used as signal sources. 
Furthermore, when the arrangement of FIG. 1 is required to be disposed on 
the rotor of a brushless synchronous machine, it is disadvantageous in 
that, an unbalance may take place in the rotational movement of the rotor 
because the rotor is mechanically moved, and the design and construction 
of the rectifier device lacks the versatility and so on. 
The present invention contemplates elimination of the disadvantages of the 
prior art practice as described above by providing a blown fuse sensor 
device for surely sensing the blowing of a fuse without the necessity of 
employing mechanical contacts or the like. 
In FIG. 2, wherein like reference numerals designate components identical 
to those shown in FIG. 1, there is illustrated one embodiment of the blown 
fuse sensor device of the present invention. The arrangement illustrated 
is different from that shown in FIG. 1 only in that in FIG. 2, a current 
transformer 18 is connected in each series circuit consisting of the 
rectifier element 10 and the fuse 12 having its primary winding formed by 
the lead interconnecting the rectifier element 10 and the fuse 12. The 
current transformers 18 each includes a secondary winding connecting 
across the primary winding of an electrically insulating transformer in 
the form of a pulse transformer 20 that includes a secondary winding 
connected to a pulse extinction detection circuit 22. 
The operation of the arrangement shown in FIG. 2 will now be described with 
reference to FIG. 3 which illustrates waveforms developed at various 
points in the arrangement of FIG. 2. With the arrangements operated in the 
normal mode, each of the rectifier elements 10 has flowing therethrough a 
current having a current waveform as shown in FIG. 3. This current 
simultaneously forms the primary current flowing through the current 
transformer 18 to induce a voltage across the secondary winding of the 
current transformer 18. The primary current flowing through the current 
transformer 18 is a unidirectional pulsating current as shown by the 
waveform as in FIG. 3. More specifically, the current begins to flow 
through the current transformer 18 at time points t.sub.1, t.sub.2 or 
t.sub.4 and rises to a saturated magnitude after which it falls to a null 
magnitude. Therefore, the secondary winding of the current transformer 18 
can induce voltage pulses in response to the rise and fall of the 
pulsating current alone as shown by the waveform b in FIG. 3. The voltage 
pulses thus induced are applied via the pulse transformer 20 to the pulse 
extinction detection circuit 22 where the voltage pulses are rectified to 
be converted to the voltage pulses as shown by the waveform c in FIG. 3. 
From FIG. 3 it is seen that the voltage pulse appearing in the pulse 
extinction detection circuit 22 results from the rise of the pulsating 
current flowing through the primary winding of the current transformer 18. 
The pulse extinction detection circuit 22 is designed and constructed so 
that it does not operate when pulse voltages are applied thereto at 
predetermined time intervals or less but produces a DC signal only when 
voltage pulses from the pulse transformer 20 are not applied thereto for a 
predetermined time interval or more. In the example illustrated, the pulse 
extinction detection circuit 22 produces the DC signal as shown by the 
waveform d in FIG. 3 only when those voltage pulses are not applied 
thereto for the predetermined time interval between time points t.sub.2 
and t.sub.5 or longer. This predetermined time interval is longer than the 
time interval between the rise of each pulsating current portion and that 
of the next succeeding one or between time points t.sub.1 and t.sub.2 or 
t.sub.2 and t.sub.4. 
From the foregoing it is seen that the pulse extinction indicator circuit 
produces no output signal so long as the arrangement of FIG. 2 is operated 
in the normal mode. 
It is now assumed that any one of the fuses 12 shown in FIG. 2 is blown at 
the time point t.sub.3 after time point t.sub.2 (see FIG. 3, waveform a). 
Under the assumed conditions, no current flows through the primary winding 
of that current transformer 18 operatively coupled to the blown fuse 12 at 
and after the time point t.sub.3. Therefore no voltage is induced across 
the secondary winding of that current transformer 18. This means that at 
time point t.sub.4 the pulse extinction detection circuit does not have 
applied thereto the voltage pulse that is to be normally applied thereto 
at that time point before time point t.sub.5 is reached. That is, the 
predetermined time interval of the circuit 22 expires. Thus the pulse 
extinction detection circuit 22 produces a DC output signal at time point 
t.sub.5 as shown by the waveform d in FIG. 3. 
The signal from the circuit 22 indicates that the associated fuse 12 has 
been fused or blown. In other words the blowing of any fuse can be 
indicated by a DC output signal from the associated pulse extinction 
detection circuit 22. 
There are a variety of known types of pulse extinction detection circuits 
22 as described above. For example, reference may be made to "'76 
Mitsubishi Semiconductor Handbook-Integrated Circuits", page 737 published 
on Apr. 15, 1975 by Seibundo-Shin-kosha. In the pulse extinction detector 
shown in FIG. 7, when an input of a pulse train from a trigger terminal is 
inputted, as a trigger pulse, to the circuit, a change in pulse frequency 
and a pulse extinction can be detected. In this case, however, it is 
required to set a delay time somewhat longer than the repetition time of 
the input of the pulse train. The entry of one trigger pulse causes an 
electric charge on a timing capacitor C.sub.A to discharge and a FLIP-FLOP 
to be set resulting in an output "H". 
Accordingly when the input pulses are maintained at the normal pulse 
spacing, the next trigger pulse enters the connection before a voltage 
across the timing capacitor C.sub.A reaches a threshold voltage. The 
timing capacitor C.sub.A again discharges to maintain the output "H". 
However, upon the occurrence of an abnormality in the input pulses, i.e., 
upon the extinction of the pulse input, the voltage on the timing 
capacitor C.sub.A reaches the threshold voltage so as to reset the 
FLIP-FLOP resulting in the output changing to "L". Integrated circuit 
M51841P Shown in FIG. 7 is available commercially from Mitsubishi Electric 
Co., Tokyo, Japan. 
The present invention will be further described in conjunction with FIGS. 
4, 5, and 6. 
In FIG. 4 the rotary shaft 100 of a brushless AC generator includes an AC 
exciter having its rotor 91 disposed thereon and its stator 92 disposed on 
a stationary portion opposite the rotor 91. The AC exciter forms a source 
of three-phase alternating current designated by the three lines connected 
to the diodes 10 in FIG. 2. A diode wheel 80 is disposed on the rotary 
shaft 100 located to the left of the exciter 90 and includes a plurality 
of series combinations of a rectifier element 10 and a fuse 12 disposed at 
equal angular intervals thereon as shown in FIG. 5. Outputs of the 
rectifier elements are connected to a rotor winding of an AC generator 
(not shown) disposed on the rotary shaft 100. The rotor winding is shown 
in FIG. 2 as comprising the load 14. 
As shown in FIG. 4 a conductor 70 connects each of the rectifier elements 
10 to its associated fuse 12 and also serves as the primary winding of the 
current transformer 18 also disposed on the diode wheel 80. 
As best shown in FIG. 6 the conductor 70 is sandwiched between two legs of 
a U-shaped magnetic core 18a of the current transformer 18 through an 
electrically insulating sleeve 18c. The core 18a is composed of a stack of 
magnetic laminations and has a secondary winding 18b wound therearound. 
The output from the secondary winding 18b is supplied to the transformer 
20 through a pair of leads 60 running along the rotary shaft 100. The 
pulse transformer 20 for each current transformer 18 and the pulse 
extinction detection circuit 22 connected thereto are disposed on a 
telemetry wheel 50 that is, in turn, disposed on the rotary shaft 100 on 
that side of the excitor 90 remote from the diode wheel 80. Outputs from 
all the pulse extinction circuits 22 are applied to an antenna 40a 
disposed on the telemetry wheel 50. The signal from the antenna 40a is 
received by an antenna 40b disposed on the stationary portion. This 
telemetry circuit may be as disclosed in U.S. application Ser. No. 
936,504, filed Aug. 24, 1978. Because this telemetry circuit forms no part 
of the present invention, further description thereof is omitted for 
brevity. Note that the components as described above are disposed on the 
rotary shaft 100 to ensure dynamic balance. 
In summary, the present invention is so constructed that a current 
transformer is connected in each series circuit including a rectifier 
element and a fuse to thereby sense the blowing of the fuse. Further the 
present invention is formed of a contactless circuit without the necessity 
of using a special fuse including a contact set and accordingly 
malfunctions due to the bad engagement of contacts which might have 
occurred previously do not occur. Also, because the present invention does 
not include any mechanically moving part, limitations as to the 
construction can be alleviated. 
While the present invention has been illustrated and described in 
conjunction with a single preferred embodiment thereof it is to be 
understood that numerous changes and modifications may be resorted to 
without departing from the spirit and scope of the present invention.