A DEVICE FOR ON LINE DETERMINATION OF SCR STATUS IN DRIVE
SYSTEMS
FIELD OF INVENTION
The present invention deals with online determination of SCR status in drive
systems. More particularly the present invention deals with line monitoring of SCR
(Silicon Controlled Rectifier) status in thyristorised drives. More particularly, the
presently disclosed device consists of only minimum simple discrete electronics
components.
Background Art:
Faults in a power supply may exist as a phase loss, an under-voltage condition, an
over-voltage condition, or other condition. Such faults may result from a variety of
reasons including breaker trips, fuse loss, inverter faults, shorted turns in a
transformer, and ground faults. Frequency or phase errors also occur when the AC
power frequency falls outside of specified ranges. Power supply faults have
deleterious impact on electrical equipment, and in many applications cannot be
tolerated.
Thyristorised controlled drives are common in todays electrical industrial
applications, Thyristors are gate pulse controlled semi conductor devices. Thynstor
does not conduct when there is no pulse in gate or pulse missing at gate, hence
creates maloperation. To find out the particular missing pulse, all the pulses are to
be checked by oscilloscope making the power circuit off. Oscilloscope Time division,
Volts/ division setting is to be done. Then one by one pulse is checked which is a
time consuming process.

Whenever there is a fault in a thyristorised drive, the general fault finding practice is
to switch off main power and to give the auxiliary power for control circuit so that
gate pulses can be released from the controller. Then pulses are observed across
gate and cathode of thyristors with the help of a oscilloscope.
In this process all the pulses are checked one by one. Pulse shape and size are
noted down. If one or more pulses are missing then fault is traced back. This is a
time consuming process. Monitoring of pulses and their comparison takes time.
Objects of invention:
Therefore the present invention is basically targeted to monitor the healthy ness of
the SCRs in on line condition. Hence fault finding during breakdown or failure of
thyristorised controlled systems becomes more easy and less time consuming. The
device is very simple in construction and easy to operate.
The principle object of the present invention is to provide a simple circuit to check
the pulses as well as to know the on line status of SCRs.
Another exemplary object of the present invention is to provide a device, wherein it
is possible to observe all the forward or reverse pulses in circuit at the same time.
Another object of the present invention is to provide facility of observation to monitor
12 or 24 pulses as per requirement.
Another exemplary object of the present invention to provide the on line status of
thyristors which was not possible prior to this invention. Moreover to check the
pulses in minimum time and to find out which pulse is missing thereby making the
fault finding procedure easy with the involvement of minimum person.

Another object of the present invention is to provide the on line status of all the
SCRs and condition of gate pulses simultaneously. If any pulse is missing then it is
found out instantly and rectification done immediately. (Fault finding chart helps in
pin pointing the particular fault)
Also the additional objectives of the present invention to reduce the fault finding time
and making the entire fault finding procedure easier with the involvement of
minimum persons.
Another embodiment of the present invention is to provide a circuit which can be
extended to monitor any number of gate pulses individually. Now a days many pulse
Transformers are have LED indication in primary side. If the pulse transformer is not
having primary side LED this can also be incorporated.
These together with other objects of the invention, along with the various features of
novelty, which characterize the invention, are pointed out with particularity in the
claims annexed to and forming a part of this disclosure. For a better understanding
of the invention, its operating advantages and the specific objects attained by its
uses, reference should be had to the accompanying drawings and descriptive matter
in which there are illustrated preferred embodiments of the invention.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
The invention will be better understood and objects other than those set forth above
will become apparent when consideration is given to the following detailed
description thereof. Such description makes reference to the annexed drawings
wherein:

Fig 1 illustrates the circuit diagram of the device for on line determination of SCR
status in accordance with the present invention.
Fig 2 illustrates the circuit diagram of the device when the input led present in pulse
transformer block in accordance with the present invention.
DETAILED DESCRIPTION
This invention is one such effort related to tryristorised drives. AC or DC motors are
heart of any electrical system. In industries mechanical work is done by electrical
control of motors. Speed control of motors by thyristors is a popular technology used
in industries. This invention is related to thyristorised drives of any electrical system.
In present highly competitive market to earn profitability cost reduction is a vital
aspect. Cost of production increases whenever there is a breakdown. The goal of
maintenance people in a production sector is to increase machine availability by
reducing breakdown time. In spite of all preventive maintenance breakdowns occur.
It is the endeavor of the maintenance people to resume production with in minimum
time. This requires quick fault finding technique and rectification procedure. Fault
finding is more difficult and time consuming than rectification activity. Delay due to
electrical problems can be reduced substantially if pin pointing of fault is possible.
The construction is very simple. Number of discrete components used is minimum.
Per thyristor only two i.e. one resistance and one LED. As gate- cathode circuit is
low resistive and sensitive one, most care is taken to reduce any spurious noise by
pulse transformer block or auxiliary circuit. This device also takes care of this point.
Hence G-K terminals are connected with twisted pair cable.
Though the circuit is very simple it's advantages are many. With the use of this
device: -

1) Status of individual SCRs in the drive can be known from the visual
indication of LEDs.
2) Individual pulse status and their comparison possible.
3) Fault finding in thyristorised drives becomes easier and less time
consuming.
4) Men power required for fault finding reduces (one person can do the total
fault finding job).
Normally the pulse Transformer output from G-K goes to gate and cathode of
thyristor respectively. Our device terminal G and K is connected to G-K respectively
as shown in figure 1. Hence our indicating device is connected in parallel to the
existing circuit of G-K terminals of pulse transformer. A parallel circuit always draws
some current from the original circuit. Here utmost attention is given to select the
design resistance so that this circuit does not draw more than 10% of the original
gate current. Generally hard firing is used in industrial applications, which is 200%-
250% of nominal gate current required to turn 'ON' the thyristor. Hence 10% of this
current (maximum) if drawn by this device does not affect normal operation of the
thyristorised drives.
If G-K of thyristor is open the LED glow increases substantially, indicating the failure
of the thyristor. If LED does not glow pulse is missing, either from firing card or from
the pulse transformer. For fault finding refer fault finding charts.
This device consists of a number of channels usually 6 Each individual circuit
consists of a specific series resistance and a LED. The series resistance value is so
selected that the glow of LED is sufficient and this circuit does not draw more than
10% of lg (Gate Current).
This device consists of 6 numbers of individual circuits. The number may increase or
decrease according to requirement. Each individual circuit consists of a series

resistance (designed value) and a LED. The series resistance value should be so
selected that the glow of LED is sufficient.
The LED glows when the gate pulse is present as it is connected parallel to the Gate
- Cathode terminals. Hence easily indicates the presence of Pulse.
The device is simple in construction and easy to operate. This device is connected
permanently in circuit, to indicate the gate pulse condition and the healthiness of the
system continuously. In case of any damage of Gate- Cathode junction of SCR, the
LED 2 glow bright LED 1 indicates the healthy condition of the firing circuit up to
primary side of pulse transformer. Hence fault finding becomes easy and less time
consuming. This device can have any number of channels according to requirement.
For every thyristor there should be provision for one channel. For Fault Finding refer
fault finding charts.
Usefulness of the innovation :
(a) Technical / Technological:
- Indicates the health of SCR in on line condition.
- Also indicates the availability of firing pulse at secondary of pulse
transformer.
- Also indicates pulse of insufficient strength (missing pulse) to trigger
the SCR
(b) Economic:
i) Low downtime.
ii) Reduction in Manpower.
fc) Impact in Industry :
Manufactures can incorporate this in their design of pulse controller circuit.

FAULT FINDING CHART WHEN INPUT LED PRESENT IN PULSE
TRANSFORMER BLOCK



The above-described embodiments of the invention are intended to be examples of
the present invention. Numerous modifications changes and improvements within
the scope of the invention will occur to the reader. Those of skill in the art may effect
alterations and modifications thereto, without departing from the scope of the
invention, which is defined solely by the claims appended hereto.

We claim:-
1. A device for on line determination of SCR status in drive systems comprising
of:
a circuit with at least one LED connected in parallel;
at least one resistance connected in series with the LED;
and a twisted cable for the interconnection.
2. A device for on line determination of SCR status in drive systems as claimed
in claim 1, wherein the number of LED and the series resistance are increased in
proportion to the number of thyristors in the circuit.
3. A device for on line determination of SCR status in drive systems as claimed
in claim 1, wherein the circuit is configured to record all the forward or reverse
pulses in circuit at the same time.
4. A device for on line determination of SCR status in drive systems as claimed
in claim 1, wherein the circuit is configured to monitor 12 or 24 pulses as per
requirement.
5. A device for on line determination of SCR status in drive systems as claimed
in claim 1, wherein the series resistance value is so selected that the glow of LED is
sufficient and this circuit does not draw more than 10% of lg (Gate Current).
6. A device for on line determination of SCR status in drive systems as claimed
in claim 1, wherein the circuit is configured to provide the on line status of all the
SCRs and condition of gate pulses in a circuit simultaneously.
7. A device for on line determination of SCR status in drive systems as claimed
in claim 1 wherein the circuit is extended to monitor any number of gate pulses
individually.

8. A device for on line determination of SCR status in drive systems
substantially as herein described with particular reference to accompanying
drawings.

The present invention deals with online determination of SCR (Silicon Controlled
Rectifier) status in drive systems, that is it deals with line monitoring of SCR (Silicon
Controlled Rectifier) status in thyristorised drives and the said device consists of only
minimum simple discrete electronics components.