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Using oscilloscope on
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Electronic
components in Thyristor drives have voltages of the order of 400 volts as
well as dc +/- 5 volts, 12 volts etc. High voltages present around thryistors
are normally derived directly from three phase supply which, many a times,
has a neutral connection also. This means that there are dangerous voltages
present around thyristors with respect to neutral. Oscilloscopes operating
from mains supply voltages have invariably an “Earth” connection in the mains
cord. Internally, this is connected to the “zero” or “common” of electronic
circuitry of oscilloscope. The BNC connectors where we connect the CRO
probes, thus, have a zero volt point getting connected all the way upto the
measuring tip of the probe. C A U T I O N
: Here comes the
major risk. When the probe is connected to, say Gate-Cathode of a Thyristor,
to check firing pulses, a dead short circuit occurs between a phase voltage
and the earth, causing a big blast in
the probe shield ! To avoid such a
situation from arising, “earth” pin of the CRO must be REMOVED when using on DC drives. This
sounds against the basics of handling electrical equipment where Earthing is
a must! However in this situation it must be done with due care in handling.
It is further important to keep in mind that there are dangerous voltages
present on CRO metal parts which must not be allowed to come in contact with
human body or other metal parts connected to ground.
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Speed control not satisfactory
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