Electrical Test Instruments MAC-20 User Manual

Electrical
Test lnstruments
MAC-20 Retrofit
Package
Sectjon ll
-
Detailed
Description
SECTION II
DETAILED
DESCRIPTION
THEORY
OF
OPERATION
High Current
Test Sets
High cunent
test sets
generally
consist of
the following:
1.
One or more
coarse taps
accomplished by:
(a)
A tapped autotransformer
providing
several
equal steps of voltage
or
(b)
A
multi-section output
transformer
2. A continuously
variable vernier
adjustment between
coarse taps
by means
of:
(a)
A variable
autotransformer
and boost
transformer,
or
(b)
A variable
autotransformer
connected to a section
of the
output trans-
former.
3. An output
transformer with
one or more isolated low voltage,
high
current sec-
ondary.
Power
control circuitry consisting
of switches, relays,
protective
devices,
etc.
Current measurement
circuitry
consisting of sensors, current
meter, and
timer.
ln
general,
a
primary
voltage
is selected
by
means
of a combination
of coarse tap
and
vernier adjustment. This
voltage is stepped
down by the output
transformer to
provide
the
desired
current into the load impedance.
For
test sets using an electromechanical
contactor,
an
AC-controlled
solid-state relay
with zero-crossing detection
is often used to energize
the main contactor. This
provides
some consistency
of output waveform, since
the contactor will always
pull
in
at some
fixed
time after the zero crossing,
and hopefully not at a multiple
of a half-cycle. This
tends to minimize
the DC offset which results when
an inductive load is
energized at or
near the voltage zero-crossing.
Many modern test sets use solid-state
SCR controllers, with
electronic circuitry to
provide precise
phase
control of initial firing angle.
Solid state contactor replacement is
a retrofit option
that
is highly recommended.
MAC-20 Unit Circuitry
The
current
measurement function
of most breaker test sets is based
on the
principle
of
an air core inductor, which may be
used to sense a magnetic field, which is
proportional
to the rate of change of current flowing in an adjacent
conductor. The output voltage
of
the
inductor is
therefore
proportional
to the
rate
of change of current in the
conductor.
For
practical purposes,
the sensing inductor is usually made in
the form of a split core,
or
"fork",
which fits closely around the current-carrying buswork.
Electrical Test lnstruments,
Inc.
Page
ll-1
Copyright(C)
1994-1998
All Rights Reserved

Electrical Test Instruments MAC-20 Specifications

General

Brand	Electrical Test Instruments
Model	MAC-20
Category	Controller
Language	English