Electrical Test Instruments PI-2500 & PI-5000
Section II - Detailed Description
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Copyright(C) 1994-2008 Electrical Test Instruments, LLC. All Rights Reserved
Page II-12
current power to the test set output circuitry through the SCR controller. The
switched output voltage is supplied to distribution blocks DB1 and DB2.
The Vernier autotransformer T1 and output transformer T2 are connected to in-
put voltage selector switch S11. This allows for connection in series for 480 VAC,
and parallel for 208/240 VAC. Vernier T1 is connected directly to the first primary
of T2, so that clockwise rotation produces an output voltage of about 0-1.5 VAC.
Relays K3, K4, and K5 connect the second primary windings for fixed outputs of
0, 1.25, or 2.50 VAC for taps 1, 2 and 3. This provides three (3) overlapping output
ranges up to 3.75. 7.5 or 15 VAC, depending on how the four secondary windings
are connected.
Output current is detected by means of an air core CT around all four main bus-
ses. The configuration switch can be set to 1000, 2000, or 4000, which divides
the signal by a factor of 4, 2, or 1. Calibration of all three modes is accomplished
with potentiometers on the switch assembly under the output face plate.
The output section is energized when a signal is applied to the trigger board of
the SCR controller. The initial phase angle is determined by a zero-crossing de-
tector in the trigger PCB, and a delay which is normally preset to 70 degrees.
There are three solid state relays, one of which powers the Vernier assembly, and
the others power the second primary on the output transformer.
The Vernier autotransformer may be adjusted manually by means of a hand-
wheel, or by activating the motor drive. Increasing output is clockwise for all
taps.
When the SCR board is energized, 12 VDC is applied to the control panel which
is used for output ON detection and indication.
Thermistor TH2 senses the temperature of the output bus, and TH1 reads the
temperature of the first primary of output transformer T1. These normally have
a resistance of 10 K ohms at room temperature, and lower values as the temper-
ature increases.
Relays K3 through K5 are energized through the indicator control panel and the
control PCB. They may be equipped with electrical interlocks to avoid damage
if all three are simultaneously energized. The relays actually have 24 VAC coils
which are driven with a pulse of 24 VDC long enough to cause them to seal in,
and then are held by a lower current through power resistors on the control
PCB. This type of contactor is not available with a DC coil, and DC was chosen to
avoid relay chatter that can occur with AC relays if line voltage momentarily
drops.
The adjustable Vernier assembly T2 in the PI-2500 consists of a stack of four 120
VAC decks, with each series connected pair connected to a 240 VAC primary on
T1A. Minor voltage imbalance up to about 2 VAC is corrected by means of paral-
leling transformer T3, and actual current in each primary circuit is monitored by
means of 50:5 current transformers T4 and T5. Resistors R1 through R4 ensure
