ETI PI-800 Instruction Manual page 14

Electrical Test Instruments MAC-21
Section III – MAC-21
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than about 160 VAC, this relay energizes to indicate connection to a voltage of
208 to 264 VAC. The Input Select Error indicator DS2 lights if it senses an incor-
rect connection.
The input voltage selector switch S1 sets the primary windings of transformer
T4 in parallel in the 120 V position, and in series in the 240 V position. When the
control power switch S3 on the front panel is pressed, and all voltage interlocks
are OK, relay K4 is energized, which provides 120 VAC control power to the entire
test set. If so equipped, this also powers the auxiliary GFI outlet in the MAC-21,
protected by a 4-ampere circuit breaker.
If all power interlocks are OK, a second set of contacts on S3 activate the main
contactor K1. This provides high current power to the test set output circuitry
through the SCR controller.
The dual vernier autotransformer T1 and T2, and output transformers T5 and T6,
are connected to input voltage selector switch S1. This allows for connection in
series for 240 VAC, and parallel for 120 VAC. Verniers T1 and T2 are connected
through paralleling transformer T3 and the POD, directly to the primaries of out-
put transformer T4, so that clockwise rotation produces an output voltage of
about 0-0.84 VAC for each secondary turn. Relays K2 and K3, energized by tap
switch S2, connect the primary windings of T5 and T6 to main power, for fixed
outputs of 0, 0.78, or 1.56 VAC for taps 1, 2 and 3, for a maximum of 2.40 VAC per
turn. The outputs may be connected in series for 4.8 and 9.6 VAC.
Output current is detected by means of air core CTs L1-L4 on each main bus.
Switch S7 on the calibration board is set to correspond to the output configura-
tion of 400, 800, or 1600 amps. Calibration of each mode is accomplished with
potentiometers on the calibration board 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.
The output of the SCR is distributed to T1 and T2, and output transformers T5
and T6 through the tap relays. This voltage is also supplied on connector J6 for
the PI-Aux Boost Unit.
The Vernier autotransformer is adjusted manually by means of a knob. Increas-
ing output is clockwise for all taps.
When T1 and T2 are energized, 120 VAC is supplied to neon lamp DS14 on the
rear panel, and 20 VAC is supplied to lamp DS2 on the front panel. This provides
a visual indication from the front and rear when output is on.
Thermistor TH1 senses the temperature of the output bus. This normally has a
resistance of 10 K ohms at room temperature, and lower values as the temper-
ature increases. This value is read by the POD and used to compute and display
the virtual temperature of the test set on LEDs DS10-DS12.
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