Electrical Test Instruments PI-4000B
Section II - Detailed Description
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Copyright(C) 1994-2004 Electrical Test Instruments, Inc. All Rights Reserved
Page II-3
The vernier uses a large wheel to adjust the output manually within the range
determined by the coarse tap, as well as a motorized control actuated by means of a
smart control switch. When the switch is pressed briefly, the vernier moves in a very
small increment for fine control. When the switch is held, the vernier moves very quickly,
so that the entire span may be traversed in less than five seconds. The approximate
vernier position is displayed on an LED indicator bar graph. For the PI-4000B, the
vernier is always turned clockwise to increase output.
The electronically controlled tap selection uses power contactors to set the coarse tap.
The unit powers up in the lowest tap position (1), and the setting may be adjusted by
means of a rocker switch on the indicator/control panel. If the switch is held, the taps
change at a rate of about one per second. Tap change is not allowed when output is on.
The indicator/control panel features two temperature indicators: one for the output bus
temperature, and the other for the overall system temperature. This is determined by a
combination of actual winding temperature, and a “virtual temperature” which is derived
from time and current duty cycle usage as monitored by a “POD”, or Programmable
Overload Device. Additional details are available in a separate manual. If either
temperature exceeds a safe operating level, the interlock is asserted and the output
section is de-energized. Additional thermostatic sensors in the output transformer
windings will also assert the interlock if unsafe temperature levels are detected.
The output voltage of the test set is provided on the indicator/control panel for
monitoring by means of a separate digital voltmeter.
Primary catastrophic overload protection for the test set is accomplished with input
fuses having high interrupting current capacity. Overload of the output system is sensed
by means of the previously mentioned “POD” on the vernier autotransformer, and
thermal sensors in each winding and on the SCR. The combination of these devices
allows full utilization of the overload capability of the test set, and eliminates costly and
cumbersome fuse replacement required in other test sets.
The output of the test set is controlled by means of a proven SCR controller. This
provides precise initial phase angle control to reduce DC offset for inductive loads and
more consistent pulse currents. The phase angle is internally adjustable.
The measurement of output current and time is accomplished by means of the proven
MAC-21, which uses highly accurate A/D circuitry and intelligent firmware to resolve
true-RMS values of distorted waveforms. It also provides features such as preset ON
times for convenient jogging of output current without overheating the breaker or
causing unwanted trips.
The entire test set has been designed for reliability, ruggedness, and ease of use.
Sophisticated electronics in the indicator/control panel and the MAC-21 provide state of
the art accuracy and convenience, while simple connections allow for easy removal and
replacement for transportation or service. All covers are removable, allowing easy
access to all internal components. A horizontal relay panel located just under the top
panel contains most of the circuitry, facilitating service and maintenance. Two large
front-mounted fans provide maximum airflow to the transformer and output bus, and
rear exhaust directs air flow away from the operator. The cabinet is designed with no
