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Perfect Harmony GENIII/e User's Manual
2.4 The Power Circuit
The basic power schematic for an 18 cell (4160 VAC) system is shown in Figure 2-14. Besides the direct operating
information received from each cell by the Fiber Optic System, input voltage, output voltage, and current are also
directly monitored. Input and output voltage information is supplied to the control boards by an attenuator system
consisting of a voltage divider and voltage clamps.
Two Hall Effect sensors placed on output phases B and C sense output motor current. Two CTs placed on input phase
B and C sense drive input current. Polarity and burden resistor values must always be maintained.
Each secondary of the power transformer T1 serves one cell only. Each cell receives modulation information through
the Fiber Optic System in a way that develops the required output voltage and frequency demanded by the load.
Unlike standard PWM systems, the voltage applied to the motor leads is developed in many small steps instead of
through a few large steps. This provides two distinct advantages: the voltage stress on the motor insulation is
dramatically reduced, and the quality of the motor currents is dramatically increased.
DANGER - Electrical Hazard! Even though each cell by itself develops no more than 690 VAC, the
voltage to ground can increase to the rated drive output.
Since each cell is fed from transformer
current distortion is dramatically reduced. Input power factor is always maintained greater than 0.94 lagging.
Each Perfect Harmony VFD cell within a specific system is identical (refer to Figure 2-15). Larger and smaller
versions of power cells differ in the size or quantity of input diodes, filter capacitors and IGBTs.
At a minimum, each cell contains a Cell Control Board and an IGBT Gate Driver Board. The Cell Control Board
performs all communication and control for each cell.
2.4.1
Monitoring of Input Power Quality
Input currents and voltages to the drive input T1 transformer are also measured and processed continuously by the
control system. Information such as efficiency, power factor, and harmonics are available to the user through the tool
suite, communication network, and some on the keypad. The input monitoring also protects against T1 transformer
secondary side faults that cannot be seen by typical primary protection relaying. Thus it is very important that the
drive input medium switchgear, if not supplied, is interlocked to the control system so that input medium voltage can
be interrupted upon the rare event of such a fault.
A Form C 250VAC/300VDC rated contact output is supplied standard for liquid cooled drives only, to trip the drive
input medium voltage circuit breaker or contactor. This contact is designated "TRIP INPUT MEDIUM VOLTAGE"
and changes state whenever the drive input power and power factor are outside hardcoded normal operating
conditions. This contact must be integrated with input switchgear to deactivate drive input medium voltage in the rare
event of a T1 transformer secondary circuit fault.
DANGER! This contact must be integrated with input switch-gear to deactivate the drive input medium
voltage upon the rare event of a secondary circuit fault.
A1A19000405A: Version 1.3
with varying degrees of phase shift (see Figure 2-14), the input VFD
T1
s s s s
Hardware Components
2-21
2
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