MNL940-677-000
REV - B
3/20/2017
13
SECTION 4
TROUBLESHOOTING
4.1
THEORY OF OPERATION
The INV-1200 series utilizes high frequency, switched mode, power conversion to convert the
incoming DC to an AC output. The first stage is a DC to DC converter which “steps - up” the input
voltage to a reg/- 200 VDC. This voltage is applied to a second high frequency
“housekeeping” supply and the output inverter. The housekeeping supply provides operating
voltages for the output inverter and alarm circuits. The output section consists of a crystal
controlled sine wave oscillator and class “D” power amplifier. The crystal derived clock frequency
is divided down and applied to a counter, PROM, D/A converter combination. The resulting sine
wave at the output frequency is applied to the power amplifier and boosted to the 120 VAC level
required on the output.
The power amplifier consists of a pulse width modulated (PWM) “half bridge” circuit utilizing IGBT
output devices. The devices are switched at a rate of 20 KHZ and modulated with the sine wave
information. The result is a 400V p/p pulse train at the output that contains the original sine wave
information. The 20 KHz “carrier” frequency is removed by a low pass LC filter. This provides an
amplified version of the original modulation signal to the output terminals. The control circuitry on
the output inverter assembly also provides output regulation and protective functions. The output
voltage RMS value is monitored and used to keep the output regulated. Output current is
measured and used to provide fold back type limiting in the event of an overload. Output short
circuits will disable the inverter stage, requiring the input power to be recycled to return proper
operation.
The alarm and bypass circuits (if provided) allow for external monitoring of input and output
status. The AC input/ bypass feature allows the AC power line to be applied to the model INV-
1200 and sent directly to the output terminals. In the event the AC is interrupted, a relay will
switch the output to the inverter. Units supplied with the D1 option will switch the line through
when the DC is not available or the inverter has a fault. The AC input is fused at 10 amps.
HOUSE
OSC. &
POWER
DC /DC
LINE FUSE
AC
DC
AC
SYSTEM BLOCK DIAGRAM
BYPASS RELAY
CONVERTER
KEEPING
CONTROL
AMP
OUTPUT
INPUT
IN
FIGURE 4-1 INV-1200 BLOCK DIAGRAM (WITH BYPASS OPTION)
