Danfoss VLT Series Service Manual page 7

Section One
SEQUENCE OF
OPERATION
NTCs
MOVs
1 - 4
VLT 3002-3004, 230V,
VLT 3002-3008, 380V/460V
When input power is first applied, the Rectifier Module converts the line voltage
into a DC voltage. The rectified output is then connected to the DC Bus filter
establishing a fixed DC Bus voltage. To limit the inrush charge current in the DC
Bus capacitors, three Negative Temperature Coefficient (NTC) resistors are added
in series with the inputs of the Rectifier Module. NTC resistors decrease in resistance
as temperature increases. Providing that the charging process proceeds normally,
the power supplies will come up and provide the Control Card with low voltage
control power. At this time the Control Card display will indicate that the unit is
ready for operation.
Following a run command and a speed reference, the Control Card delivers three
Pulse Width Modulation (PWM) signals to the ILD Card. The ILD Card in turn
receives these three signals and creates the six individually isolated gate drive
signals. These gate pulses are fed directly to the Insulated Gate Bi-polar Transistor
(IGBT) output power devices. The IGBTs are switched on and off to develop the
PWM waveform which is ultimately delivered to the motor. As the unit operates,
the ILD Card monitors the unit's operational status. Currents and voltages out of
specified limits or excessive temperatures will result in the ILD Card responding to
the fault. The ILD Card sends the appropriate fault message to the Control Card
and in virtually all cases causes the unit to trip. Section 2 of this manual describes
the fault messages and provides direction in determining the cause and the solution
for the fault.
Rectifier
Module
MOV
MOV
VLT 3006-3032, 230V,
VLT 3011-3052, 380V/460V
When power is first applied, the normally open Bus Contactor forces the input line
current to flow through the Positive Temperature Coefficient (PTC) resistors. PTC
resistors increase in resistance as the temperature increases. The
VLT 3502-3504, 230V
VLT 3502-3508, 380V/460V
Bus Coil
Bus
Capacitors
VLT 3508-3532, 230V
VLT 3511-3562, 380V/460V
+VDC
–VDC