Application Specific Features
7.7 Synchronous Transfer
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
167
6. Once the Motor Line Contactor is closed and acknowledged at the VFD, the VFD will
disable its output.
7. Once the VFD Output is disabled, the run request should be removed and the VFD output
contactor must be opened. This completes the Up Transfer process, and the Up Transfer
Request digital input can be removed.
7.7.5 Down Transfer Implementation
Down transfers are accomplished by transferring a motor from the line to VFD control. This is
accomplished by using the voltage/frequency present at the motor terminals as a velocity
reference, which is automatically implemented by the NXG control when a "Down Request"
is received. Once the VFD output frequency is matched to the motor, the VFD output phase
also needs to be matched with a predetermined leading phase to ensure the power flow is
out of the VFD when both the VFD output and the motor line contactors are closed (both
must be closed at one time to ensure bumpless transfer). When the de-synchronization is
complete, the VFD output contactor remains closed (i.e., motor connected to the VFD), and
the motor line contactor is then opened to end the transition. At this point, the VFD controls
the motor.
A typical Down Transfer sequence is summarized as follows:
1. The motor is running directly from the line, VFD is idle, the VFD output contactor must be
open, and the external speed reference must be set to the speed desired upon
completion of the down transfer (remember to account for motor slip if bumpless down
transfer is desired).
2. If the VFD indicates that it is capable of performing a down transfer (i.e., sufficient output
voltage available, drive energized, and ready to run), the down transfer request is issued
to the VFD.
3. Once the drive acknowledges the down request, the VFD output contactor is closed, and
the VFD run request must be set.
Note
The VFD output is not actually enabled at this time. The run request enables the VFD
output phase locked loop (PLL) to monitor and lock to the line voltage/frequency. Since
the VFD output contactor is closed while the motor is connected to the line, the line
voltage and frequency are available via the drive’s output sensors.
4. Once the VFD PLL maintains phase/frequency and voltage lock (typically 5-10 seconds),
the VFD output is automatically enabled with a small frequency offset to ensure that
current flows from the VFD to the line/motor.
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