Application Specific Features
7.7 Synchronous Transfer
Product User Manual
166 Operating Instructions, Version AE 12/2009, A5E01454341C
velocity reference and adjusting the VFD output voltage to match the line. Both functions are
automatically implemented by the NXG control when an "Up Request" is received. The
active acceleration ramps and torque limits set in the control are used during this period.
Note
Mismatched line/motor voltages are permitted when careful consideration is given to the
configuration of the VFD (including step up/down transformers between the line and the
motor).
Once the VFD output frequency is matched to the line, the output phase also needs to be
matched with a pre-determined leading phase to ensure the power flow is out of the VFD
when both the VFD output and motor line contactors are closed (both must be closed at one
time to ensure bumpless transfer). When the synchronization is complete, the motor line
contactor remains closed (i.e., motor connected to the line), the VFD output is disabled, and
the drive output contactor is then opened to end the transfer.
A typical Up Transfer sequence is summarized as follows:
1. Start the VFD as a normal running drive with desired speed command. The drive must be
in the "RUN" state to initiate transfer, and the "VFD Transfer Permissive" signal must be
set true. In addition, the digital input flag "VFD Output Contactor Status" must be set true.
2. Initiate the transfer with the "Up Transfer" request digital input when an up transfer is
desired. A timer should be used to establish a maximum time period for the transfer to
complete. Transfer timeouts are rarely encountered and are usually due to an unstable
source, a varying load, or improperly tuned drive.
Note
The VFD will attempt to perform the Up Transfer as long as the "Up Transfer Request"
signal is set or until a VFD stop command is issued. If a VFD fault occurs at any time prio
to closing the Motor Line Contactor, the external controller must reset its Up Transfer
state machine and clear all Transfer related digital inputs sent to the VFD. As a rule, an
Up Transfer typically will be completed within 10-20 seconds after the VFD output
reaches the rated line frequency. Should a timeout occur, the transfer should be
terminated and the NXG control will revert to its previous state, using the speed reference
that was active prior to initiating the Up Transfer sequence.
3. The VFD will ramp the output to match the measured line frequency.
Note
The VFD will automatically ramp the output to rated frequency upon initiating the Up
Transfer Sequence. There is no need to change the speed reference prior to issuing an
"Up Transfer request."
4. The VFD ramps to line frequency and phase-locks for a minimum a 5 seconds. Note that
the NXG control includes provisions for offsetting the phase should a configuration exist
where the VFD Medium Voltage input is not the same phase as the line voltage to be
used at the motor.
5. Once synchronized, the VFD issues a permit signal, "Up Transfer Permit", indicating that
the Motor Line Contactor can be closed (VFD Output contactor also remains closed
during this stage).
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