Application Specific Features
7.7 Synchronous Transfer
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
163
7.7 Synchronous Transfer
7.7.1 Introduction
The synchronous transfer feature is used to avoid line start mechanical/electrical strain in
constant-speed applications. The VFD soft starts the motor(s), and then NXG control
matches line/load electrical characteristics, allowing "bumpless" synchronous transfer.
Note
Beyond the core drive, synchronous transfer requires additional hardware: output reactor
and switchgear. A PLC is recommended in multi-motor applications.
● "Up Transfer" - The process of transferring a VFD-controlled motor to the line, and then
decoupling the motor from the drive.
● "Down transfer" - The process of transferring a line-energized motor to VFD control, and
then decoupling the motor from the line.
To achieve successful up/down transfers, the output voltage of the VFD must match or
exceed the amplitude of the line. If the line is unstable with frequency and/or voltage
variations, the VFD may not be able to synchronize, and therefore transfer is inhibited.
Note
In transfer applications, where a synchronous motor is used, the VFD must have control of
the field supply.
Note
Synchronous Up/Down transfer is not available in V/HZ or OLTM control modes.
WARNING
Improper phase sequence may result in a synchronous transfer related VFD fault.
7.7.2 VFD Synchronous Transfer Implementation
Synchronous transfer is inherent to NXG control. To optimize this feature, Siemens
engineering should always be involved (regardless of scope of supply) in the switchgear
configuration and logic sequencing for both equipment and personnel safety. Siemens
engineering can supply switchgear and reactors as part of the drive, or provide
recommendations as needed.
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