Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 48)

5-30 DESCRIPTION OF OPERATION
1557-UM050F-EN-P – June 2013
Transfer to Bypass
When the motor is running on the drive and a synchronous transfer to bypass is required, the transfer
takes place as described below:
1. Before a transfer to bypass is requested, the drive must be given control of the open bypass
contactor. Since the drive is running, it will already have control of the output contactor. The "transfer
to bypass", "transfer to drive", and "bypass contactor closed" inputs and the "close bypass contactor"
output must all be off at this time.
2. The drive is given a "transfer to bypass" command, which must remain active until the synchronous
transfer is complete. If the transfer command is removed before the bypass contactor is requested to
close, the drive will abort the transfer and return to normal running. When the drive receives the
transfer command, it accelerates the motor up to line frequency. If the drive is unable to achieve
synchronous speed, it may be necessary to increase parameter Torque Limit Motoring.
3. When the motor reaches synchronous speed, the synchronizing regulator is activated. The
synchronizing regulator adjusts the drive speed reference as required, to synchronize the motor to the
line with the motor voltage leading the line voltage by an angle specified by parameter Synchronous
Transfer Lead Angle. This parameter can be used to compensate for any phase difference between
the bypass voltage and the drive input voltage. The response of the synchronizing regulator is
controlled by parameter Synchronizing Regulator Gain. If the phase error tends to oscillate, it may be
necessary to adjust parameters Synchronizing Regulator Gain or Speed Regulator Bandwidth.
4. When the phase error between the motor voltage and line voltage has remained less than the value
specified by parameter Synchronizing Error Maximum for the time interval specified by parameter
Synchronizing Time, the drive activates its "close bypass contactor" output. If the drive is unable to
synchronize within the time specified by parameter Sync Transfer Time, the synchronous transfer is
aborted.
5. After a time delay specified by parameter Synchronous Transfer Off Delay, the drive shuts off. It is
very important that parameter Synchronous Transfer Off Delay is set to the correct value. This
should be slightly less than the contactor closing time. If this time delay is set too short the motor
voltage could drift out of phase with the bypass voltage during the time the motor is not under power,
producing a large current and torque transient when the bypass contactor closes, and possibly
causing protective equipment to operate. If the time delay is set too long, a drive overcurrent fault
may occur because the drive is unable to control its output voltage and frequency once the bypass
contactor has closed.
CAUTION: If parameter Synchronous Transfer Off Delay is set
incorrectly, damage may occur to the drive, motor, couplings, and
driven load if a transfer to bypass is attempted.
6. When the "bypass contactor closed" input indicates that the bypass contactor has closed, the drive
deactivates the "close output contactor" output. When the output contactor opens, the drive is
disconnected from the motor, leaving the output filter capacitors charged to line voltage. It may take
several minutes for the capacitors to discharge.
7. Control of the closed bypass contactor is taken away from the drive.
8. The "transfer to bypass" command is removed, causing the drive to deactivate its "close bypass
contactor" output. The synchronous transfer is now complete and the motor is running on bypass.