Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 47)

DESCRIPTION OF OPERATION 5-29
1557-UM050F-EN-P –June 2013
Synchronous Transfer
Synchronous transfer is an optional feature of the Bulletin 1557 ac drive which allows a motor to be
transferred between the drive and a fixed frequency supply in either direction without stopping and with a
very short interruption of power to the motor. Compared to a simple non-synchronous transfer in which
power to the motor is interrupted for a significant length of time, the transient drop in motor speed is much
less with synchronous transfer.
In order to perform a synchronous or non-synchronous transfer both a drive output contactor and a
bypass contactor are required. The name "bypass" indicates that the function of this contactor is to
connect the motor directly to the fixed frequency supply, bypassing the drive.
Synchronous transfer also requires the optional second discrete I/O board to produce the three inputs
and one output used for this function. The three inputs are the "bypass contactor closed" status input,
and the "transfer to bypass" and "transfer to drive" command inputs. The one output is "close bypass
contactor".
In most single motor and all multiple motor applications, a programmable controller is used for overall
control of the synchronous transfer operation. Because of the risk of damage to the motor and drive if the
bypass contactor is closed at the wrong time, close co-ordination is required between the drive and the
programmable controller for the successful execution of a synchronous transfer. In particular, when
transferring from drive to bypass, the time interval between the drive shutting off and the bypass contactor
closing must be accurately controlled. Typically, the PLC gives control of the bypass contactor to the
drive before performing the transfer, and takes back control after the transfer is completed.
CAUTION: Since the programmable controller and not the drive
controls the output and bypass contactors, the transfer command
must always go via the PLC and never directly to the drive from
another controlling device (e.g. a Remote I/O adapter).
Since the drive does not have any means of measuring the input voltage of the bypass contactor, the
drive input voltage is used as the phase reference for synchronization. Independent verification of the
phase rotation and phase angle of the bypass voltage relative to the drive input voltage is therefore
absolutely essential.
CAUTION: If the phase rotation and phase angle of the bypass
voltage compared to the drive input voltage are not correct, severe
damage may occur to the drive, motor, couplings, and driven
equipment if a transfer to bypass is attempted.
As a precaution, a synchronous transfer will not be performed if the phase sequence of the line voltage is
not positive.