Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 49)

DESCRIPTION OF OPERATION 5-31
1557-UM050F-EN-P –June 2013
Transfer to Drive
The drive must synchronize itself with the motor voltage while the motor is still running on bypass, in order
to perform the transfer to drive before the motor flux decays. The output contactor must therefore be
closed with the motor still connected to the line in order to pre-charge the output filter capacitors.
When the motor is running on bypass and a transfer to drive is required, the transfer takes place as
described below:
1. The drive is given a "transfer to drive" command. Initially, both the "close output contactor" and
"close bypass contactor" outputs will be off. When the drive receives the transfer command, it
activates its "close bypass contactor" output and runs its output frequency up to rated line frequency.
The programmable controller must wait for at least 2 seconds to allow the drive to ramp up to rated
frequency.
2. The drive is given control of the closed bypass and open output contactors.
3. The drive is given a normal start command, which causes it to activate its "close output contactor"
output.
4. After the "output contactor closed" input indicates that the output contactor has closed, there is a
short delay to allow the output filter capacitors to charge. The drive then deactivates its "close bypass
contactor" output.
5. When the "bypass contactor closed" input indicates that the bypass contactor has opened, the drive
goes to run mode. As the drive brings the motor torque up to the level required by the load, the motor
speed will drop slightly before returning to line frequency. A drop in speed is unavoidable because
the drive cannot know in advance what torque will be required.
6. The "transfer to drive" command is removed. The synchronous transfer is now complete and the
motor is running on the drive. The drive retains control of the output contactor for protection
purposes.