Xylem IM167 R8 Aquavar CPC (Centrifugal Pump Control) - Manual (Page 9)

9
POWER DISTRIBUTION SYSTEMS
The type of transformer and the connection configuration feeding a drive plays an important
role in its performance and safety. The following is a brief description of some of the more
common configurations and a discussion of their virtues and shortcomings. Always ask what
type of power system the site has before sizing the drive.
Delta / Wye with grounded Wye neutral:
This configuration is one of if not the most common. It provides rebalancing of unbalanced
voltage with a 30 degree phase shift. Depending on the output connections from the drive to
motor, the grounded neutral may be a path for common mode current caused by the drive
output.
Delta / Delta with grounded leg:
Another common configuration providing voltage rebalancing with no phase shift between
input and output. Again, depending on the output connections from the drive to motor, the
grounded neutral may be a path for common mode current caused by the drive output.
Ungrounded secondary:
Grounding of the transformer secondary is essential to the safety of personnel as well as
the safe operation of the drive. Leaving the secondary floating can permit dangerously high
voltages between the chassis of the drive and the internal power structure components.
In many cases this voltage could exceed the rating of the input MOV protection devices of
the drive causing a catastrophic failure. In all cases, the input power to the drive should be
referenced to ground. If the transformer can not be grounded, then an isolation transformer
must be installed with the secondary of the transformer grounded.