Rockwell Automation 23P PowerFlex DC Stand Alone Regulator and Gate Amplifier - Manual (Page 46)

46 Rockwell Automation Publication 23P-UM001D-EN-P - July 2012
Chapter 3 Gate Amplifier Installation and Wiring
Key Grounding/Bonding Points Outside of the Cabinet
The high frequency grounding/bonding outside of the cabinet should be treated
in a manner similar to a power supply mounted on a ground plane with all of the
components that it supplies, mounted on the same ground plane. In the case of
the drive system, the transformer can be thought of as the power supply for the
downstream components; AC entry bay, drive cabinets, and motors. The high
frequency grounding/bonding of the components is effectively like mounting
them on a virtual ground plane. All components from the power transformer to
the motor needs to viewed as a whole system.
All currents used to run the drives and motors originate at the power transformer.
As the power is being controlled by the drive system to regulate the amount of the
energy supplied to the motor, high frequencies are generated through the
switching action of the drive power components. The capacitive coupling of the
high frequencies to the metal that is within proximity to the conductors creates
ground currents (ic=Cdv/dt). The major components of the system, motor,
conduit, and cabinet will all be “charged” by this capacitive coupling effect. The
ground currents seek a return path back to the source, the drive, and ultimately
the transformer. If a known good path is not provided, more currents will flow
through the machine and other undesirable paths. The power transformer
provides a magnetic isolation barrier that prevents the ground currents from
going into the power grid. In the transformer there is capacitive coupling between
the transformer core and the windings. Attachment of the transformer frame
(which is attached to the core) to the drive cabinet(s) will complete the ground
current path for these components through a known path.
• Motor to cabinet - the connections for the motor ground must be made
directly to the frame of the motor, not the conduit box. The connection
between the motor frame and the conduit box does not provide a good
high frequency conduction path.
• Drive cabinet to bus power entry cabinet.
• Power entry cabinet to power transformer case/frame.
If insufficient grounding/bonding is suspected, then additional wiring must be
added (as shown in the system diagram that follows) to ensure that there is a
proper path for electrical noise.
Welding cable is needed for grounding/bonding where high frequency noise
currents are present.
Table 10 - Recommended Welding Cable for High
Frequency Grounding/Bonding on page 47 provides an overall guideline for the
wire size to use for the high frequency bonding between the major system
components shown in
Figure 18 - DC Drive High Frequency Grounding /
Bonding Configuration on page 47 .