070.610-IOM (JUL 21)
Page 18
RWF II Rotary Screw Compressor Units
Installation
Figure 21: Control power ground circuit
NOTICE
Structural grounding can also result in multiple
ground potentials and is also a relatively poor con-
ductor. Therefore, this is not an acceptable method
for proper operation of electronic equipment.
There must be a ground for the three-phase power wir
-
ing. This must be sized in accordance to the NEC and any
local codes relative to the highest rated circuit overload
protection provided in the circuit. The manufacturer may
require a larger ground conductor than what is required by
the NEC for proper steering of EMI from sensitive circuits.
This conductor must also be insulated to avoid inadvertent
contact at multiple points to ground, which could cre
-
ate Ground Loops. In many installations that are having
electronic control problems, this essential wire is usually
missing, is not insulated, or improperly sized.
NEC size ratings are for safety purposes and not neces
-
sarily for adequate relaying of noise (EMI) to earth ground
to avoid possible interference with sensitive equipment.
Therefore sizing this conductor 1 – 2 sizes larger than
required by code will provide better transfer of this noise.
Johnson Controls-Frick requires that the ground conductor
meet the following:
• Stranded copper
• Insulated
• One size larger than NEC requirements for conventional
starters
• Two sizes larger than NEC requirements for VFD starters
• Conduit must be grounded at each end
• This circuit must be complete from the motor to the
starter continuing in a seamless manner back to the
plant supply transformer (power source).
For Direct Coupled, Package Mounted Starters, the ground
between the motor and the starter may need to be made
externally. See
. The connection on the starter
end must be on the starter side of the vibration isolators.
Be certain the connection is metal to metal. Paint may
need to be removed to ensure a proper conductive circuit.
The use of counter-sunk star washers at the point of con-
nection at each end will maximize metal to metal contact.
Figure 22: Motor And starter grounding
VFD applications
The primary ground conductor that accompanies the
three-phase supply must be stranded copper, insulated
and two sizes larger than the minimum required by the
NEC or any other applicable codes. This is necessary due
to the increased generation of EMI, which is a character
-
istic of a
VFD output to the motor when compared to a
conventional starter.
For VFD applications, isolation of the control power,
analog devices, and communications ground from the
three-phase ground within the starter and the electronic
control panel may be necessary. This is due to the higher
noise (RFI/EMI) levels generated between the VFD output
and the motor, relative to a conventional starter. If these
grounds are left coupled by a common back-plate in the
starter/drive, this noise can be direct coupled to the con-
trol power, analog device, and communications grounding
and may cause unexplained behavior and possible damage
to components.
To install correctly, run a separate, properly sized (10 or 8
AWG typically) insulated ground along with and taken to
ground with, the three-phase ground at the
three-phase
supply transformer (plant). This will require that the three-
phase ground and the control power ground be electri
-
cally isolated except for the connection at the plant supply
transformer.
This style of grounding should steer the noise (EMI/RFI)
to earth ground, reducing the potential for it to affect the
sensitive equipment, which could occur if the grounds
were left coupled.
NOTICE
If all other recommendations for grounding are fol-
lowed, this process should not be necessary.
