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Diagnostics Optimization CT MODBUS RTU Technical Data
Installation and System Design Guide 101
Issue Number: 1
4.8 Shield, Ground connections
Shielding considerations are important for PWM drive installations due to the high voltages and currents present in the output (motor) circuit with a
very wide frequency spectrum, typically from 0 to 20 MHz.
Encoder connections:
• Use a cable with the correct impedance
• Use a cable with individually shielded twisted pairs
• Connect the cable shields to 0V at both the drive and the encoder, using the shortest possible links (“pigtails”)
• Preferably the cable should not be interrupted. If interrupted, ensure the absolute minimum length of “pigtail” in the shield connections at each
interruption. Preferably, use a connection method which provides substantial metallic clamps for the cable shield terminations
The above applies where the encoder body is isolated from the motor and where the encoder circuit is isolated from the encoder body. Where there is
no isolation between the encoder circuits and the motor body, and if in any doubt, the following additional requirement must be observed in the
interests of best possible noise immunity.
• The shields must be directly clamped to the encoder body (no pigtail) and to the drive grounding bracket. This may be achieved by clamping of
the individual shields or by providing an additional overall shield which is clamped
N
The recommendations of the encoder manufacturer must also be adhered to for the encoder connections.
Motor cable: Use a motor cable with an overall shield. Connect the shield of the motor cable to the ground terminal of the motor frame using a link
that is as short as possible and not exceeding 50 mm (2 in) long. A full 360 ° termination of the shield to the terminal housing of the motor is beneficial.
Brake resistor cable: The optional braking resistor should also be wired with shielded cable. If un-shielded wire is required refer to section 4.17.5
Shielding requirements for the braking circuit on page 120 for guidance.
Control cables (including encoder): If the control wiring is to leave the enclosure, it must be shielded and the shield(s) clamped to the drive using
the grounding bracket. Remove the outer insulating cover of the cable to ensure the shield(s) make contact with the bracket but keep the shield(s)
intact until as close as possible to the terminals. Refer to Figure 4-40 Grounding of signal cable shields using the grounding bracket on page 120.
Figure 4-16 Feedback cable, twisted pair and Figure 4-17 Feedback cable connectionsillustrate the preferred construction of cable and the method of
clamping.
Figure 4-16 Feedback cable, twisted pair
N
In order to guarantee maximum noise immunity for any application double shielded cable as shown should be used.
Figure 4-17 Feedback cable connections
Use the grounding bracket and grounding clamp supplied with the drive to terminate all shielded cables at the drive.
Twisted
pair
cable
Twisted pair shield
Cable
Cable overall shield
Cable
Cable
shield
Twisted
pair
shield
Cable
shield
Twisted
pair
shield
Connection
at motor
Connection
at drive
Ground clamp
on shield
connection
to 0V
Shield
connection
to 0V
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