78 Unidrive M / HS Frame 7 to 10 Power Installation Guide
Issue Number: 5
Feedback device cable shielding
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.
The following guidance is divided into two parts:
1. Ensuring correct transfer of data without disturbance from electrical noise originating either within
the drive or from outside.
2. Additional measures to prevent unwanted emission of radio frequency noise. These are optional
and only required where the installation is subject to specific requirements for radio frequency
emission control.
To ensure correct transfer of data, observe the following:
Resolver connections:
• Use a cable with an overall shield and twisted pairs for the resolver signals
• Connect the cable shield to the drive 0V connection by the shortest possible link ("pigtail")
• It is generally preferable not to connect the cable shield to the resolver. However in cases where
there is an exceptional level of common-mode noise voltage present on the resolver body, it may
be helpful to connect the shield there. If this is done then it becomes essential to ensure the
absolute minimum length of "pigtails" at both shield connections, and possibly to clamp the cable
shield directly to the resolver body and to the drive grounding bracket.
• The cable should preferably not be interrupted. If interruptions are unavoidable, ensure the
absolute minimum length of "pigtail" in the shield connections at each interruption.
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")
• The cable should preferably not be interrupted. If interruptions are unavoidable, 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 in case of doubt, the following additional requirement must be observed. This gives
the 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.
In some cases single shielding of each pair of differential signals cables, or a single overall shield
with individual shield on the thermistor connections is sufficient. In these cases all the shields should
be connected to ground and 0V at both ends.
If the 0V is required to be left floating a cable with individual shields and an overall shield must be
used.
The recommendations of the encoder manufacturer must also be adhered to for the
encoder connections.
In order to guarantee maximum noise immunity for any application double shielded cable
as shown should be used.
Unidrive M frame7 to 10 Power Installation Guide issue5.book Page 78 Tuesday, May 24, 2016 11:52 AM
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