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46
Unidrive User Guide
www.controltechniques.com Issue Number: 9
The following guidelines should be followed for all installations to
minimise the risk of disturbing any other equipment in the vicinity of the
drive.
The earthing / grounding and clearance sections should be followed for
all installations as good practice.
Earth / Ground connections
The diagram below indicates the grounding method which should be
used in all standard installations using an grounded secondary AC
supply.
Clearances
The diagram below indicates the clearances which should be observed
around the drive and related ‘noisy’ power cables by all sensitive control
signals / equipment.
Figure 4-9 Drive clearances
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 back plate, located as close as possible
to the drive.
•
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 back plate, located as close as possible to the
drive. 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.
N
In order to guarantee maximum noise immunity for any application
double screened cable as shown should be used.
In some cases single shielding of each pair of differential signals cables
or a single overall shield with an 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.
Figure 4-10 and Figure 4-11 illustrate the preferred construction of cable
and the method of clamping. The outer sheath of the cable should be
stripped back enough to allow the clamp to be fitted. The shield must not
be broken or opened at this point. The clamps should be fitted close to
the drive or feedback device, with the ground connections made to a
ground plate or similar metallic ground surface.
The ground loop impedance must conform to the
requirements of local safety regulations.
The drive must be grounded by a connection capable of
carrying the prospective fault current until the protective
device (fuse, etc.) disconnects the AC supply.
The ground connections must be inspected and tested at
appropriate intervals.
WARNING
Optional braking resistor and overload
Do not place sensitive
(unscreened) signal circuits
in a zone extending
300mm (12”) all around the
Drive, motor cable, input
cable from RFI filter and
unscreened braking resistor
cable (if used)
300mm
(12in)
This does not apply to a motor thermistor
cable. The motor thermistor cable must
be shielded.
NOTE
NOTE
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