Eurotherm Drives 584S, Product Manual, Page: 62 / 176

CHAPTER 3 - INSTALLATION PROCEDURE
3-20 584S FREQUENCY INVERTER
Motor Cable-length Limitations
Screened/armoured cable has significant capacitance between the conductors and the screen which increases linearly
with cable length. Typically this is 200 pF per metre but this will vary with cable type and current rating. Long cable
lengths may have the following undesirable effects:
- Tripping on 'over current' as the cable capacitance is charged and discharged at the switching frequency,
- Producing increased conducted emissions which degrade the performance of the EMC filter due to saturation.
EMC compliance is only guaranteed up to a maximum cable length of 50m (type 4, 5, 6 and 7).
- Causes RCDs (Residential Current Detection) to trip out due to increased high frequency earth current.
- Produces increased heating inside the EMC AC supply filter from the increased conducted emissions.
Eurotherm Drives only guarantee the thermal performance of the filters up to a specified cable length of 150m
with screened cable.
These effects can be overcome by adding chokes at the output of the 584S/620 drive module. In applications where
multiple motors are connected to a single drive, minimise the length of screened/armoured cable connected to the
drive by using a single length of cable to a star junction point, from where all the other motor cables are attached.
Maintain the integrity of the shield. If the cable is interrupted to insert contactors or other components, the screen
must be connected over the shortest possible route. Table A1 in the appendix gives information on the recommended
output chokes for use with long cables, cables connected in parallel, or when EMC output filters are used with cables
greater than that specified for EMC compliance.
Output filters can also be used to achieve EMC and filter thermal conformance with longer cable lengths than that
specified. These output filters also ensure a long motor life by reducing the high dV/dt and over voltage stresses
applied to the motor windings by inverters. These filters should be mounted as close to the 584S/620 drive module
as possible. Refer to Eurotherm Drives for the selection of suitable filters.
Other Layout Considerations
The proximity between the source and victim circuit has a large effect on radiated coupling. The electromagnetic
fields produced by drive modules falls off rapidly with distance from the cabling/enclosure. It should be remembered
that the radiated fields from EMC compliant drive systems are measured at least 10m from the equipment over the
frequency band 30 to 1000 MHz (as required by EN55011, referenced by the generics and the drive product specific
standard). Any equipment placed closer to the drive system than this will see larger magnitude fields, particularly
very close to the drive. No magnetic/electric field sensitive equipment should be placed within 0.25m of the
following parts of a drive system:
- 584S/620 Drive module
- EMC output filters
- Input or output chokes/transformers
- Cable between 584S/620 Frequency Inverter and Motor (even when screened/armoured)
- Connections to external braking chopper and resistor (even when screened/armoured)
- AC/DC brushed motors (due to commutation)
- DC link connections (even when screened/armoured)
- Relays and contactors (even if they are suppressed)
Often the coupling between electrically 'noisy' and 'sensitive' cables is a problem. This can be minimised by
separating parallel runs by at least 0.25m, and minimising the length of parallel runs. For long parallel runs (>10 m)
the separation should be increased proportionally. For example if the parallel runs were 50 m then the separation
would be (50/10)
×
0.25 m = 1.25 m.
In addition the coupling between two cables which must cross is minimised if they cross over at 90
°
. Hence sensitive
cables should cross the cables to the motor, DC link and braking chopper circuit at 90
°
, and should never be run
close to them or in parallel for any great length.
Never run supply, DC link or motor cables in the same bundle as the signal/control and feedback cables, even if they
are screened.
From experience the following equipment is defined as particularly sensitive and care must be taken in the
installation:
- Any transducers which produce low level analogue outputs (<1 volt) e.g. load cells, strain gauges,
thermocouples, piezoelectric transducers, anometers, LVDT's
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