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70 Unidrive M Regen Design Guide
Issue Number: 4
4.6 Exceeding maximum cable length
If the maximum cable length specified is exceeded, the increased circulating currents caused by the extra cable capacitance will have an effect on the
other parts of the system. This will necessitate additional components to be added to the standard arrangement. Refer to section 4.6.1 Available
preventative measures on page 72.
When the maximum Regen system cable length is exceeded the effect of the additional capacitance of this cable to ground can become significant.
At every switching edge the capacitance must be charged and subsequently discharged on the falling edge. This leads to a capacitive high frequency
current flowing in the common mode, i.e. returning through the ground connections, which must be supported by the EMC filter, inverter output and for
a Regen system the Regen inductor.
This charging current is sensed by the drive current sensing circuit and may affect the drive current control system or protection, causing a loss of
torque or over-current trip. It may also have heating effects on the EMC filter and Regen inductor.
The effect of the charging current on the standard inverter can be managed by connecting an inductor in series with the output, and where necessary
additional filter capacitors between the DC bus and ground. For a Regen system the inductor is effectively already present in the system, the Regen
inductor, and capacitance to ground at the DC bus is not permitted because of the PWM action of the Regen input stage.
When operating with a Unidrive M Regen system with a total system cable length that exceeds the maximum cable lengths listed in Table 4-11 to
Table 4-14 the following considerations must be made:
1. Exceeding the maximum allowed cable lengths in a Unidrive M Regen system can result in both heating of the Regen inductor and also heating
and saturation of the EMC filter if fitted due to the increased common mode currents.
2. Cable types should also be considered along with the increased losses generated in the long cable application, these can be calculated for the
given cable type and length to ensure these do not exceed the rating of the Regen inductor.
Cable types should be considered with the long cable applications due to the increased losses generated with certain cable types. The absolute
maximum allowable cable length (cable length allowed with long cable modifications) can be limited if high capacitance cables are used. The high
capacitance cables result in increased cable-induced losses which can exceed the Regen inductor rating. Most cables have an insulating jacket
between the cores and the armor or shield; these cables have a low capacitance and are recommended. Cables that do not have an insulating jacket
tend to have high capacitance; if a cable of this type is used the absolute maximum cable length could be limited. (Figure 4-14 shows two types of
cables). Typical cable capacitance for a shielded cable containing four cores is 130 pF/m (i.e. from one core to all others and the shield connected
together).
Figure 4-15 Cable construction influencing the capacitance
Long cable lengths if routed through a plant can be a source for radiated emissions due to the PWM present on all three cables sections A, B and C.
Refer to Figure 4-14, Unidrive M Regen system cables. In addition there can be increased common mode currents due to the increased cable length
and capacitance. The correct choice of screened cable and cable segregation should be followed.
The absolute maximum cable length achievable for the Regen system will be limited by the increased losses and the rating of the Regen inductor.
Excessive cable lengths will result in unacceptable losses in the Regen inductor.
Normal capacitance
Shield or armour
separated from the cores
High capacitance
Shield or armour close
to the cores
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