64 Unidrive M / HS Frame 7 to 10 Power Installation Guide
Issue Number: 5
4.8.2 High-capacitance / reduced diameter cables
The maximum cable length is reduced from that shown in Table 5-23 Maximum motor cable
lengths on page 107 if high capacitance or reduced diameter motor cables are used.
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 maximum cable length is half that quoted in the
tables, (Figure 4-8 shows how to identify the two types).
Figure 4-8 Cable construction influencing the capacitance
The maximum motor cable lengths specified in section 5.1.21 Maximum motor cable lengths on
page 107 is shielded and contains four cores. Typical capacitance for this type of cable is 130 pF/m
(i.e. from one core to all others and the shield connected together).
4.8.3 Motor winding voltage
The PWM output voltage can adversely affect the inter-turn insulation in the motor. This is because of
the high rate of change of voltage, in conjunction with the impedance of the motor cable and the
distributed nature of the motor winding.
For normal operation with AC supplies up to 500 Vac and a standard motor with a good quality
insulation system, there is no need for any special precautions. In case of doubt the motor supplier
should be consulted.
Special precautions are recommended under the following conditions, but only if the motor cable
length exceeds 10 m:
• AC supply voltage exceeds 500 V
• DC supply voltage exceeds 670 V i.e. regenerative / AFE supply
• Operation of 400 V drive with continuous or very frequent sustained braking
• Multiple motors connected to a single drive
For multiple motors, the precautions given in section 4.8.4 Multiple motors should be followed.
For the other cases listed, it is recommended that an inverter-rated motor be used. This has a
reinforced insulation system intended by the manufacturer for repetitive fast-rising pulsed voltage
operation.
Users of 575 V NEMA rated motors should note that the specification for inverter-rated motors given
in NEMA MG1 section 31 is sufficient for motoring operation but not where the motor spends
significant periods braking. In that case an insulation peak voltage rating of 2.2 kV is recommended.
If it is not practical to use an inverter-rated motor, an output choke (inductor) should be used. The
recommended type is a simple iron-cored component with a reactance of about 2 %. The exact value
is not critical. This operates in conjunction with the capacitance of the motor cable to increase the
rise-time of the motor terminal voltage and prevent excessive electrical stress.
Normal capacitance
Shield or armor
separated from the cores
High capacitance
Shield or armor close
to the cores
Unidrive M frame7 to 10 Power Installation Guide issue5.book Page 64 Tuesday, May 24, 2016 11:52 AM
To Next Page
Loading...
Need help?
General
