56
Digitax HD M75X Series Installation and Technical Guide
Issue Number: 5
Table 4-8 Motor cable size and maximum lengths
4.7.2 High-capacitance / reduced diameter cables
The maximum cable length is reduced from that shown in Table 4-8 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 4-7 shows how to identify the two types).
Figure 4-7 Cable construction influencing the capacitance
The maximum motor cable lengths specified in Table 4-8
are shielded and contain 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).
4.7.3 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
It is recommended that an inverter-rated motor be used taking into account the voltage rating of the inverter. This has a reinforced insulation system
intended by the manufacturer for repetitive fast-rising pulsed voltage operation.
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.
Model
Number of
input
phases
Output cable
All switching frequencies
mm
²
AWG
01200022
1
0.75
24
50 m
01200040
1
0.75
22
01200065
1
0.75
20
02200090
1
0.75
16
02200120
1
0.75
16
03200160
1
1.5
14
01200022
3
0.75
20
50 m
01200040
3
0.75
18
01200065
3
0.75
16
02200090
3
1.0
14
02200120
3
1.5
12
03200160
3
2.5
12
01400015
3
0.75
20
01400030
3
0.75
20
01400042
3
0.75
18
02400060
3
0.75
16
02400080
3
0.75
14
02400105
3
1.5
14
03400135
3
2.5
12
03400160
3
2.5
12
Normal capacitance
Shield or armour
separated from the cores
High capacitance
Shield or armour close
to the cores
Summary of Contents for Digitax HD M75 Series
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