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Affinity User Guide
75
Issue Number: 5 www.controltechniques.com
High-capacitance cables
The maximum cable length is reduced from that shown in Table 4-6,
Table 4-7, Table 4-8 and Table 4-9 if high capacitance 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 4-11 shows how to identify the two
types.)
Figure 4-11 Cable construction influencing the capacitance
The cable used for Table 4-6, Table 4-7, Table 4-8 and Table 4-9 is
shielded and contains four cores. Typical capacitance for this type of
cable is 130pF/m (i.e. from one core to all others and the shield
connected together).
4.8.2 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 500Vac 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 10m:
•
AC supply voltage exceeds 500V
•
DC supply voltage exceeds 670V
•
Operation of 400V drive with continuous or very frequent sustained
braking
•
Multiple motors connected to a single drive
For multiple motors, the precautions given in section 4.8.3
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 575V 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 2.2kV 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.
4.8.3 Multiple motors
Open-loop only (not RFC mode)
If the drive is to control more than one motor, one of the fixed V/F modes
should be selected (Pr
5.14
= Fd or SrE). Make the motor connections
as shown in Figure 4-12 and Figure 4-13. The maximum cable lengths in
Table 4-6, Table 4-7, Table 4-8 and Table 4-9 apply to the sum of the
total cable lengths from the drive to each motor.
It is recommended that each motor is connected through a protection relay
since the drive cannot protect each motor individually. For connection, a
sinusoidal filter or an output inductor must be connected as shown in
Figure 4-13, even when the cable lengths are less than the maximum
permissible. For details of inductor sizes refer to the supplier of the drive.
Figure 4-12 Preferred chain connection for multiple motors
Figure 4-13 Alternative connection for multiple motors
4.8.4 /
Δ
motor operation
The voltage rating for
A
and
Δ
connections of the motor should always
be checked before attempting to run the motor.
The default setting of the motor rated voltage parameter is the same as
the drive rated voltage, i.e.
400V drive 400V rated voltage
200V drive 200V rated voltage
A typical 3 phase motor would be connected in
A
for 400V operation or
Δ
for 200V operation, however, variations on this are common e.g.
A
690V
Δ
400V.
Normal capacitance
Shield or armour
separated from the cores
High capacitance
Shield or armour close
to the cores
Motor protection
relay
Chain connection (preferred)
connection
Inductor
Motor protection
relay
Summary of Contents for Affinity
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