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Cable, Motor Lengths
The length of cable between the drive and motor may be limited by various
application parameters. The 2 primary areas of concern are Reflected Wave
and Cable charging.
The Reflected Wave phenomenon, also known as transmission line effect,
produces very high peak voltages on the motor due to voltage reflection.
While Allen-Bradley drives have patented software that limits the voltage
peak to 2 times the DC bus voltage and reduce the number of occurrences,
many motors have inadequate insulation systems to tolerate these peaks.
See
Reflected Wave on page 2-127
Refer to
Figure 2.3
for measuring cable lengths when concerned about
Reflected Wave. Each individual motor must be considered based on the
lead length for that motor only. Diagram A shows 2 motors, each 300 feet
from the drive. Motor protection decisions are based on 300 feet cable
length (not 600 ft). If the motors need protection at this distance, then both
motors must be dealt with individually. Diagram B shows 1 motor at 50 feet
and one at 550 feet. It is likely that the motor that is close to the drive (50
Ft) will not need protection, but the motor farther from the drive (550 Ft)
may. Again, each motor must be considered individually based on its
distance from the drive.
Cable charging occurs because of the capacitance, phase-to-phase or
phase-to-ground, inherent in the length of cable. The current that is used to
charge the cable capacitance detracts from the overall current capability of
the drive and reduces the availability of torque producing current for the
motor. This can result in poor motor performance, motor stalls under full
load and nuisance drive overcurrent tripping.
Refer to
Figure 2.3
for measuring cable lengths when concerned about
cable charging. In this case, it is the total amount of cable connected to the
drive that must be considered. Diagram A shows 2 motors, each 300 feet
from the drive. The drive must be capable of supplying enough current to
charge the total length (600 ft) plus the needed current to produce necessary
torque in the motors. If the motors will not receive the desired current due to
cable charging, then the drive size should be increased to supply needed
current. Diagram B shows 1 motor at 50 feet and one at 550 feet. Again, the
drive must be capable of supplying enough current to charge the total length
(600 ft), plus the needed current to produce necessary torque in the motors.
In fact, diagrams A, B, C and D will all require the same cable charging
installation guidelines because they all have total cable lengths of 600 feet.
Cable, Motor Lengths
for more details.
2-31
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