Rockwell Automation SP600 AC Drive User Manual Version 1.0 - Manual (Page 62)

4-8 SP600 AC Drive User Manual
Initially, the cable is in a fully charged condition. A transient
disturbance occurs by discharging the cable for approximately 4 ms.
The propagation delay between the inverter terminals and motor
terminals is approximately 1 ms. The small time between pulses of
4 ms does not provide sufficient time to allow the decay of the cable
transient. Thus, the second pulse arrives at a point in the motor
terminal voltage’s natural response and excites a motor overvoltage
transient greater than 2 pu.
The amplitude of the double pulsed motor overvoltage is
determined by a number of variables. These include the damping
characteristics of the cable, bus voltage, and the time between
pulses, the carrier frequency, modulation technique, and duty cycle.
Figure 4.3 shows the per unit motor overvoltage as a function of
cable length. This is for no correction versus the modulation
correction code for varied lengths of #12 AWG PVC cable to 600
feet for a 4 kHz and 8 kHz carrier frequencies. The output
line-to-line voltage was measured at the motor terminals in 100 feet
increments.
Without the correction, the overvoltage increases to unsafe levels
with increasing cable length for both carrier frequencies.
The patented modulation correction code reduces the overvoltage
for both carrier frequencies and maintains a relatively flat
overvoltage level for increasing cable lengths beyond 300 feet.
Figure 4.3 – Motor Overvoltage as a Function of Cable Length
No Correction vs Correction Method at 4 kHz and 8 kHz Carrier
Frequencies - Vbus = 650, fe = 60 Hz
Cable Length (Feet)
per Unit Vout/Vbus
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
2.5
2.6
100 0 200 400 600 300 500
No Correction 4 kHz Carrier
Corrected 4 kHz Carrier
No Correction 8 kHz Carrier
Corrected 8 kHz Carrier