Allen-Bradley PowerFlex 70 Reference Manual page 150

Reflected Wave
150
The above figure shows the inverter line-to-line output voltage (top trace) and
the motor line-to-line voltage (bottom trace) for a 10 HP, 460V AC inverter, and
an unloaded 10 HP AC induction motor at 60 Hz operation. 500 ft. of #12
AWG cable connects the drive to the motor.
Initially, the cable is in a fully charged condition. A transient disturbance occurs
by discharging the cable for approximately 4ms. The propagation delay between
the inverter terminals and motor terminals is approximately 1ms. The small time
between pulses of 4ms 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 over-voltage transient greater than
2 pu. The amplitude of the double pulsed motor over-voltage 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.
The plot below 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 cable to 600 feet for 4 and 8 kHz carrier frequencies. The
output line-to-line voltage was measured at the motor terminals in 100 feet
increments.
No Correction vs Correction Method at 4 kHz and 8 kHz Carrier
Frequencies - Vbus = 650, fe = 60 Hz
2.6
No Correction 4 kHz Carrier
2.5
Corrected 4 kHz Carrier
No Correction 8 kHz Carrier
2.4
Corrected 8 kHz Carrier
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
0 100 200 300
Cable Length (Feet)
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.
To determine the maximum recommended motor cable lengths for a particular
drive refer to Cable, Motor Lengths on page
Rockwell Automation Publication PFLEX-RM001H-EN-P - June 2013
400 500 600
61.