Rockwell Automation Reliance electric 6SB401 Series, User Manual, Page: 55 / 274

Wiring Requirements for the Drive
4-7
The reflected wave phenomenon, also known as transmission line
effect, produces very high peak voltages on the motor due to
voltage reflection. Voltages in excess of twice the DC bus voltage,
(650 V DC nominal @480 V input) result at the motor and can
cause motor winding failure.
While Reliance Electric 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.
The correction software modifies the PWM modulator to prevent
PWM pulses less than a minimum time from being applied to the
motor. The minimum time between PWM pulses is 10
microseconds. The modifications to the PWM modulator limit the
overvoltage transient to 2.25 per unit volts line-to-line peak at 600
feet of cable.
400 V Line = 540 V DC bus (max) x 2.25 = 1200 V
480 V Line = 715 V DC bus (max) x 2.25 = 1600 V
600 V Line = 891 V DC bus (max) x 2.25 = 2000 V
Parameter 56 is used to enable or disable this feature. Refer to the
parameter description in chapter 11 for more information.
Figure 4.2 shows the inverter line-to-line output voltage (top trace)
and the motor line-to-line voltage (bottom trace) for a 10 HP, 460 V
AC inverter, and an unloaded 10 HP AC induction motor at 60 Hz
operation. 500 ft. of #12 AWG PVC cable connects the drive to the
motor.
Figure 4.2 – Inverter and Motor Line-to-line Voltages
<T
α
1670 Vpk
5 0 10 15 20 25
Time ( sec)
30 35 40 45 50
500
V/div
0
500
V/div Inverter
Motor
0
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