Reflected Wave - Allen-Bradley PowerFlex 70 Reference Manual

Reflected Wave

118 [Trim Out Select]
Specifies which speed references are to be trimmed.
x x x x x
15 14 13 12 11 10
Bit #
Factory Default Bit Values
For example, % selected, Max Frequency = 130, Speed Reference = 22 Hz, Trim
Reference = 20%. 4.4 Hz will be added to the Speed Reference.
% not selected, Max Frequency = 130, Speed Reference = 22 Hz, Trim Reference
= 20%. 26 Hz will be added to the Speed Reference.
[Compensation]
The pulses from a Pulse Width Modulation (PWM) inverter using IGBTs are
very short in duration (50 nanoseconds to 1 millisecond). These short pulse
times combined with the fast rise times (50 to 400 nanoseconds) of the IGBT,
will result in excessive over-voltage transients at the motor.
Voltages in excess of twice the DC bus voltage (650V DC nominal at 480V
input) will occur at the motor and can cause motor winding failure.
The patented reflected wave correction software in the PowerFlex 70/700 will
reduce these over-voltage transients from a VFD to the motor. 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 over-voltage transient to 2.25 per unit volts line-to-line peak at 600 feet
of cable.
400 V Line = 540V DC bus x 2.25 = 1215V
480 V Line = 650V DC bus x 2.25 = 1463V
600 V Line = 810V DC bus x 2.25 = 1823 V
The software is standard and requires no special parameters or settings.
500
V/div
<Tα
0
500
V/div
0
0 5 10 15 20
Time (µsec)
Rockwell Automation Publication PFLEX-RM001H-EN-P - June 2013
1 =Trimmed/%
x x x x x x x x
0 0 0
0 =Not Trimmed/Add
9 8 7 6 5 4 3 2 1 0
x =Reserved
* Vector firmware 3.001 & later.
Inverter
1670 V pk
Motor
25 30 35 40 45 50
Reflected Wave
117
119
120
149