Reflected wave – Rockwell Automation 20B PowerFlex 70, PowerFlex 700 Reference Manual User Manual
Page 149
Reflected Wave
Rockwell Automation Publication PFLEX-RM001H-EN-P - June 2013
149
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.
Reflected Wave
[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.
118 [Trim Out Select]
Specifies which speed references are to be trimmed.
117
119
120
0
0
x
0
x
x
x
x
x
x
x
x
x
x
x
x
10
0
1
2
3
4
5
6
7
8
9
11
12
13
14
15
1
=Trimmed/%
0
=Not Trimmed/Add
x =Reserved
Bit #
Factory Default Bit Values
Trim Ref A
Trim Ref B
Ad
d or % *
* Vector firmware 3.001 & later.
<T
α
1670 Vpk
5
0
10
15
20
25
Time (µsec)
30
35
40
45
50
500
V/div
0
500
V/div
Inverter
Motor
0