Rockwell Automation 1395 Digital DC Drive FRN 5-10.10/9.30 - Manual (Page 235)

Chapter 7
Programming Parameters
7-69
Parameter 744 - Bridge Switch Delay
Internal units : None
Minimum Value: 0
Maximum Value: 75
Default Value: 2
Description: This parameter allows the user to set up a programmable
delay which would begin after the drive's Zero Current Detector tells the
drive to switch between bridges. A load with higher than normal
inductance could mean that an undesirable level of current still exists
when the drive attempts to change bridges. This is true for both forward
to reverse and for a reverse to forward bridge change. This delay would
help to insure that the armature circuit current has additional time to
reach zero before the bridge change occurs.
The units used for Parameter 744 will be a number "n" ranging from 0 to
75. Where " n" represents the number of "time increments" to delay the
bridge change. The length of these time increments will vary for 60Hz
or 50Hz systems.
For a 60 Hz system:
Time delay in seconds = n(2.78x10
-3 ) +1.1x10 -3 where 2.78 ms is the
conduction time @ 60Hz.
For a 50 Hz system:
Time delay in seconds = n(3.30x10
-3 ) +1.1x10 -3 where 3.33 ms is the
conduction time @ 50Hz.
The 1.1 ms in the above formula is the standard time delay used for
normal armature inductance. As you can see in the above formula, the
number programmed into Parameter 744 is the integer number of
conduction periods added to the normal 1.1ms delay.
NOTE: For DC motor armature time constants in the range of 1 to 100
milliseconds, Parameter 744 should be set to zero. (2 is the default value)
In order to determine the desired time delay some additional application
information is required and the following equation solved.
t
d
= 0.03(I
0
)(L
a
)/V
d
Time delay in seconds
where:
I
0
= Rated armature bridge amps of the 1395
L
a
= Inductance of the load in Henrys, when the load current is 0.03(I
0
)
V
d
= 1.169(V
11
) where:
V
11
= the lowest average line to line voltage input to the 1395 at a
particular installation.
Example: Assume a load with a 10 Henry inductance, a drive rated a 75
amps, and a 460VAC line that could dip to 90% of nominal.
t
d
= 0.03(75)(10)/1.169(414) = 0.0649 would be the desired delay for this
application. Using this value in the equation for calculating the time
delay for Parameter 744@ 60Hz:
0.04649 = n(2.78x10
-3 ) +1.1x10 -3 and rearranging to solve for n:
n = (0.04649 - 1.1 x 10
-3
)/(2.78 x 10
-3
) = 16.33. Rounding to the next
highest integer, 17 would be the value programmed into Parameter 744.
If additional safety margin is desired, the number 20 might be suitable
for this application.
Added in Firmware
Version 9.20