Rockwell Automation SD3000 Drive Configuration, Programming - Manual (Page 89)

Appendix B
Continued
The SCR diagnostic tunables are summarized below. The first step in the diagnostics is to update a running
average of the current feedback over the last 12 SCR firings. The next step in the routine is to calculate the
error as follows:
Error =
(Running-Average - Current Feedback) * 100
Running-Average
If the calculated error is less than SCR_DBAND%, then the error is set to zero. Higher values in
SCR_DBAND% can keep small errors from causing nuisance faults.
SCR_DBAND% should be increased to reduce nuisance warnings that may occur due to small imbalances
between the SCRs. For example, if SCR_DBAND% is set to 20, all imbalances less than 20% are ignored.
To keep track of the error for every SCR, there are 12 integrators (only six would be used for non-regenerative
drive applications), which are selected based on which SCR pair has just fired. On every scan the integrators
decay to zero using the following formula:
Integrator( i ) = Integrator( i ) * SCR_DECAY%/100
SCR_DECAY% cannot be less than one, and should not be greater than 99. Making SCR_DECAY% smaller
makes the error decay faster.
SCR_DECAY% indicates how much error is retained from calculation to calculation. It should be decreased
when nuisance warnings are occuring due to large cyclic loads. During step changes in current, some of the
SCRs in the bridge will carry more or less current than the average until a steady state is achieved. During
these periods, the error integrator will integrate in the positive direction even though there is no real current
sharing problem. SCR_DECAY% can keep the error integrator from slowly integrating up to the warning point
over time due to these transient conditions. Entering a value of 99 would result in the error integrator being
decreased by 1% of its value every time it is calculated for a particular SCR. A value of 1 would result in the
error integrator being decreased by 99% of its value.
On every scan, the integrators build up by the following formula:
Integrator( i ) = Integrator( i ) + Error * SCR_GAIN% /1 00
SCR_GAIN% determines how fast error builds up. The value should be increased if warnings need to be
annunciated quickly, for example, in the case of a Power Module being run close to current or temperature
limits. The default setting (100) may result in the lapse of several seconds before the problem is recognized
and annunciated. Note that in most cases, other SCRs in the bridge that must make up for the loss of one
SCR will not be adversely affected because their thermal time constants are on the order of minutes. A value
of 1 in this variable means that 1 /100 of accumulated error will be carried over to the next calculation. A value
of 100 (the default) means that the gain is 1 (100/1 00). A value of 500 (the maximum) means that the gain is 5
times the error (500/1 00).
When Integrator(i) becomes greater than SCR_TRIP%, a diagnostic bit is set to indicate that a problem has
been detected. Making SCR_TRIP% larger raises the detection threshold for errors. SCR_TRIP% determines
the threshold of integrated error that will cause a warning to be annunciated. The higher the value (maximum
= 3000), the more tolerant the system will be of SCR errors. The lower the value (1500 minimum and default),
the earlier the system will annunciate SCR errors.
B-5