Operation: CP1SD/T/L Version
86
CENTRON® Polyphase Meter Technical Reference Guide
5
The current ramps (directly or through zero) to the LL current level at unity power
factor.
6
A pulse from the meter starts the LL test and another pulse ends it.
Most test boards use jogging (slewing) immediately following a FL or PF test to shorten the
time required for the next test to start. The energy used for jogging may be more than
enough to cause the next pulse from the meter even before the ramping of current or
changing of phase angle is completed. If the trigger to start the next test is armed and ready
during the jogging or transition to the next test level, an unexpected pulse may cause the test
to start too soon. This obviously will result in erroneous readings. Some settling time is
necessary for the test board power, the reference standard, and the meter under test to
stabilize after the change to a new test level.
Most test boards provide a settling time (programmable or fixed) and will not recognize
another test pulse following the completion of a test until the jogging, ramping, and settling
time have all transpired. The CENTRON Polyphase meter needs a settling time of about
three to five seconds after the new test level has been reached before the test starts.
Recommendations
Erroneous test results caused by the problems previously described can probably be
corrected by implementing one of the following suggestions. Even if there are no bad
readings, Solution 3 can cut the total test time significantly without sacrificing test
verification certainty. The suggested solutions are:
1
Change the test sequence to avoid jogging before the light-load test.
2
Upgrade the test board to meet the requirements listed previously.
3
Program the meter and test board for a small test constant. This will avoid jogging and
also give the added benefit of shorter test time.
Solution 1
This is the preferred solution, since it results in shorter test times and can be implemented
simply by programming the meter for a smaller test constant and settling the test board
accordingly.
The CENTRON Polyphase and most other solid-state meters have the capability of being
programmed for a much smaller test constant (Kt), such as one-tenth or one-twelfth of the
energy required for one “disk revolution” of the meter. With the test pulses running 10 to 12
times faster, there is the possibility of shortening the test time considerably, but not by a
factor of 10 or 12. It still takes a finite amount of time to obtain meaningful results.
Solution 2
Change the test sequence so that the LL test is first, followed by the PF test and then the FL
test. This should prevent all jogging from occurring between tests and will probably
eliminate the erroneous readings. This is the quickest solution to implement since it requires
no changes to the test board or the meter.
