Danfoss VLT HVAC Drive FC 100 Service Manual page 97

6 Test Procedures
Main rectifier circuit test part I
1.
2.
Each reading should show infinity. The meter will start at a low value and slowly climb towards
infinity due to capacitance within the adjustable frequency drive being charged by the meter.
Incorrect reading
With the Part I test connection, the SCRs in the SCR/diode modules are reverse biased so they
are blocking current flow. If a short circuit exists, it would be possible that either the SCRs or the
diodes in the soft charge rectifier are shorted. To isolate between SCRs or the soft charge rectifier,
perform the Soft Charge Rectifier Test.
Main rectifier circuit test part II
1.
6
2.
Incorrect reading
With the Part II test connection, even though the SCRs in the SCR/diode modules are forward
biased by the meter, current will not flow through the SCRs without providing a signal to their
gates. The upper diodes in the soft charge rectifier are forward biased so the meter reads the
voltage drop across those diodes.
If an open reading were present, it would indicate the upper diodes in the soft charge rectifier are
open. It could also indicate that one or more of the soft charge fuses are open. It could further
indicate that the soft charge resistor is open. To isolate between the three possibilities, perform
the Soft Charge Fuse Test and Soft Charge Rectifier Test.
A short circuit reading indicates either one or more of the upper soft charge rectifier diodes are
shorted or the SCRs are shorted in the SCR/diode module. To isolate between SCRs or the soft
charge rectifier, perform the Soft Charge Rectifier Test.
Main rectifier circuit test part III
1.
2.
Incorrect reading With the Part III test connection, the diodes in the SCR/diode modules are
forward biased as well as the lower diodes in the soft charge rectifier. The meter reads the diode
drops. If a short circuit exists, it would be possible that either the diodes in the SCR/diode modules
or the lower diodes in the soft charge rectifier are shorted. To isolate between SCRs or the soft
charge rectifier, perform the Soft Charge Rectifier Test.
Although an open reading is possible, it is unlikely since that indicates that both the diodes in the
SCR/diode modules and the lower diodes in the soft charge rectifier are open. Should that occur,
replace both diodes.
6-6
Connect the positive (+) meter lead to the positive (+) DC bus connector MK105 (A) on
the power card.
Connect the negative (–) meter lead to terminals L1, L2, and L3 in sequence.
Reverse meter leads by connecting the negative (–) meter lead to the positive (+) DC
bus connector MK105 (A) on the power card.
Connect the positive (+) meter lead to L1, L2, and L3 in sequence. Each reading should
show a diode drop.
Connect the positive (+) meter lead to the negative (-) DC bus connector MK105 (B) on
the power card.
Connect the negative (–) meter lead to terminals L1, L2 and L3 in sequence. Each reading
should show a diode drop.
MG.90.L1.22 - VLT
High Power Service Manual for VLT
® is a registered Danfoss trademark
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FC Series
Adjustable Frequency Drives