Checking Neutral Point (Np) Diodes - ABB PCS6000 Service Manual

6. Reconnect the capacitor wires (torque 5.5 Nm).
IMPORTANT! Because the respective diodes of each phase are connected in parallel
(V11C||V21C||V31C and V12C||V22C||V32C) a faulty clamp diode will cause the multimeter
to show the other two diodes as faulty as well. If the newly installed diode is tested and
shows a value below 0.22 V, check and replace the diodes connected in parallel first,
before doubting the quality of the new diode. In the worst case, all three diodes have to
be replaced to find the faulty one.

9.9.4. Checking neutral point (NP) diodes

1. Carry out the preparation procedure according to section 9.9.1, Preparation for
checking power diodes, page 129.
2. Switch the multimeter to the diode test function.
3. Hold the two probes to the side of the coolers directly above and below the diode
to be tested.
IMPORTANT! Pay attention to the correct polarity of the diode and the probes. The plus
probe must always be connected to the anode (1) of the diode and not the cathode (2).
1
2
+ -
4. The multimeter should indicate a value of ~0.22 V across each diode.
Compare the values of all diodes in both stacks. Normally, only one diode is defective
and the voltage should be lower than across all others.
5. If the shown value is still below 0.22 V the diode needs to be replaced according to
section 10.6.5.3, Replacing neutral point (NP) diodes, page 157.
IMPORTANT! Because the respective diodes of each phase are connected in parallel
(Vx1N||Vx1D||Vx2D and Vx2N||Vx3D||Vx4D) a faulty clamp diode will cause the multimeter
to show the other two diodes as faulty as well. If the newly installed diode is tested and
shows a value below 0.22 V, check and replace the diodes connected in parallel first,
before doubting the quality of the new diode. In the worst case, all three diodes have to
be replaced to find the faulty one.
PRODUCT
PCS6000
DOCUMENT KIND
Service manual
CHECKING DIODES, IGCTS AND IPS
DOCUMENT ID.
3BHS600000 E80
REV. LANG. PAGE
F en 131/272