Service Manual
Test Procedures
7 - 27
3x3 UPS
Issue 2 - November 2001
7.8.7 - Checking the Hinet power circuit capacitors.
Failure of one or more capacitors in the power circuits, can normally be determined by a visual
inspection. A defective capacitor will normally have distortion to the top of the capacitor can, the terĆ
minals may have splayed apart and in extreme circumstances the cooling oil may leak from the conĆ
tainer.
The primary and secondary DC bus capacitors C1 & C2; C7 & C8; should have a thorough visual examĆ
ination.
If the capacitors show no external signs of damage each capacitor may be checked individually for an
internal open or short circuit.
Capacitor short circuit checks
1.
Transfer the load to the Maintenance Bypass supply.
2.
Totally powerĆdown the UPS.
3.
Allow sufficient time for the capacitors to fully discharge, verify with DC voltmeter (DVM) that:
a)
DC bus capacitors are safe to touch.
b)
Half bridge capacitors are safe to touch.
4.
Isolate each capacitor by disconnecting each terminal connection.
5.
Using an ohmmeter (DVM on Resistance range) check the following:
a)
Connect the ohmmeter between the capacitor terminals and verify that the capacitor
charges up (eventually the meter will indicate OL).
b)
Measure the resistance between each capacitor terminal and ground (capacitor body
earth) ć this should indicate open circuit in each case.
6.
Replace any capacitor that fails the above checks.
Capacitor open circuit checks
1.
Transfer the load to the Maintenance Bypass supply.
2.
Totally powerĆdown the UPS.
3.
Allow sufficient time for the capacitors to fully discharge, verify with DC voltmeter (DVM) that:
a)
DC bus capacitors are safe to touch.
b)
Half bridge capacitors are safe to touch.
4.
Isolate each capacitor by disconnecting each terminal connection.
5.
Connect a shorting link across both capacitor terminals, and at the same time connect an
ohmmeter (DVM) across the shorted terminals (i.e. meter should indicate 0 Ohms ć
shortĆcircuit).
6.
Remove the shorting link and note the time taken by the meter indication to rise from 0
Ohms to OL as the capacitor charges up to the meter voltage ć this should be of the order of
20 seconds on a suitable meter range.
7.
Perform this check for the DC bus capacitors and compare the results. If any capacitor has a
seriously reduced time constant compared to the others then it should be replaced.
8.
Repeat the check for any suspect capacitors.