9 Common Fault Analyses of UPS
Oct 24, 2024
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1. When the UPS output is normal with mains power, but the buzzer sounds continuously without mains power, and there is no output.
Fault Analysis: This indicates a fault in the battery and inverter section.
The following steps can be used to check:
A. Check the battery voltage to see if the battery is undercharged. If the battery is undercharged, check if the fault is with the battery itself or the charging circuit.
B. If the battery operating voltage is normal, check if the inverter drive circuit is functioning properly. If the drive circuit output is normal, the inverter is damaged.
C. If the inverter drive circuit is not functioning properly, check if the waveform generation circuit has a PWM control signal output. If there is a control signal output, the fault is in the inverter drive circuit.
D. If the waveform generation circuit has no PWM control signal output, check if its output is blocked by the protection circuit. If so, identify the protection cause.
E. If the protection circuit is not working and the operating voltage is normal, but the waveform generation circuit has no PWM waveform output, the waveform generation circuit is damaged. The troubleshooting sequence can also be reversed, which may help find the fault faster.
2. The battery voltage is low, but after charging for more than ten hours, the battery voltage still does not rise.
Fault Analysis: This indicates a fault in the battery or charging circuit. The following steps can be used to check:
A. Check if the input and output voltages of the charging circuit are normal.
B. If the charging circuit input is normal but the output is not, disconnect the battery and measure again. If it is still not normal, the charging circuit is faulty.
C. If the charging circuit input and output are normal after disconnecting the battery, the battery is likely damaged due to long-term non-charging, over-discharge, or reaching its lifespan.
3. A pair of power amplifier transistors in the inverter section are damaged. After replacing them with the same model, they burn out again due to excessive current.
The reasons for excessive current are:
A. Overcurrent protection failure. When overcurrent occurs in the inverter output, the overcurrent protection circuit does not function.
B. Fault in the PWM component, resulting in asymmetric complementary waveforms. One has a longer conduction time, and the other has a shorter conduction time, causing imbalance in both arms, or both arms conducting simultaneously, leading to transistor damage.
C. Significant difference in power transistor parameters. Even with symmetric input waveforms, the output will be asymmetric. This waveform, after passing through the output transformer, causes magnetic bias, i.e., unbalanced magnetic flux, which accumulates and leads to transformer saturation and a sudden increase in current, burning the power transistors. Once one is burned, the other follows.
4. After turning on the UPS, there is no display on the panel, and the UPS does not work.
Fault Analysis: The fault is likely in the mains power input, battery, mains power detection section, and battery voltage detection circuit:
A. Check if the mains input fuse is burned.
B. If the mains input fuse is intact, check if the battery fuse is burned, as some UPS systems shut down all outputs and displays when they cannot detect the battery voltage.
C. If the battery fuse is intact, check if the mains power detection circuit is functioning properly. If the mains power detection circuit is not functioning properly and the UPS does not have a no-mains-power startup function, the UPS will shut down all outputs and displays.
D. If the mains power detection circuit is functioning properly, check if the battery voltage detection circuit is normal.
5. When mains power is connected, each time the UPS is turned on, the relay repeatedly actuates, the battery voltage low indicator on the panel lights up continuously, and the buzzer sounds continuously.
Based on the above fault phenomena, it can be determined that the fault is caused by the low battery voltage, resulting in the UPS failing to start. Remove the battery, perform equalizing charge (charging all batteries in parallel), and if it still does not succeed, replace the battery.
6. A standby UPS works normally with mains power, but without mains power, the inverter has output, but the output voltage is low, and the transformer emits a loud noise.
Fault Analysis: The inverter output indicates that the final stage drive circuit is basically normal, and the transformer noise indicates asymmetric operation of the push-pull circuit's two arms. The following steps can be used to check:
A. Check if the power is normal.
B. If the power is normal, check if the output signal of the pulse width output circuit is normal.
C. If the pulse width output circuit output is normal, check if the output of the drive circuit is normal.
7. When mains power is normal, turning on the UPS causes the inverter operation indicator to flash, and the buzzer to emit intermittent sounds. The UPS can only work in inverter mode and cannot switch to mains mode.
Fault Analysis: The inability to switch from inverter power supply to mains power supply indicates a fault in the inverter power supply to mains power supply conversion section. The following should be checked:
A. Check if the mains input fuse is damaged.
B. If the mains input fuse is intact, check if the output of the mains rectifier filter circuit is normal.
C. If the mains rectifier filter circuit output is normal, check if the mains power detection circuit is normal.
D. If the mains power detection circuit is normal, check if the control output for inverter power supply to mains power supply conversion is normal.
8. In a standby UPS, when the load is close to full, mains supply is normal, but the battery fuse blows when battery power is used.
Fault Analysis: The battery fuse blowing indicates excessive battery current. The following steps can be used to check:
A. Check if the inverter is shorted.
B. Check if the battery voltage is too low.
C. If the battery voltage is too low, check if the battery charging circuit is normal.
D. If the battery charging circuit is normal, check if the battery voltage detection circuit is functioning properly.
9. The UPS can only be powered by mains power and cannot switch to inverter power supply.
Fault Analysis: The inability to switch from mains power to inverter power supply indicates a fault in the mains power to inverter power supply conversion section. The following should be checked:
A. Check if the battery voltage is too low and if the battery fuse is intact.
B. If the battery part is normal, check if the battery voltage detection circuit is normal.
C. If the battery voltage detection circuit is normal, check if the control output for mains power to inverter power supply conversion is normal.
