6 Commonly-seen Faults of UPS

When there is electric power, the UPS output is normal; when there is no electric power, the buzzer will give a long blow and there is no output.
Fault analysis: Judged by the phenomenon, the accumulator and the inverter are in fault.
Check the accumulator’s voltage to see whether the accumulator is inadequate charged, charge whether the accumulator itself is in fault or the charging circuit is in fault;
If the accumulator working voltage is normal, check whether the inverter’s drive circuit functions normally. If the drive circuit outputs normally, it means that the inverter is damaged;
If the inverter’s drive circuit cannot work normally, it should be checked whether the circuit generated by the waveform outputs PWM control signals. If yes, it means the inverter’s drive circuit is in fault;
If the waveform-generated circuit outputs no PWM control signals, it should be checked whether the output is blocked by the protective circuit. If yes, clarify why the protective circuit is operating;
If the protective circuit’s operating voltage is normal but the waveform-generated circuit outputs no PWM waveform, it means the waveform-generated circuit has been damaged.
The accumulator’s voltage is small, but after dozens of hours of charging at the startup state, the accumulator’s voltage cannot reach the required level.
Fault analysis: Judged by the phenomenon, the accumulator or the charging circuit is in fault. Check whether the output circuit output and input is normal;
If the charging circuit input is normal but the output is abnormal, disconnect the accumulator for remeasurement of the accumulator. If it is still abnormal, it suggests that the charging circuit is in fault;
If the charging circuit’s input and output are both normal after disconnection of the accumulator, it means that the accumulator has been damaged because of long-term no charge or expiration.
One pair of power amplifier transistors of the inverter at the power level, after changing the transistor of the same type, might be burned after operating for a period of time because of excessive current. Causes of excessive current:
The overcurrent protection fails. When overcurrent occurs to the inverter’s output, the overcurrent protective circuit does not work.
The PWM components are in fault, and the output two circuits mutually supplement their waveform asymmetry. One has a long “on” time and the other has a short “on” time. As a result, two arms cannot work normally and two tubes will be damaged. 
After UPS is on, there is no display on the panel, and the UPS cannot work.
Fault analysis: Judged by the fault phenomenon, the fault is with the electric power input, accumulator and electric power detection, and accumulator voltage detection loop.
Check whether the electric power input fuse is burned;
If the electric power input fuse is in a good state, check whether the accumulator fuse is burned;
If the electric power fuse is in a good state, check whether the electric power detection circuit works normally;
If the electric power detection circuit is in a good state, check whether the accumulator’s voltage detection circuit is normal.
A standby UPS works normally when the UPS has the electric power. When there is no electric power, the inverter has output but the output voltage is considerately low. At the same time, the transformer will emit large noises.
Fault analysis: When the inverter has output, it means that the final-stage driver circuit should be maintained normal. If the transformer has noises, it means the two arms of the push-pull circuit are asymmetric.
Check whether the power is normal;
Check whether the power is normal;
If the power is normal, check whether the output circuit and output signal of the PWM are normal;
If the pulse output circuit outputs normally, check whether the output of the drive circuit is normal.
When the load is close to the full load, the electric power supply of the standby UPS is normal. When the accumulator provides power, the accumulator’s fuse is blown.
Fault analysis: The accumulator’s fuse is blown, indicating a large power supply current of the accumulator.
Check whether the inverter is penetrated through;
Check whether the accumulator’s voltage is too low;
If the accumulator’s voltage is too low, check the accumulator’s charge circuit to see whether it is normal;
If the accumulator’s charging circuit is normal, check whether the accumulator’s voltage and circuit function normally.