Service manual

Always On UPS Systems Inc

5. RECTIFIER

The main function of a rectifier is to convert the AC input power to DC power. The DC

power is then used to charge the batteries, supply the DC bus, which in turn supplies the

inverter.

The rectifier design used in the 10kVA to 60kVA UPS’s include a 6-pulse full controlled

rectifier. Power Factor Correction (PFC) has been added to maintain a high input power

factor, independent of the load power factor. This 6 pulse rectifier smoothes the current

waveform and reduces the harmonic content reflected back to the utility. The control circuit

regulates the DC bus to within 1% of the nominal voltage. Soft walk-in circuitry

(approximately 20 seconds) and current limiting circuitry are used to prevent over current or

surge currents from affecting any part of the UPS system.

Additionally, over and under-voltage protection is added to improve reliability and to

shutdown the rectifier in the event of abnormal conditions. The DC bus is adjustable to allow

for different types and capacities of batteries. The power components used within the rectifier

are specially designed and selected to handle extreme ranges of high voltage and high current

(–25% to +25%).

In order to further improve the power factor and reduce harmonic current drawn by the

rectifier, Always “On” UPS’s above 80kVA incorporate a 12-pulse full controlled rectifier

(also available as an option on the 10 to 60kVA UPS’s) to reduce the total harmonic current

to less than 12%. A phase shift transformer is added with input inductors to achieve this

higher level of performance while maintaining our high input power factor. An optional 5

harmonic filter can be added to further reduce the total harmonic distortion current to less

than 9%.

The total harmonic current can be decreased to less than 7% with the inclusion of the 18-

pulse full controlled rectifier. This option is recommended for the larger (80kVA and up)

UPS systems.

Figure 5.1

Rectifier Block Dia