UPS SIZING: RECTIFIER AND INVERTER

Monday, May 30, 2011

The sizing of the Rectifier and Inverter follows once the overall UPS size has been determined. Most of the time, the suppliers will only ask the UPS VA rating and they will do the rest. However, as an engineer, knowing how to determine the corresponding rectifier and inverter size will help you evaluate the practicality of the specifications given by the supplier. For that reason, this post includes the sizing calculations with an example for easy learning.


Rectifier or Charger

The rectifier should be properly sized to satisfactorily perform these two tasks:  Supply the inverter at full load (Ir) and charge the batteries at the maximum charge current (Ic). Therefore, the rectifier DC load current (Idc) is the sum of Ir and Ic. In equation form: Idc = Ir + Ic

The design DC load current is the current drawn by the inverter from the rectifier at full load.

Ir = S/Vdc

where:

Ir = Design DC full load current (A)
S = UPS VA rating
Vdc = nominal battery / DC link voltage

Since a 1000 VA UPS was selected, then,

Ir = 1000 VA / 120 V = 8.33 A.

Meanwhile, the maximum battery charging current can be computed as follows:

Ic = (C X f)/t

where:

Ic = maximum DC charge current (A)
C = selected battery capacity (Ampere-hour or Ah)
f = battery recharge efficiency (typically 1.1)
t = minimum battery recharge time (hours)

In the example, for a 60 Ah battery as calculated in UPS Sizing: Battery Capacity, and recharge time of 2.25 hours,

Ic = (60 Ah X 1.1) / 2.25
Ic = 29.33 A

Thus, the total minimum DC rectifier/charger current is:

Idc = 8.33 + 29.33 = 37.7 A

Select the next standard rectifier rating that exceeds the total minimum DC current above.

Use a 40-Ampere Rectifier.

Inverter Sizing

The inverter must be rated to continuously supply the UPS loads. Therefore, the inverter shall be sized based on the selected UPS VA rating.

For a three-phase UPS:
Iac = S / (1.732 X Vo)

For a single-phase UPS:
Iac = S/Vo

where:

Iac = design AC full load current (A)
S = UPS VA rating
Vo = nominal AC output voltage (line-to-line voltage for a three phase UPS)

As an example, a single-phase 1000 VA UPS shall have an inverter size of:

Iac = 1000 VA / 120 V
Iac = 8.33 A

Select the next standard inverter rating that exceeds the design AC load current.

Use a 10-Ampere Inverter.

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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.