Rectifier Efficiency Formula for Cathodic Protection

Rectifier efficiency compares useful DC output power to AC input power. It is useful for evaluating rectifier performance, troubleshooting abnormal power consumption, and understanding whether a CP power source is operating reasonably.

Formula

DC output power = VDC × ADC

Efficiency (%) = (DC watts / AC watts) × 100

TermMeaningCommon Unit
VDCRectifier DC voltage outputvolts DC
ADCRectifier DC current outputamps DC
DC wattsUseful DC output powerwatts
AC wattsAC input powerwatts

Worked Example 1: Basic Efficiency

A rectifier has 20 VDC and 8 ADC output. AC input power is measured as 250 watts. Estimate efficiency.

DC watts = 20 × 8 = 160 W

Efficiency = (160 / 250) × 100

Efficiency = 64%

The estimated operating efficiency is 64 percent.

Worked Example 2: KWH Meter Disc Method

If a watt-hour meter provides a meter constant and disc revolutions over a measured time interval, first calculate AC watts from the meter data, then compare the rectifier DC watts to that AC input power.

AC watts = K × N × 3600 / T

Efficiency = (VDC × ADC / AC watts) × 100

Use seconds for the measured time interval when applying the 3600-second conversion.

When Efficiency Matters

  • Comparing rectifier performance before and after repairs.
  • Investigating abnormal utility consumption.
  • Checking whether low DC output is caused by circuit resistance, tap setting, failed components, or power supply issues.
  • Supporting CP 2, CP 3, and CP 4 level troubleshooting practice.

Interpretation Notes

A low calculated efficiency does not automatically identify one failed component. It should trigger additional checks of AC input, tap settings, rectifier stack condition, circuit resistance, DC output stability, ripple, shunts, panel meters, and external loading.

Common Mistakes

Confusing Power and Energy

Watts are power. Kilowatt-hours are energy over time. Convert correctly before calculating efficiency.

Using Nameplate Output

Use measured VDC and ADC, not nameplate capacity.

Ignoring Circuit Condition

Efficiency should be interpreted alongside output current, voltage, tap setting, and circuit resistance.

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