EXPERT TIP #3: DIAGNOSING & FIXING A ‘BROKEN’ RECTIFIER
Understanding Rectifiers
Most cathodic protection (CP) rectifiers produced today are extremely reliable. Yet, like any manmade device, failures can and will occur. For CP rectifiers, in some instances, they will continue to operate despite component failure.
Most CP rectifiers in service today are air-cooled, single-phase transformer units. While they may appear complicated to provide needed DC current for cathodic protection to a steel structure, there are only two components required to have a fully functional rectifier:
- Step-down transformer - to reduce the incoming AC power to a lower, usable voltage
- Diode or rectifier stack - four diodes configured into a “full-wave bridge” rectifier stack
In most CP rectifiers, there will be circuit breakers, surge suppressors, meters, component frames, panels and an enclosure. Understanding the purpose and function can help a CP technician properly maintain and troubleshoot the rectifier.
The more components in a rectifier, the greater the chance of a failure. Some failures completely disable the rectifier while other failures have little or no effect on the actual capability of the unit to produce current. An example would be a defective meter or broken meter switch.
Rectifier Failure
As an example, an operator has a rectifier that has been operating at 20 volts and 20 amps for an extended time. Suddenly, the unit begins operating at 10 volts and 10 amps. While not completely broken, it is not operating as normal.
Two conditions (below) can cause this issue. Changes to the tap settings or reconfiguring the transformer primary AC input will not fix the problem.
Possible Causes
- The AC input voltage has reduced by 50% (unlikely). A check of the AC input voltage to the rectifier would eliminate this unlikely situation.
- One of the four rectifier stack diodes have failed in an “open” condition (most likely). The rectifier still operates, but in a “half-wave” condition, thus only producing half the voltage output and half the current output.
Troubleshooting the diode stack will not be covered in this tip. However, there is an easy way to confirm that there is an open diode. Simply measure the output frequency in Hertz (Hz). (Wait! This is a DC rectifier so it should not have an output frequency, shouldn’t it?). Actually, the typical full-wave bridge does not provide true DC output. Instead, it is a “unidirectional cycling voltage”. Therefore, it has an “AC component” to the DC output.
See the waveforms below. A normally operating rectifier will have an output frequency of 120 Hz. In the case of an open diode, the output frequency will be only 60 Hz. Most digital multimeters (DMM), as well as clamp-on ammeters, can measure frequency in Hz.
When using a DMM to test this possibility, set the meter to measure “DC volts” and connect the test leads across the output lugs of the rectifier. Press the Hz button to read the output frequency of the rectifier.
When using a DC clamp-on instrument, measure the output in DC amps. At the same time, press the Hz button or switch the meter to Hz function and read the output frequency.
Correcting this issue will require the replacement of one or possibly two diode(s). With this fix, your rectifier will be fully function again.
| 60 Hz AC |  |
| 60 Hz half wave |  |
| 120 Hz full wave |  |