A Current Affair: Examining the "No Amperage" Phenomenon

Article From: Products Finishing, , from JP Tech

Posted on: 8/1/2005

If you are performing a coating process that requires the use of a rectifier, you may have experienced the "no amperage" problem. Here's a look at the phenomenon and some solutions…

Rectifiers, which are used to convert alternating current (AC) into direct current (DC), are a critical—but often under appreciated—part of any electrocoating or electroplating operation.

And while rectifiers, for the most part, operate according to some very basic principles, they can occasionally pose some unique challenges for the people using them.

One of these challenges is the "no amperage phenomenon" that occurs when the rectifier is putting out voltage but no amperage. Although this is often an easily diagnosed and corrected situation, there are some circumstances in which the solution is much more complex.

To understand what is happening, it is necessary to first acknowledge the relationship between voltage, amperage, and the size of your load (resistance):

The relationship between voltage and amperage (current) can be compared to water traveling through a garden hose. Whereas the pressure of water traveling through a hose is representative of voltage, amperage would be compared to the flow of that water. By increasing the pressure of the water, you increase its flow.

A rectifier connected to an electrocoating, plating, anodizing or electropolishing tank that does not have a load in it will not generate any amperage. To continue with the garden hose analogy, it is like closing the nozzle of the hose. The load is extremely important as it provides the means to bridge the gap between the positive (anode) connection and the negative (cathode) connection. The bath provides resistance until the application of sufficient voltage to cause the ions to flow across the gap from the anode to the cathode.

At a given voltage, the larger the load the more amperage will be drawn until the process reaches it's maximum potential between the bath, the load, and the rectifier. In other words, a rectifier only produces voltage. Amperage is determined by the load. 

Diagnosing the Problem

When experiencing the "no amperage phenomenon," the first thing that needs to be determined is if there is really no amperage. A rectifier's ammeter is typically driven by a 50 mv signal (sometimes 100 mv) from the shunt. The shunt is a fixed resistor (which looks like two rectangular pieces of brass with thin plates connecting them) that is placed on the output of the rectifier. This generates a small signal proportionate to the amperage that is flowing through it.

For instance, if you have a 4,000 amp rectifier putting out 4,000 amps, the shunt will generate a 50 millivolt (mv) signal which will drive the 4,000 amp meter to read 4,000 amps. If this same rectifier is only providing 2,000 amps, the shunt will generate only half of the 50 mv (25 mv), which should drive the meter to read 2,000 amps. All readings from the meter should be in proportion to the ratio of rectifier output divided by capacity.

To determine if this is the case, use a multimeter set to "DC voltage" and read the signal across the back of the ammeter. If you are reading between 0 and 50 mv on your multimeter and the rectifier's ammeter is not registering, then you have either a bad connection or bad meter. Clean the connections or replace the meter.

If you are not reading any millivolts at the meter, try to perform a millivolt reading at the shunt. If you get a reading there, then you have either bad connections or wiring from the shunt to the ammeter. Replace as needed.

If you have determined that there is really "no amperage", you should investigate further. The most common cause of no amperage is a bad contact somewhere between the rectifier and the load. This can be in the buswork, at the saddle, or even where the anodes sit on the bar. To determine if this is the case, measure the voltage between the load and the anode bar. If you have little or no voltage, there is a bad connection somewhere and you must find it and correct it. If the voltage at the tank is within 5% of the rectifier's voltage, then it is unlikely that you have a connection issue.

A primitive, but easy-to-use technique for determining if the rectifier is capable of developing its rated amperage is to "dead short" the rectifier. Do this by clamping a piece of copper bus or cable of sufficient size to handle the amperage across the output busses of the rectifier and then turning the unit up slowly while watching the clamped piece closely. If the amperage goes up quickly and the clamped piece becomes hot, then the rectifier is probably working correctly and your problem lies in the connecting bus or the bath itself. (Editor's note: This technique – while effective – may in fact cause a weak component to fail such as the diodes or even the main transformer of the rectifier. If you do not feel comfortable performing this test, seek the assistance of a qualified professional.)

If you have a condition of less than full voltage output along with no amperage, you may have a failure of one or more components within the rectifier. These causes are difficult to diagnose without more information but may include:

  • blown diodes
  • blown SCRs
  • tapswitches set incorrectly or with a bad tap
  • shorted powerstat
  • missing AC phase
  • defective SCR firing board
  • defective main transformer

If one or more of these components is creating your problem, either your in-house or an independent repair service will be needed to diagnose and correct the situation.

Craig Henry is the vice president of JP Tech, Inc., a Wisconsin-based firm specializing in the sales, service and engineering of rectifiers, controllers and meters for the electrocoating and plating industries. He also co-authors "Dr. Rectifier" an Internet-based column (www.drrectifier.com), which dispenses free advice to those with rectifier issues.

 


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