Rectifier Maintenance

by John Morelli
Rapid Power Corporation
Dynapower Corporation
South Burlington, VT. USA

 

Today’s modern rectifiers are filled with sophisticated electronic controls, programmable logic controllers, power control equipment, power conversion devices, and cooling systems. To achieve a long lasting system, manufacturers invest many hours of engineering, research and development, manufacturing and testing to insure a high quality product. Once delivered, it is the owner’s responsibility to maintain the system directly using in-house personnel; a factory authorized service shop or factory service team. A small amount of scheduled preventive maintenance can pay large benefits in reducing unscheduled down time. 

In this article, I would like to touch on a few points that are vital to good preventive maintenance. The amount of maintenance required by any equipment is directly related to the environment the equipment is placed in, the use of the system, the original specifications that the equipment was designed to, and the amount of regular maintenance performed. A seldom maintained system may fail when you need it most.

In order to perform proper maintenance, personnel performing the maintenance should have a basic understanding of the type of rectifier system, basic knowledge of the operation of a rectifier system, type of control, and clearly understand hazards and ensure safety precautions are observed.

The most common rectifier control types used in the metal finishing industry can be categorized in one of the following control types and are available in single and three phase power configurations:

No control:

   

This type of rectifier provides limited output control and regulation. It is used where a single level of voltage is required with limited regulation.

Thyristor (Thyristor (SCR) control):

This type of control is the one most commonly used in the modern metal finishing shop, where automatic control and/or remote operation are desired. The thyristor or SCR is a solid-state semiconductor device which acts as a switch, conducting when the gate lead receives a current pulse during the time the SCR's anode is more positive than is cathode, i.e. when they are forward biased. The SCRs will continue to conduct for as long as they are forward biased. The signal at the gate lead need only be in the proper time and of only a few milliamperes to allow current to flow from anode to cathode. With no gating signal applied, the SCR fully blocks the thru voltage allowing full range control of the rectifier.

 

Manufacturers today configure SCRs using one of two typologies for providing either primary or secondary control rectifier control. In the primary system, SCRs are installed in the primary line between the input supply and the primary winding of the main power transformer. They are configured back-to-back to vary the input voltage to the transformer. The secondary rectifier assembly contains a set of diodes to perform the voltage conversion to DC. Variation of the input voltage, by adjusting the phase angle that the SCRs are fired at, alters the AC voltage on the primary of the transformer. Because the turns-ratio of the transformer is fixed, altering the primary voltage has a linear effect on the secondary transformer voltage and hence leads to a change in the output DC value. In a secondary SCR control arrangement, the SCRs perform both the control and conversion of the DC output. In each of these systems, the electronics and power controls are similar. The output DC wave shape observed is identical when units are providing the same output voltage and current.  Given the waveshape observed is identical, the measured AC RMS ripple on the DC signal is also identical between the two typologies.

Switchmode Power Supplies (SMPS):

 

Switchmode technology has been used for a time in smaller applications, but only within the past 10 years has the semiconductor industry been able to manufacture components large enough to allow power ratings for the metal finishing industry. These supplies are all solid-state in design and incorporate a high speed switching regulator. The “switch" is either on or off. In high power metal finishing applications, the switching device of choice is now the Isolated Gate Bipolar Transistors (IGBT). Because the sizing of magnetic components is dependent on volt-seconds, fast switching, high frequency allows designers to greatly reduce the physical size and weight of all internal components, such as the transformers and filters, and eliminate power-robbing components, such as resistors. With the rising cost of copper, these power supplies are very cost effective simply because of the reduced copper content. Switchmode technology offers inherent low-loss conversion from AC to DC. This high-efficiency design reduces energy consumption compared to the same output ratings of SCR type rectifiers. Manufacturers are now producing these power supplies in ranges up to 10,000 amps in a small, efficient, reliable package.

Chopper

Switching Rectifier incorporating line frequency transformers (IGBT Chopper): This is the most recent technology to be applied to the conversion of AC to DC, and is used in large high current systems. The chopper system employs two (2) conversion technologies and was developed to take advantage of key features of each. The input stage of the chopper uses a no-control diode bridge to convert the incoming AC to DC providing constant high power factor and low harmonic distortion on the incoming AC. The DC voltage is fed to a high power electronic switch, the Insulated Gate Bipolar Transistor (IGBT), that has the ability to turn the DC on and off to the load. The IGBT circuit also has support components using a capacitor to store energy and a free-wheeling diode to return energy as the chopper does its work. The effective DC output of the chopper is the average value of the DC from the diode bridge turned on and off by the IGBT duty cycle. An example of this is a DC of 100VDC switched with a 50% duty cycle (on 50% time, off 50% time) will have an effective DC voltage of 50VDC and a 33% duty cycle will be 33VDC and so on. Control of the IGBT gating duty cycle allows the chopper output DC to be regulated throughout the range of the buss DC voltage. A key feature of this IGBT switching is that it operates at a much higher switching frequency that is independent of the incoming AC line. This results in a much lower ripple voltage across the full range of control.

The combined features of high constant power factor on the input and complete DC voltage regulation with low DC ripple on the output is very beneficial to applications that have stringent power quality issues and high product quality issues.

 

Brief descriptions of the six major parts to a modern rectifier are:

1. The functional control components - provide a means of controlling the input voltage to the rectifier and operating the control functions and features; i.e. starting the rectifier, controlling the time the rectifier is on, counting amp-hours.
2. The monitoring system - senses faults within the system and provides system protection; i.e. temperature alarms, over current, coolant leaks, overload alarms.
3. The electronics - control flow of output voltage to provide desired levels of load current, i.e. interface with customers PLC or DCS, control the electronic firing of the thyristors or power components.
4. The power transformer - used to convert AC utility power to a voltage suitable for metal finishing and the conversion circuit type. The construction of the transformer can vary depending on the overall design of the system. This includes air cooled windings of VPI technology, liquid cooled where direct water or oil based coolant is circulated internal to the current carrying conductors, oil immersed or cast epoxy filled with air ducts or liquid cooling paths.
5. The power conversion section - this section controls the flow of a voltage from the power transformer (AC) to controlled direct current (DC) that the system can use.
6. The cooling system - direct air cooled, liquid cooled, water or oil, or a combination of both cooling systems.

The first step to good maintenance for any rectifier system is to keep the equipment clean. The metal finishing environment is a harsh environment for any material, especially electrical equipment. The moist air common in plating environments causes airborne dust and dirt to stick to the interior components. This, coupled with the elements of salts and acid, becomes conductive and corrosive and can cause electrical shorts and material deterioration.

For the general types of equipment listed below, the following maintenance steps can help prolong equipment life, prevent unscheduled down time, and help identify minor repairs before they escalate.

A semi-annual preventative maintenance program is a combination of inspection, testing, and cleaning. It is ideal for supporting extended warranties, or as part of an on-going maintenance program. 

Inspection Checklist for Direct Air-Cooled Systems

1. Secure all power sources – use applicable LOCKOUT/TAGOUT procedures.
2. Vacuum all loose debris.
3. Wipe down the exterior.
4. Inspect exterior of cabinet for integrity.
5. Clean base.
6. Wipe down the interior components using a vacuum and a soft brush.
7. Clean ventilation openings.
8. Clean the air inlet and outlet screens and filters.
9. Clean the cooling fan blade and motor.
10. Check the motor bearings by rotating the fan blades and feeling for ease of rotation and sounds of bearing noise.
11. For air cooled heat sinks, use a stiff brush to remove the deposits embedded within the cooling fins of the heat sink.
1. For severe deposits, the heat sink may require removal to be washed.
2. In extreme cases, the heat sink may require machining or replacement.
12. The transformer steel and coils should be wiped down. A stiff non-metalic brush can be used, with care, to clean the air passages within the coils. Pay particular attention to the bottom of the coil where contaminants tend to collect.
13. Wipe down all copper buss.
14. Check all electrical and mechanical connections for tightness.
1. Contactor covers may require removal to examine the electrical contacts. If the contacts are pitted, they may be refinished or replaced as needed.
2. Check for free movement of internal parts.
3. Check wire and cable connections for condition and tightness.
4. Use caution if using any solvents to clean or lubricate parts as solvents can be flammable.
15. Check the condition of fuses and fuse holders and clean or replace as required.
16. Check the condition of all wire and cable terminals and correct as required. Loose and/or dirty connections are a major cause of unexpected failures.
17. Electronic controls can be cleaned using a soft brush and canned air to gently clean debris from surfaces. Moisture and conductive debris directly on the surfaces of electronics can cause control problems and failure.
18. Check the condition of the metering shunt and sensing wires. Look for signs of cracking where the resistive material meets the copper supports. Cracking can cause incorrect output current readings.
19. In severe cases of prolonged lack of maintenance, contact your equipment manufacturer for guidance. Major maintenance requires specialized techniques and equipment.

Inspection Checklist for Liquid-Cooled Systems

Since liquid cooled equipment is generally provided in a sealed enclosure, they will withstand the harsh environment better than direct air-cooled units. Although for the most part, the maintenance is similar to direct air-cooled equipment mentioned above, the emphasis is to the liquid cooling system.

20. Check equipment of any signs of coolant leaks, hoses, valves, pump seals, etc.
21. Check for aged, brittle, or severely discolored hoses and tubes.
22. Inspect all hoses for wear, hose connections and manifolds for signs of leaks and/or corrosion.
23. Check pump motor for bearing noise.
24. Check coolant for cleanliness, signs of rust and color. Drain, flush, and refill with new coolant (if required).
25. Inspect main system reservoir for leaks and sediment.
26. On pure water systems, test for conductivity. Replace DI filter if required.
27. Check and clean the particle filter.
28. Check operation of liquid solenoid valve if used.
29. Check operation of protection thermals for coolant modulation and thermal protection.
30. Inspect transformer and transformer leads for cracks and bad joints.