Power control during the plasma-spray-coating finishing process is important to achieving uniform results on the product.

Figure 1. A typical example of a silicon-controlled rectifier (SCR) plasma power supply, this unit is rated for 400 VDC at 600 ADC output.

Plasma spray coating is used in a range of finishing applications, including semiconductors, metals and architectural glass. Plasma spray coating is a process of thermally spraying one substance (usually in a powder form) onto another. For instance, a jet engine’s turbine tips are plasma spray coated with a high temperature ceramic coating that ultimately serves as the bearing surface.

Power control during coating is important to achieving the most uniform results. A pulse-width modulated (PWM) DC power supply can improve coating uniformity, ease the use of different coating materials, and offer better deposition rates. Other advantages include increased efficiency and productivity. How does this pulse-width modulated DC power supply for plasma coating work?

Figure 2. This pulse-width modulation plasma power supply rated for 100 VDC at 2,000 ADC output.

Power Supply Types

Two types of DC power supplies are used in most plasma spray coating applications: silicon-controlled rectifier (SCR) and pulse-width modulation. Because SCR and pulse-width modulated plasma power supplies use electronics for modulation rather than magnetics, they provide higher power factors and require less power source kVA. Each type of control offers advantages and disadvantages.

The SCR-based DC power supply is the most common in use today (figure 1). These power supplies typically have a low power factor (PF) because the operating voltage is about half of the open-circuit voltage, which is equal to an approximate power factor of 0.5 PF. This higher open-circuit voltage requirement is the main reason for the higher kVA requirements.

Figure 3. One-line diagrams of SCR and pulse-width modulation plasma power supplies show that the only real difference is the addition of a DC capacitor bank and a transistor on the pulse-width modulation type.

The pulse-width modulated DC power supply is steadily increasing in popularity (figure 2). A pulse-width modulation DC power supply has lower kVA requirements, which help reduce the incoming power requirements, and a high power factor (about 0.9 PF), which is significant enough to help reduce overall electrical operating costs. Its decreased harmonic currents also are becoming more important due to the IEEE 519 standard, “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.” (This standard slowly is being adopted by industries trying to minimize harmonics in their electrical power distribution systems.) In addition, the pulse-width modulated DC power control has reduced output ripple, which helps create a smoother coating.

When comparing the SCR and pulse-width modulation one-line diagrams (figure 3), note that the only real difference is the addition of a DC capacitor bank and a transistor. On the SCR type, the SCR actually does the control. On the pulse-width modulation type, the SCR is used to charge the DC capacitor bank and the transistor regulates the DC current.

Plasma spray coating is used in a range of finishing processes, including semiconductor fabrication and for coating architectural glass.

Positive Results

Processors that have installed pulse-width modulated DC power supplies for their plasma coating processes have reported good results. Smoother coatings, reduced rework and higher yields are among the positive outcomes. The biggest improvement, though, is the reduced powder-coating material requirements. There are reports of reductions as high as 20 percent or more. This is significant because the powder coating material is one of the highest cost items.

The pulse-width modulated plasma power supply has a higher initial cost; however, the overall operating cost is lower than a conventional SCR-controlled plasma power supply. This lower operating cost may be significant enough to offset the additional upfront cost. Benefits such as reduced ripple that helps reduce electrode wear helps increase production yields and lowers product cost. In addition, the extra benefit of reduced harmonic currents helps minimize the potential for plant power distribution problems.