Improving Plasma Spray Coating Control
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?
Power Supply TypesTwo 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.
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.
Positive ResultsProcessors 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.