Electrostatic Powder Coating
Voltage VS. Current – How do they relate?
Voltage and current have an inverse relationship when there is a limited amount of energy supply. This is the case with electrostatic powder coating guns. Voltage is electrical potential to do work, while current is measuring the movement of electricity, or work being done. Once the potential is used, it becomes current; thus, there is less potential or energy left. Both voltage and current are useful settings and indicators for electrostatic guns.
"KV" (kilovolt = 1000 volts) is the unit of measure indicating how much electrical charge the gun can produce. The current (in micro amps) is the unit of measure indicating how much charge is moving from the gun’s electrode to the air/powder exiting the gun and also the substrate. The gun should be set at the maximum KV value available, as long as it does not produce an undesirable finish. This is because a high KV setting generates the highest potential to charge powder than a lower setting, thus yielding better transfer efficiency. KV should be lowered only when it becomes necessary to reduce back ionization or other undesirable affects that cannot be controlled otherwise. It may also be necessary to lower KV when re-coating a part in order to reduce the chances of back ionization caused from charging the cured finish beneath.
The current feedback is a better indication than voltage of what the electrostatic charge is doing. This is because current tells you how much of the charge is moving or working instead of the charge’s potential. Current can sometimes be limited with electrostatic powder coating systems, depending on the type of gun control unit. Limiting current is a very good way to control the electrostatic charge produced by the gun. It is also the optimal way to achieve high transfer efficiency, while working closely to the substrate in an effort to overcome the Faraday Cage Effect. For first-pass coating of complex parts, KV should be set to maximum with the current limited to approximately 25 to 30 micro amps.
It should be noted that the 100 KV setting is also a limiting setting built into the power supply. You will likely not have 100 KV while spraying a part. When spraying a part, the potential (voltage) is being realized due to the load on the power supply and will be lower than the actual setting. As the gun-to-part distance gets smaller the current load will increase. As the current load reaches the limit set point, the KV will fall rapidly. This abrupt fall of the KV is what helps prevent back ionization when working close to the substrate.
The bottom line is that both voltage and current are very important factors in the electrostatic powder coating process. Using a control unit that has the ability to monitor both of these settings is a definite advantage and gives the user maximum flexibility and control.