Why use a transformer?
Control system featuring phase angle fired SCR power controller and isolation type stepdown transformer
We’ve been asked why we control the power to molydisilicide heating elements using not only a phase angle fired SCR power controller, but also an isolation type stepdown transformer. After all, the SCR will do a good job of regulating the volts and amps required to fire the load, and the transformer is an added expense.
Here’s the reason: Molydisilicide heating elements have virtually no resistance on startup. So very low voltage and high amperage are required to fire the elements without blowing them away. This can be accomplished by using a soft start, high amperage SCR, but then you must have greater line power amps. This can be a real problem in many laboratory settings where the available incoming power can be as low as 30 amps.
Consider an example. Suppose you have a benchtop furnace that uses six heating elements in series with a power requirement of 48 amps and a nine volt drop per element. Using only a power controller, the system will require 48 amps at the wall. But coupled with an isolation type (for safety) stepdown transformer, a 15 amp power controller can be used.
But what about the added cost of the transformer? This can be offset by the lower cost of the lower amperage power controller.
Learn more about Deltech control systems, ETL certified to conform to UL508A standards by Intertek.