Current limiting is required for heater elements that have a high hot-to-cold ratio for an extended duration, such as:

Molybdenum
Silicon carbide (e.g., Globar®)
Carbon graphite

Current limiting is available on single-phase and three-phase, three-leg models only, requires the heater diagnostics option, and requires use of phase angle control.

Why is current limiting required?
How does current limiting work?
Watch current limiting on an oscilloscope

Current limiting protects the power controller and heater from being destroyed by high inrush current during startup.

Some heater elements show large changes in resistance with temperature or with age. If full power is applied when the element is cold, high inrush current will destroy the SCRs and fuses, and might damage the heater.

Heater elements such as molybdenum, silicon carbide and carbon graphite require minutes, or even hours, to warm up. Current limiting limits the current for as long as it takes the heater element to warm up.

Upon power up, current limiting initiates during a soft start sequence. The Power Series slowly increases the output power while not exceeding a specified current limit. Upon reaching the current limit, the Power Series holds constant the output power level.

A current transformer continues to monitor the current flow. As the load current decreases (as happens when the heater element warms up), the Power Series allows more output voltage (and thus more current) until the requested power level is achieved.

Watch a video clip of current limiting on an oscilloscope.

The video shows a two-second soft start followed by current limiting. It then shows another soft start followed by current limiting. In this video, the input command signal is requesting 100 percent power. However, the current limit is set to 4A, so the Power Series limits the output current to 4A.

Click here if the video clip will not play or if you cannot hear the sound.