Use phase angle control for:

Transformer-connected loads such as low-voltage heaters.

Variable-resistance heaters such as tungsten, molybdenum, carbon graphite and silicon carbide (e.g., Globar®).

Phase angle control is a field-selectable control mode. It is available on single-phase and three-phase, three-leg models only.

Why is phase angle control required?
How does phase angle control work?
Watch phase angle control on an oscilloscope

Phase angle control is required for two types of applications:

Variable-resistance heaters require soft start and current limiting to prevent high inrush current from damaging equipment. Soft start and current limiting are available only in phase angle mode.

Transformer-connected loads have a transformer between the Power Series and the load. Phase angle control ensures that the transformer always receives alternating positive and negative AC voltage pulses. If the transformer were to receive two pulses of the same polarity, it could become saturated, which could cause overheating, high current and blown fuses.

The phase angle control method provides a limited portion of each AC voltage cycle to the load, based on the percentage of power requested. The output is switched on for a portion of every AC half cycle.

The illustration below shows phase angle firing at 50 percent power:

Watch a video clip of phase angle control on an oscilloscope.

In the video, the power starts at 0 percent, gradually increases to 100 percent, then gradually decreases to 0 percent. The entire process is then repeated.

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