IEC 60145: The Variac — How the Adjustable AC Source in Every Lab Works

The Variac — How That Adjustable AC Source in Every Lab Works

IEC 60145 specifies contact voltage regulators (Variometer/Variac). A Variac is an autotransformer with a carbon brush sliding on the winding surface, continuously varying output voltage — 0 V to 110% input voltage, smoothly adjustable.

Key limitations: (1) The brush must not short adjacent turns — brush width must be narrower than inter-turn spacing. (2) Output current contains waveform distortion from brush contact resistance (predominantly odd harmonics), affecting precision measurements. (3) Variacs provide no galvanic isolation — input and output are electrically connected (autotransformer principle); an isolation transformer must follow if isolation is required.