Ferroresonant Transformers also known as constant voltage transformers (CVT). Ferroresonant designs allow the primary voltage to vary while the output voltage is held constant. Additionally, ferroresonant transformers can be designed to provide various degrees of current limit. Standard output ferros are typically used voltage regulating rectifier transformers. Sine-wave ferros use a harmonically compensated secondary winding, producing a waveform that is nearly sinusoidal. Ferroresonant inverter transformers are found primarily in uninterruptible power supplies.
Northlake Engineering offers custom ferroresonant designs from 25VA to 10KVA for operating frequencies of 50Hz, 60Hz and 50/60Hz.