Isolation transformer or voltage stabiliser — which problem are you solving?

Two different failures, two different devices

An automatic voltage controller corrects the magnitude of the supply: it holds output voltage steady while the input drifts. It does nothing about the shape of the waveform or about disturbances riding on the neutral.

An isolation transformer corrects reference and coupling: a double-wound construction with an electrostatic shield between primary and secondary breaks the galvanic path, attenuates common-mode noise and gives the load a locally derived neutral. It does not regulate voltage magnitude.

Choose by symptom

Map what you actually observe on the shop floor to the corrective device:

• Lamps dimming and motors running hot or stalling on start — voltage magnitude problem, use a voltage controller.
• Drives tripping on earth fault or DC bus faults with no measurable voltage excursion — coupling and neutral problem, use an isolation transformer.
• PLC or instrumentation resets and erratic analogue readings — common-mode noise, use an isolation transformer.
• Power factor penalty on the bill — neither; use an APFC panel.
• Both wide voltage swings and sensitive electronics — a controller upstream with isolation dedicated to the sensitive panel.

What to specify

For isolation transformers, state the kVA, primary and secondary voltage, insulation class, expected ambient temperature, and whether an electrostatic shield with a dedicated shield earth is required — the shield is what delivers the noise attenuation, so it should never be left as an optional extra.

For voltage controllers, state kVA, the input voltage window from measured data, whether the feeder is balanced or unbalanced, cooling type, and the protections required, including bypass arrangement for maintenance.