AC Stabilisers and Voltage Fluctuation: When You Actually Need One

Between 7pm and 11pm in many parts of Kalyan, Dombivli and the older Thane wards, supply drops to 185–200V. An AC compressor designed for 230V tries to start, fails, retries, and repeats that cycle until either the supply recovers or the winding gives out. This is the most avoidable compressor failure there is.

What low voltage actually does to a compressor

A compressor motor draws current inversely with voltage. At 185V instead of 230V it needs roughly 24% more current for the same work. That current heats the winding, and the internal overload trips to protect it — which is the click you hear.

Each failed start is a heat cycle. A few dozen a week for a season is enough to degrade insulation. The failure then appears as a compressor that trips after twenty minutes of running, months later, and gets blamed on the unit.

Signs you are in a low-voltage pocket

• Lights dim noticeably when the AC compressor starts.
• The AC clicks repeatedly before settling into a run.
• Cooling is adequate in the afternoon but poor between 7pm and 11pm.
• The outdoor fan runs but the compressor does not.
• Neighbours report the same pattern with their units.

Two of these together, recurring at the same hours, is enough to justify a measurement rather than a guess. A technician can read supply voltage at the isolator in a minute.

When a stabiliser is the right purchase

Buy one if your supply measured at the AC point drops below about 200V under load, or rises above 250V. Both directions damage the compressor; high voltage is rarer but more destructive.

A 1.5-ton AC needs a stabiliser rated for at least 4 kVA, and 5 kVA is the safer buy because startup current is several times running current. For a 2-ton unit, 5–6 kVA. Undersizing is worse than not buying one — a stabiliser that trips on every compressor start will be bypassed by a frustrated owner within a week.

When you do not need one

Modern inverter ACs contain a wide-input power stage that tolerates roughly 145–265V. If your unit is an inverter model bought in the last five years, the manufacturer’s manual will state the operating range, and in most Mumbai installations that range is adequate without an external stabiliser. Adding one is harmless but redundant.

The exception is a building with severe, frequent surges — often where a lift motor or a welding workshop shares the supply. There, a surge protector at the distribution board protects every appliance in the flat, which is better value than protecting one AC.

Stabiliser or servo, and what they cost

For most Kalyan and Dombivli flats with evening sag, a 4–5 kVA static stabiliser at the AC point is the proportionate answer. Servo units belong in buildings with genuinely wild supply.

The cheaper fix: check the wiring first

Before buying anything, have the AC point checked. A loose neutral, an undersized cable from the meter, or a shared circuit with a geyser will cause voltage drop inside your own flat, and a stabiliser cannot correct a problem that originates after it.

We find loose terminals at the isolator surprisingly often, and tightening them fixes a "low voltage" complaint outright. Ask for the voltage reading at the meter and at the AC point — a large difference between the two means the wiring, not the supply, is the fault.