AC Capacitor Guide: What It Does and How to Tell It Is Failing

AC capacitor: the short answer

An AC capacitor is the cylindrical part, usually in the outdoor unit, that gives the air conditioner’s compressor motor and outdoor fan motor the push they need to start and keeps them running smoothly. When it weakens, the AC typically hums, struggles to start, blows warm air or trips its MCB, even though the rest of the unit is healthy.

It is one of the cheapest parts in an air conditioner and one of the most common to fail, because it sits in a hot outdoor box and works through every start, every day of summer. The right replacement is simple to name: the same microfarad value, written as µF or MFD, that is printed on the old capacitor’s label.

This guide covers what the capacitor does, what MFD means, what a dual capacitor is, the signs of a bad one, where it sits in the outdoor unit and the MRPs of the AC-size capacitors we make in Mumbai. Ceiling fan capacitors, the small end of the same range, have their own fan capacitor guide. Replacing an AC capacitor is a technician’s job: it holds charge after the power is off and sits beside other live parts.

What does a capacitor do in an AC?

In an air conditioner the capacitor supplies an out-of-step current to a second winding of the compressor motor and the outdoor fan motor, which gives each single-phase motor a turning push to start in the right direction and helps it run smoothly once it is up to speed.

A typical split AC has capacitors for up to three motors: compressor the largest value, outdoor fan a smaller value, and sometimes indoor blower a small one inside the indoor unit. Some outdoor units combine the first two in one can, the dual capacitor described below.

The compressor is the heavy load. Starting it against refrigerant pressure takes a strong push, and a capacitor that has lost part of its value gives a weaker one. The motor then takes longer to start, draws more current while it tries, and runs hotter. That is why a tired capacitor shows up first on the hottest afternoons, when the AC starts most often.

The same principle runs a ceiling fan with a much smaller capacitor; our ceiling fan power consumption guide explains how a BLDC fan does without one.

What does MFD mean on an AC capacitor?

MFD on an AC capacitor means microfarads, the same unit as µF, so a label reading 36 MFD and one reading 36 µF describe the same value; the label also usually shows a tolerance and a voltage rating, and a replacement must match the microfarad value and be rated for at least the same voltage.

What to read on the label: µF or MFD value tolerance, often a ± figure voltage rating terminal markings

The “AC” in “AC MFD capacitor” refers to alternating current, not to air conditioners, so the same name covers fan, motor and AC-unit capacitors. Our fan capacitor guide explains the term in full. Our catalogue lists our sixteen capacitors by µF value; it does not print a voltage rating against them, so the technician checks the rating on the part itself against the old one.

What is a dual capacitor in an AC?

A dual capacitor is two capacitors built into one can, one for the compressor and one for the outdoor fan motor, sharing a common terminal, so it has three terminals usually marked C for common, HERM or H for the compressor and FAN or F for the fan, and its label gives two values, the larger for the compressor.

When a dual capacitor fails, often only one half has weakened. The symptoms point to which: fan stopped, compressor running suggests the fan side, fan running, compressor humming the compressor side.

We make single-value capacitors only, not dual ones. A single part does not replace a dual can like for like; whether two single capacitors can stand in for one dual is the technician’s decision on the unit in front of them.

What are the signs of a bad AC capacitor?

The common signs of a bad AC capacitor are an outdoor fan that does not spin or spins slowly, a humming outdoor unit that does not start, a compressor that clicks on and off, room-temperature air from the indoor unit, an MCB that trips when the AC starts, and a capacitor that looks swollen, cracked or leaking.

The signs, roughly in the order people notice them: takes longer to cool outdoor fan slow or still humming, no start clicking on and off MCB trips at start bulged top

Not every one of these is the capacitor. A dirty outdoor coil, low refrigerant, a failing contactor or a low supply voltage can look similar, which is why the technician measures the capacitor with a meter before replacing it. A capacitor that reads well below the value on its label is weak, even if it looks normal.

If the MCB keeps tripping at start-up, our MCB for 1, 1.5 and 2 ton AC guide covers the other causes, including an MCB that is wrongly rated for the AC.

Where is the capacitor in an AC outdoor unit?

In a split AC the capacitor for the compressor and outdoor fan sits inside the outdoor unit, usually in the electrical compartment behind a small cover near the supply terminals, mounted with a clamp beside the contactor, while a smaller blower capacitor, where fitted, sits in the indoor unit’s control area.

Getting to it means removing a panel on a unit that may be mounted on a wall bracket or a ledge outside a window, often several floors up. Inside the same compartment are the supply terminals and the contactor, both live when the AC is powered.

This is the strongest reason the job belongs to a technician. Even with the AC’s MCB off, a capacitor can hold a charge for some time, and it has to be discharged safely before its terminals are touched. A technician also checks that the right MCB has been switched off, because AC points are sometimes fed from a different board from the one people expect.

An isolator or DP switch near the outdoor unit, where fitted, gives the technician a local off point; our isolator vs MCB guide explains what an isolator does and does not protect.

Which µF capacitor does my AC need?

Your AC needs exactly the µF value printed on the capacitor it already has, or in the unit’s service documents, because the value is set by the motor and compressor the maker fitted, not by the tonnage, so two ACs of the same tonnage can need different capacitors.

AC compressor capacitors commonly sit at the large end of the range, outdoor fan capacitors lower down. Our range covers 2.25 µF to 50 µF in sixteen values, with 20, 25, 30, 36, 40 and 50 µF at the top.

The common question of a 1.5 ton AC’s capacitor is answered in our fan capacitor guide: tonnage is cooling capacity, not an electrical rating, so any tonnage-to-µF chart is a guess. What the technician should do is read the old label, measure the old part, and fit the same µF at the same or higher voltage rating.

AC capacitor price: what do ours cost?

Our AC-size capacitors cost ₹170 MRP for 20 µF, ₹175 for 25 µF, ₹220 for 30 µF, ₹256 for 36 µF, ₹270 for 45 µF and ₹300 for 50 µF, codes 511 to 516 in our VIYONA range, packed 25 to a box and sold through electrical shops that stock our range.

VIYONA 516 is a 50 µF AC MFD capacitor with an MRP of ₹300. A replacement bill also includes the technician’s visit and labour, which we do not set. Ask the technician to name the µF and show you the old part. See the VIYONA AC MFD capacitor page for the full range.

Fan capacitor vs AC capacitor: what is the difference?

A fan capacitor and an AC capacitor do the same job, starting and running a single-phase motor, and differ mainly in value and size: a ceiling fan commonly uses a small value of a few microfarads, while an AC compressor commonly uses tens of microfarads in a much larger can.

So a fan capacitor will not do in an AC, and the reverse is just as wrong. A ceiling fan that hums, by contrast, often points to its small capacitor or regulator; our guide on fan humming noise sorts those causes.

Can a weak capacitor damage the AC compressor?

A weak capacitor can shorten a compressor’s life, because the motor struggles longer at every start, draws more current while it does, and runs hotter, so replacing a capacitor that has measured low is a cheap repair that removes repeated strain from the most expensive part of the AC.

What a weak capacitor does to the motor, as general practice: longer starts higher start current more heat thermal overload trips

Most compressors have internal overload protection, so a struggling one usually cuts out rather than burning immediately. The pattern people notice is the AC stopping and starting on a hot day. Left long enough, repeated overload trips and heat are hard on the motor’s windings.

Low supply voltage makes the same start harder. Whether your AC needs a stabiliser is covered in our voltage stabiliser for AC and fridge guide. The capacitor itself costs ₹170 to ₹300 at MRP in the AC sizes we make, far less than a compressor.

AC capacitor replacement: what the technician does

An AC capacitor replacement starts with the AC switched off at its MCB and proved dead, then the old capacitor is safely discharged, photographed for its connections, measured, and replaced with one of the same µF and at least the same voltage rating, connected exactly as before and clamped firmly in place.

The steps, in order: MCB off, prove dead Discharge the old capacitor Photograph the wiring Measure the old part Fit same µF, same or higher voltage Clamp and test run

After the test run, a good technician watches the outdoor fan spin up and the compressor start cleanly, and checks that the unit cools. Ask for the old part back; it confirms what was replaced. Any wiring at the AC’s own point, from the board to the unit, is a licensed electrician’s work.

Electricians and AC technicians who buy by the box can order our capacitors from our dealers across India; we sell in full boxes only. Our fan capacitor guide lists every code with its box quantity.