A three-phase package unit on a Sharjah workshop roof that hums, fails to start, and then trips its overload is not automatically a dead compressor. One of the three supplies can be missing. The motor cannot build a rotating field, it sits there drawing a high current, the overload heater cooks, and the unit goes quiet. Resetting it asks the same windings to do the same thing again. Air-Con.ae, the marketing name of Burj Al Qasimiah AC units fix cont., measures all three legs before anyone condemns the compressor. The price of a contactor, a repair at the isolator, or a compressor is written on site before the part is fitted.
Package units in Industrial Area 6 and the surrounding sheds see this more often than a villa split does, because the supply is three-phase and the roof isolator, the contactor, and the building distribution can each drop a single leg. The indoor fan may still run. That fools people into thinking the refrigeration circuit has simply “lost gas”.
What single-phasing feels like from the ground
You hear a hum from the package unit for a few seconds, sometimes a buzz from the contactor, and then a click as the overload opens. The supply air never goes cold. After a wait, someone on the floor resets the breaker or the red button and the hum returns. The casing of the compressor, if you could touch it safely, would be hotter each time. On the second or third reset the overload opens faster. Staff call it a compressor that “almost starts”. It is a motor that is not being given three phases.
A compressor that has already been running can lose a phase too. It may keep turning for a while and then overheat, which is worse than a clean failure to start, because the remaining windings carry current they were not meant to carry alone. Either way, repeated resets are how a compressor that could have been saved becomes a compressor that has to be replaced. We ask how many times it was reset before we open the panel. The answer changes how carefully the windings are tested afterwards.
Where a phase goes missing
The building supply can be short one leg, in which case other three-phase machines in the shed misbehave as well and the package unit is a victim. A fuse in the roof isolator can be open on one pole. A contactor can have one set of contacts burnt so that two legs pass and the third does not, even though the coil pulls in and the unit sounds as if it is trying. A loose lug at the compressor terminal can do the same thing only when the machine is asked to start. These are different repairs. They look identical from the sales floor: hum, no cooling, a hot overload.
Measure all three legs before the compressor is blamed
With the unit isolated, we look at the isolator, the contactor faces, and the compressor terminals for heat marks and a loose connection. Then we measure. Phase to phase at the incoming side of the isolator should be three readings close to one another. A collapsed reading is a supply or isolator problem, and the compressor is not condemned for it. If the incoming side is healthy, we measure again on the outgoing side of the contactor while it is closed, which is the test that catches a burnt pole. Only when all three legs are present at the compressor terminals do we treat the motor itself as the suspect.
- Incoming isolator short one leg: the building supply or the isolator is the job. The compressor is not replaced into the same fault.
- Incoming healthy, one contactor pole dead: replace the contactor, then test the compressor before it is asked to start again.
- All three legs present and the compressor still will not turn: then winding resistance, insulation, and a compressor quote.
We do not ask the storeman to hold the reset while we listen. A single-phasing compressor is damaged by that habit. If the overload is already hot, it is left to cool, and the electrical fault is found before the next start is attempted. Voltage readings and the location of the missing leg are written on the job note so a later visit does not start by ordering a compressor from a phone description of a hum.
After the phase is restored
A compressor that has hummed and tripped more than once is checked before it is put back to work. Winding resistances are compared with each other. Insulation to earth is checked. A motor that still balances and still insulates can be run, watched, and left, with the contactor or the lug that caused the trouble already replaced. A motor that is burnt, shorted between windings, or down to earth is quoted as a compressor, and the quote includes the reason the old one died. Fitting a new compressor onto a contactor with a missing pole destroys the new one the first afternoon someone resets it.
The overload and the breaker are not used as a speed control. If they are tripping on a compressor that now has three healthy phases, the running current is compared with the nameplate. High current with correct voltage sends us to airflow, a dirty condenser, or a failed fan on the package unit, which are real causes of an overload and are cheaper than a compressor when they are the actual cause. Gas is not added to a motor that was single-phasing. The electrical repair comes first, then pressures if the unit runs and the air is still wrong.
What you should stop doing while you wait
Leave the isolator off if the unit has already tripped twice. Another reset is another locked-rotor attempt on a missing phase. Note whether any other three-phase machine in the building is down. That single fact tells us to look at the main distribution before we spend an hour on a healthy contactor. Do not wedge the overload closed. A compressor that cannot trip will fail in a more expensive way than a compressor that is allowed to stop.
Booking the visit
Call +971 50 257 5128 and say it is a three-phase package unit, that it hums and trips, and how many times it has been reset today. Name the industrial district and whether the roof access is open. The workshop is in Industrial Area 6, opposite the cricket stadium. Package-unit work is quoted after we can see the isolator and the error, not from the hum alone. Any contactor, lug repair, or compressor is priced in writing before it is fitted.
The same measurement applies to a rooftop unit that serves a shop rather than a shed. The commercial scope is on our commercial AC services in Sharjah page. If other machines in the building are also dead on one phase, tell us that first. We would rather arrive knowing the main supply is the suspect than condemn a compressor that never had three legs to run on.
After the three legs are actually present
When all three phases are back and the voltages match within a few volts, the compressor is still tested before it is blamed or cleared. Insulation to earth and a resistance balance between windings tell you whether the earlier single-phasing already cooked the motor. A compressor that ohms out open, or that trips on overload within a minute of a clean supply, is finished. Resetting it again to “see if it holds” is how the terminals burn. We stop and quote a replacement or a new unit, with the readings on the page.
If the windings are balanced and the current on each leg is even and under the nameplate, the supply was the fault and the machine can run. We still look at the contactor and the overload that sat in the heat while the unit hummed. A pitted contactor is replaced rather than left to weld shut next week. Call +971 50 257 5128 and say whether the package unit hums without starting. The electrical test is done before any compressor price is discussed.
The air handler and the ducted machine are covered as central AC services in Sharjah.
Keeping the same fault from returning is AC maintenance in Sharjah.
