Air Cond Compressor Condition

Expert opinions may be obtained by contacting the manufacturer of the equipment directly.

Presumably it is on a maintenance contract that includes emergency breakdown cover?

The life of the motor would very much reduce. In a DOL Start, the motor starting current is typically 6 to 8 times the rated current of the motor. And, hence, the heating will be 36 to 64 times than normal. This is unnecessary thermal stress on the motor. And, a thermally stressed motor's life would definitely reduced drastically.

A solution could be to use soft starters instead of DOL Starter for the motor.

There should be a way to adjust the minimum time latencies between starts and stops to a rational level on this 'energy saving device'. If there isn't any, it's just poorly designed and is not suitable for an A/C compressor, or any electric motor for that matter. S.M.

" It is found that with this saving device, the air cond consumption (kwh) drops 30%"

Gigabolt, Is this statement information you measured or is it from the MFG? I find it hard to believe that any device would reduce the kwh's 30%

Followup question, if you are measuring it, are you sure you're measuring kwhrs and not kva / kvars?

Hope the device is just supposed to be a more accurate thermostat and OP didn't embarassingly drove me into a PF 'economizer' scum conversation. S.M.

I didn't think of the possibility of a more accurate thermostat--that could save power. I guess we'll have to see his reply.

The function of this device is by installing on additional sensor at the ac evaporator to sense the outgoing air temp. the sensor is then connected to a controller to cut off the compressor when the set temperture is achieved and allows the blower fan to blow the cool air from the cooling coil. once the cooling coil temp rise, then the controller will cut in the compressor again. With this the compressor work based on the the air temp out from evaporator not from room return air.

i have measure using kwh meter using (fluke 1735 data logger) and it gave 30% reduction before and after fixing, but noticed that the compressor start and stop cycle more frequent. that worries me.

Thanks! (I think others have (or can/will) addressed your concern about more frequent starts--in general, it is detrimental to the life of the motor.

Maybe (as someone else suggested) increased hysteresis on the additional sensor (or its controller) can reduce the number of starts while maintaining the 30% power saving.

it is through measurement using kwh meter, before and after fixing the device. The only concern is the ac start and stop cycle too often.

Apart from the foregoing considerations, any AC unit that starts/stops so frequently is grossly oversized, or is under control of an overly sensitive thermostat or pressure switch. It would take more system details to pin this down better.

I was called in to diagnose an energy management system design. The designer did not understand the essential function of hysteresis in any thermal control. Hysteresis creates a dead-band temperature range to allow an appropriate cycle time. It competes with comfort due to the temperature excursion. If you have control over it bump it up to between 2 and 5 degrees. If not, complain to the manufacturer. Possibly they can upgrade the code if it is software based.

Your only other option would be to mount the sensor in a place where it will take longer to sense the temperature change. Thermostats mounted on the wall actually have anticipators to speed up the response due to the mass of the wall. The air stream is the worst place for a sensor unless the system is carefully designed to take this into consideration.

The function of this device is by installing on additional sensor at the ac evaporator to sense the outgoing air temp. the sensor is then connected to a controller to cut off the compressor when the set temperture is achieved and allows the blower fan to blow the cool air from the cooling coil. once the cooling coil temp rise, then the controller will cut in the compressor again. With this the compressor work based on the the air temp out from evaporator not from room return air.

i have measure using kwh meter using (fluke 1735 data logger) and it gave 30% reduction before and after fixing, but noticed that the compressor start and stop cycle more frequent. that worries me.

Congratulations, you just reinvented the "discharge air sensor". I've seen a thousand of these in commercial A/C.

To expand on the discharge air sensor. The discharge air sensor is a very common component in commercial A/C. It is usually located near the end of the main supply trunk of a building (before it ducts start to branch off). The main purpose for this sensor is for "loading" of the compressor(s). If the supply temperature is too high or low this is the sensor that sends a signal to either increase or decrease compressor capacity. If you have a VAV and DDC system it has one of these sensors. On a DDC system it will give out a constant reading and is always shown on the systems status page.

On smaller systems the "return" temperature dictates loading. On medium and larger systems the "supply" temperature dictates compressor staging or loading (both mean the same thing).