Why does the air conditioner compressor fail to start and make buzzing

Update:30 Dec 2021
Summary:

1. Measure compressor voltage and coil resistance   On- […]

1. Measure compressor voltage and coil resistance
   On-site inspection, the user uses the air conditioner normally, feels natural wind at the air outlet of the indoor unit by hand, and goes to the outdoor unit for inspection.
It is found that the outdoor fan is running but the compressor is not running. As shown in the left picture of Figure 2, use the AC voltage block of a multimeter to measure the 2N (power neutral line) and 3CM (compressor power supply) terminal voltages on the outdoor unit wiring terminals. Normally it is AC 220V. The measured voltage indicates that the main board of the indoor unit has output power.
Disconnect the power supply of the air conditioner, use a multimeter to block the electricity, measure the 2N and 3CM terminal resistance (equivalent to measuring the compressor common terminal and the running winding), the normal value is about 30, and the measured result is infinite, indicating that the compressor coil circuit has an open circuit fault .
2. Cool down the compressor
   Ask the user that the air conditioner has been turned on for a period of time, and the temperature of the outdoor unit casing corresponding to the compressor is very high. It is roughly judged that the contact of the overload protector inside the compressor is disconnected.
Remove the outdoor unit shell, touch the compressor shell to burn your hands, make sure that the internal overload protector is disconnected due to the over-temperature contact protection, put a towel on the upper part of the compressor, use cold water to cool down, and measure the resistance of the 2N and 3CM terminals at the same time, When the normal resistance value is received from infinity, it indicates that the internal overload protector contact has been closed.
   Description: The compressor's internal overload protector is connected in series to the common end of the compressor coil, located on the upper top shell. When cold water is used to cool the compressor, putting a towel on the top can quickly close the overload protector contact.
3. The compressor does not start
   After measuring the resistance of the 2N and 3CM terminals to be normal, power on and turn on, as shown in the left picture of Figure 4, the compressor makes a "humming" sound for about 30s, and stops for about 20s to make a "humming" sound again.
   Use a multimeter to measure the 2N and 3CM terminal voltages when the compressor is started.
   From 218V AC (the voltage when there is no sound, that is static) to 199V (the voltage when the compressor makes a "humming" sound, that is, dynamic), it indicates that the power supply is normal.
  Using a multimeter to measure the current of the compressor with the AC current block, it is close to 20A, and it is judged comprehensively that the compressor cannot be started.
 4. Check the compressor capacitor
  Under the premise of normal power supply voltage, the most common reason why the compressor can not start is that the starting capacitor has no capacity damage. Remove the capacitor and measure it with a multimeter. The actual measurement is only a few microfarads, and it is determined that the capacitor has no capacity damage.
5. Maintenance measures
   Replace the compressor capacitor. After the replacement, power on, the compressor runs, and the air conditioner starts to cool. The compressor current is measured again to be 4.4A, and the fault is eliminated.
 6. Summary
① Compressor capacitor damage, which accounts for a large proportion of uncooling failures, usually occurs after 2 to 3 years of use.
②If the user reports a failure of no refrigeration, the user should be informed not to turn on the air conditioner, because if the cause of the failure is the compressor capacitor damage or the system lack of fluorine failure, the compressor temperature will be too high and the built-in overload protector will be disconnected. During maintenance, the compressor should be cooled down to increase the maintenance time.
③In the actual maintenance, if the fault is that the compressor cannot start and makes a "buzzing" sound, it is generally not necessary to measure and replace the compressor capacitor directly to eliminate the fault; the new replacement capacitor capacity error is within 20% of the original capacitor capacity. Can be used normally.