Figure 1: A maintenance worker is handing a refrigeration compressor that is damaged.
A compressor is a kind of driven fluid machinery that elevates low-pressure gas to high pressure. It is the heart of the refrigeration system. It inhales refrigerant gas with low temperature and low pressure from the suction pipe, compresses the piston driven by the motor, and then emits refrigerant gas with high temperature and high pressure to the exhaust pipe to provide power for the refrigeration cycle.
The motor drives the compressor directly, which makes the crankshaft rotate, drives the connecting rod to make the piston reciprocate, thus causing the cylinder volume change. Due to the change of pressure in the cylinder, the inlet valve makes the air flow from the air filter into the cylinder.
During the compression process, the cylinder volume reduces, and the compressed air flows from the exhaust pipe, check valve to storage tanks through the exhaust valve. When the exhaust pressure achieves the rated pressure of 0.7MPa, it is controlled by a pressure switch and automatically stops. When the pressure of the gas storage tank drops to 0.5MPa - 0.6MPa, the pressure switch is connected and started.
If the motor is burnt out, you should check the contactor. Contactor and phase loss, or voltage anomalies are an important but often forgotten cause of motor damage.
This article will take you to look at the contactor and power supply phase deficiency and voltage anomaly faults. I hope that it is helpful for you to solve the compressor coil problems.
An AC contactor is one of the important components in the motor control loop, and its improper selection may damage the compressor. It is extremely important to choose the contactor correctly according to the load.
AC contactors must be able to meet stringent conditions such as fast cycling, sustained overloading and low voltage. They must have an area large enough to dissipate the heat generated by the load current. And the contact material should be resistant to bonding under high current conditions such as starting or localizing.
Figure 2: An AC contactor.
The contactor should be closed at 80% of the lowest voltage marked on the nameplate.
When using a single contactor, the current rating of the contactor must be greater than that on the motor nameplate, at the same time, the contactor must be able to withstand the motor blocked current.
When you use two contactors, the blocked rating of the winding of the contactor must be equal to or greater than that of the half winding of the compressor.
The rating current of the contactor shall not be lower than the rating current indicated on the compressor nameplate. Small or poor-quality contactors can not withstand the current when the compressor starts, blocks or of low voltage. Therefore, the contactor is prone to single-phase or multiphase contact chatter, welding or even falling off, which damages the motor.
The contactor contact chatter will frequently start and stop the motor. The motor starts frequently, causing the huge starting current and heat that will aggravate the aging of the insulation layer of the winding.
Each time the motor starts, the magnetic torque causes the motor windings to move slightly and rub against each other. If there are other faults, it is easy to cause short circuits between windings.
In addition, the contactor coil is prone to failure. If the contact coil is damaged, it is easy to be in single-phase state.
You should note that after the motor burns out, you must check the contactor. In case of the contact adhesion of the AC contactor, whether there is an output, the compressor will operate. If the compressor runs without protection for a long time, it will inevitably fail.
When replacing a compressor, you must find the cause of the compressor failure. You must check if the contactor is normal, if there is a lack of phase and adhesion phenomenon. If you only replace the compressor without finding out the cause, the new compressor may also be damaged.
Figure 3: Compressor start windings.
Abnormal voltage and phase can easily destroy the motor. The range of power supply voltage shall not exceed ±10% of the rating voltage, and the voltage imbalance between three phases shall not exceed 5%. Large power motor must be powered independently to prevent its low voltage when other large power equipment on the same line starts and runs.
The power line of the motor must be capable of withstanding the rating current of the motor. If a phase loss occurs while the compressor is running, it will continue to run, but with a large load current.
The motor windings can quickly overheat and the compressor is normally thermally protected. When the motor winding is cooled to the set temperature, the contactor will be closed. However, the compressor can not start up, and there is a blockage, and forms a cycle of the "blocked - thermal protection - blocked".
In order to prevent the compressor from operating out of phase, compressors with three-phase power supply are equipped with phase sequence protectors. The role of the phase sequence protector is to avoid the reverse operation of the compressor with the wrong phase sequence. Besides, it is to prevent the operation lacking phase.
Now the differences in the motor windings are very small, and when the power supply is three-phase balanced, the difference in the phase current is almost negligible.
Ideally, the phase voltage is always equal, as long as a protector is connected to any phase, it can prevent motor damage due to over-current. In practice, however, it is difficult to ensure that the phase voltage is balanced.
The calculation method of voltage unbalance percentage:
Maximum deviation of phase voltage from average three-phase voltage/average three-phase voltage.
For instance, the three-phase power supply of nominal 380V, the voltage values measured in the compressor terminals are respectively 380V, 366V, 400V. You can calculate that the average three-phase voltage is 382V and the maximum deviation is 20V, so the percentage of voltage unbalance is 5.2%.
As a result of the voltage imbalance, the load current imbalance during normal operation is 4-10 times the voltage imbalance. In the above example, a 5.2% unbalance voltage may cause a 50% current imbalance.
It is stated that the percentage of temperature rise in the phase windings due to unbalanced voltage is about twice the square of the percentage of voltage imbalance. In the above example, the voltage imbalance is 5.2%, and the percentage increase of winding temperature is 54%. The result is that one phase winding overheats while the other two windings are at normal temperature.