Compressor Protection Measures

When the compressor is running, some abnormal conditions may occur, such as: too high discharge pressure, too low suction pressure, insufficient oil pressure, overheating of the motor, excessive liquid into the cylinder, etc. When abnormal conditions occur, the compressor will be damaged if there are no protective measures.

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Built-In Motor Protection

1.Overheating

The internal temperature of the motor will not exceed the allowed value if the motor is designed well and operated under the specified conditions, but the internal temperature of the motor will exceed the allowed value if the motor is operated under too high or too low voltage or in high temperature environment, and the temperature will be too high if the motor is started frequently.

In order to prevent the motor from overheating, in addition to proper use and maintenance, an overheating relay can be installed.

The overheating relay can be installed inside the winding, called built-in temperature relay, or outside the motor, called external temperature-current relay.

When the internal temperature of the motor exceeds the specified value, the bimetal of the built-in temperature relay trips the contact due to deformation and the motor stops running. When the temperature inside the motor drops below the specified value, the contacts are reset and the circuit is turned on again.

External temperature-current relay: a disc-shaped bimetal and a heat sink are connected in series between two terminals. When the current inside the motor is too high, the heating of the bimetal by the heater makes it in the dashed position in the figure, the contact will be tripped and the motor will stop rotating. The external temperature-current relay protects the motor from overheating due to excessive current. But in some cases, the overheating of the motor is not caused by the excessive current, but by the insufficient low temperature gas flowing through the motor, for example, the refrigerant circulation is very small due to the blockage of the refrigeration system, the motor cooling is extremely poor, resulting in internal overheating, the built-in temperature relay plays a role in protecting the motor at this time.

2.Phase loss

Phase loss of three-phase motor will cause the motor to be unable to start or overload. In order to protect the motor from the damage of phase loss, an overload relay is used, which consists of a mechanical moving part and an electromagnetic switch part. The mechanical movement part has four terminals, two at the top and two at the bottom. The upper and lower terminals are equipped with heaters between them.

When the phase is missing, the winding overload current of other phases passes through the heater, the bimetal disc is heated and deformed, pushing the pressure plate, which in turn makes the overload relay contact on the electromagnetic switch jump open, no current in the excitation coil, the magnetic contactor is no longer closed and the motor stops rotating. The overload relay is also used for normal three-phase motor to protect the motor when the current is too high.

3.Phase to phase unbalance

Unbalanced voltage between phases leads to unbalanced current of three phases. In the phase with the highest current, the proportion of temperature rise increase is about twice the square of the voltage unbalance proportion. For example, 3% of voltage unbalance produces about 18% of temperature rise. The measures to protect the motor from phase unbalance are the same as those taken in case of phase deficiency.

Temperature Protection

The temperature mentioned here refers to the exhaust temperature and the case temperature.

1、Exhaust temperature

High exhaust temperature leads to decomposition of refrigerant, aging of insulation material, carbonization of lubricating oil, damage to gas valve, and blockage of capillary tube and filter drier. The main protection method is to use the thermostat to sense the exhaust temperature, the thermostat should be placed near the exhaust port, when the exhaust temperature is too high, the thermostat action, cut off the circuit.

If the exhaust temperature is too high caused by the hot gas bypass, the method of shutdown should not be used, should be used to spray liquid cooling.

2、Case temperature

Case temperature will affect the life of the compressor. Case temperature is too high may be caused by the condenser heat transfer capacity is not enough, so you should check the condenser landscape or water, water temperature is appropriate, the refrigeration system mixed with air or other non-condensable gases, condensing pressure will rise, the case overheating; suction temperature is too high, the case is easy to overheat, in addition, the motor overheating will also overheat the case.

The fundamental way to avoid overheating of the casing and to protect the compressor is to properly deal with the various problems mentioned above and to install a temperature protector on the casing at the same time. The most commonly used case temperature protector is an external temperature-current protector. It is placed in a suitable place on the casing. When the casing temperature is too high, the disc-shaped bimetal feels the temperature and deforms, making the contacts in the circuit trip and the compressor stops.