maintaining the start relay of the compressor

Figure 1: The maintenance worker is repairing the start relay of the refrigerator.

Refrigerators have been necessary for people’s daily life, and there are probably some problems with your refrigerators after a long time of use. So this article will let you know one of the causes of the failing refrigerator - a bad start relay of the compressor.

Start Relay Fault Phenomenons

If the power supply voltage is within the allowable fluctuation range, the compressor will be unable to operate normally if it is started continuously more than three times.

The power supply voltage is normal, and the compressor and the motor are under good conditions. We hear no sound of the contact after the refrigerator is powered, but hear the compressor noise for around 10 seconds. And after a period of time, this phenomenon is repeated.

The power supply voltage is normal, as well as the motor. After powered on, the compressor makes no sound. The suction current is 0, and the thermal protection contact jumps away after 3 seconds.

The power supply voltage and the motor are both normal. The compressor hums after powered on, and the suction current is more than 5 A. After 3 seconds, the start contact is constantly connected, and then the thermal protection contact jumps away.

several motor ptc start relays of the compressor

Figure 2: Refrigerator compressor start relays.

Start Relay Failure Causes

The above phenomenons are caused by the start relay faults. There are several main reasons for the start relay failure.

The starter contact is dirty, burned, or uneven. And the starter and thermal protector are not connected to the compressor motor tightly.

The start relay is stuck in the contacts.

The failure is usually caused by the loose wiring of the starter and the open current coil of the starter.

The starter contacts fail to close.

In addition, if specifications of start relays and thermal protectors are different, it will also cause the compressor start failure or frequent start and stop. so in the replacement of new components, you should pay attention to the match between the components and the compressor.

bimetal thermal switches and thermal protectors

Figure 3: Thermal switches and thermal protectors.

Why the Relay does not Close

The air compressor applying clean energy has been widely used in various industries, and this kind of equipment failure will be inevitable in common use. In the following, we will tell you the reason why the relay in the air compressor system does not close.

In the air compressor starter, the start and stop of the motor are completed by the contact of the intermediate relay in the instrument control cabinet. The air compressor starts and stops automatically under the control of the system pressure.

When the system pressure reaches the set upper limit of 0.75 MPa, the unit unloads and runs. If the running time reaches the set time and the system pressure has not fallen below the lower limit of 0.4MPa, the unit will automatically unload and stop.

If the gas consumption is large, the unit will stop without unloading. And then it is loaded, and unloading operation, which is the cycle. These controls are completed by the pressure relay, time relay, and intermediate relay in the instrument control cabinet. The air compressor unit operates under automatic control. In addition to the two patrols operators of every shift, the unit is in an unsupervised state.

The reason why the temperature of the auto-transformer is high and fire accidents is that the main contactor of the motor does not close during the starting process, and the motor cannot run at full pressure. But it runs with the auto-transformer, which causes fire.

allen-bradley contactor 100s-e40ky31c

Figure 4: Allen-Bradley Contactor

There are two reasons why the intermediate relay does not close.

The air compressor motor is 138 KW. The main contactor is an AC contactor whose armature is heavy, leading to severe vibration when the air compressor starts. As a result, it will cause the coil lead head to vibrate or the armature to be stuck.

In general, when the air compressor runs normally, the open contacts of the intermediate relay shall normally and the closed contacts of the auto-transformer star contactor make the relay close. But this failure will make it open. The motor runs with an auto-transformer, which turns into a reactor and causes an accident when the coil is heating up.

The time relay in the control loop is damaged so that the motor always runs through the decompression of the auto-transformer, which causes the auto-transformer to get hot and burn out.


From this article, we have known the causes of the refrigerator start relay failures, which causes the refrigerator not to cool down. In addition, we have introduced some reasons why the relay cannot close.

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