What are the reasons for the damage to the compressor coil?

When the motor burns out, the contactor must be checked. Contactors and phase loss/voltage anomalies are an often-forgotten cause of motor damage. Today we will talk about the problems of contactor and power supply phase loss and abnormal voltage, and hope to help everyone.

1. AC contactor

The AC contactor is one of the important components in the motor control circuit, and unreasonable selection can destroy the best compressors. It is extremely important to select the correct contactor according to the load. AC contactors must be able to withstand harsh conditions such as rapid cycling, continuous overload and low voltage. They must have a large enough area to dissipate the heat generated by the load current, and the contact material must be chosen to prevent welding during high current conditions such as startup or stall.

The contactor must meet the following requirements:

1) The contactor must be able to close at 80% of the minimum nameplate voltage.

2) When using a single contactor, the rated current of the contactor must be greater than the rated current of the motor nameplate, and at the same time, the contactor must be able to withstand the motor stall current.

3) When two contactors are used, the split-winding stall rating of each contactor must be equal to or greater than the compressor half-winding stall rating.

Others to note:

1. The rated current of the contactor cannot be lower than the rated current on the nameplate of the compressor. Contactors with small specifications or poor quality cannot withstand the high current impact of compressor startup, locked rotor and low voltage, and are prone to single-phase or multi-phase contacts shaking, welding or even falling off, causing motor damage.

2. The contactor with contact vibration will frequently start and stop the motor. Frequent starting of the motor, huge starting current and heat will aggravate the aging of the winding insulation. The magnetic torque causes the motor windings to move slightly and rub against each other with each start. If other factors cooperate, it is easy to cause short circuit between windings. In addition, shaking contactor coils are prone to failure. If the contact coil is damaged, a single-phase state is likely to occur.

Here, we stress again that when the motor burns out, the contactor must be checked. After the contacts of the AC contactor are adhered, the compressor will work regardless of whether the main board has output or not. If there is no protection for a long time, "streaking", the compressor damage is inevitable. When replacing the compressor, be sure to find the cause of the damage to the compressor, and be sure to check whether the AC contactor is normal and whether there is phase loss or adhesion. It is not possible to just replace the compressor without investigating the cause, which will inevitably lead to the secondary damage of the newly replaced compressor.

Second, the power supply phase loss and abnormal voltage

Abnormal voltage and phase loss can easily destroy any motor, the power supply voltage variation range cannot exceed ±10% of the rated voltage, and the voltage imbalance between the three phases cannot exceed 5%. High-power motors must be powered independently to prevent low voltage when other high-power equipment on the same line starts and runs.

The motor power cord must be capable of carrying the rated current of the motor. If the compressor is running when a phase loss occurs, it will continue to run but with a high load current. The motor windings can overheat very quickly and normally the compressor is thermally protected. When the motor winding cools down to the set temperature, the contactor will be closed, but the compressor will not start up, the rotor will stall, and it will enter the dead cycle of "stall-thermal protection-stall".

In order to prevent the compressor from running without phase, the compressors with three-phase power supply are equipped with a phase sequence protector. The phase sequence protector has two functions. One is to prevent the compressor from running in reverse with a wrong phase sequence; phase operation.

Now the difference between the motor windings is very small, and the difference in phase current can be ignored when the three phases of the power supply are balanced. Ideally, the phase voltages are always equal, and a protector on either phase can prevent damage from overcurrent. It is practically difficult to ensure the balance of the phase voltages.

The motor power cord must be capable of carrying the rated current of the motor. If the compressor is running when a phase loss occurs, it will continue to run but with a high load current. The motor windings can overheat very quickly and normally the compressor is thermally protected.

When the motor winding cools down to the set temperature, the contactor will be closed, but the compressor will not start up, the rotor will stall, and it will enter the dead cycle of "stall-thermal protection-stall".

The calculation method of the voltage unbalance percentage is: the ratio of the maximum deviation value of the phase voltage and the average value of the three-phase voltage to the average value of the three-phase voltage.

For example, for a nominal 380V three-phase power supply, the voltages measured at the compressor terminals are 380V, 366V, and 400V, respectively. It can be calculated that the average value of the three-phase voltage is 382V, and the maximum deviation is 20V, so the voltage unbalance percentage is 5.2%. As a result of the voltage unbalance, the unbalance of the load current in normal operation is 4-10 times the voltage unbalance percentile. In the previous example, a 5.2% unbalanced voltage may cause a 50% current unbalance.

The National Electrical Manufacturers Association Motor and Generator Standards publication states that the percentage of phase winding temperature rise due to unbalanced voltage is approximately twice the voltage unbalance percentile squared. In the previous example, the number of voltage unbalance points is 5.2, and the percentage of winding temperature increase is 54%. The result is that one phase winding is overheated and the other two windings are normal.