Why does the motor still burn out even though a thermal overload relay has been installed?

The motor still burns out even after the thermal overload relay has been installed, which may be due to various factors that cause the relay to fail to effectively perform its protective function. The following are the common reasons and analysis:Chint Thermal Overload Relay NXR-38

I. Improper selection or setting of the thermal overload relay

1. Incorrect matching of the rated currentChint thermal overload relay 

   - The rated current of the relay is set too high (such as being greater than the rated current of the motor), resulting in the relay not operating during an overload. For example, if the rated current of the motor is 10A and the relay is set to 15A, when there is an overload, the current does not reach the threshold, and the protection cannot be triggered.

   - The influence of the motor starting current is not considered: The instantaneous current during starting may reach 5-7 times the rated current. If the relay does not have a delay function or starting protection, it may misjudge a normal start and respond slowly when there is a real overload.CHINT circuit breaker price

2. Mismatch between the type and the motorCHINT contactor price

   - For a three-phase motor, a three-phase overload relay is not used, or a single-phase relay is used in a three-phase system, resulting in the inability to detect a phase loss (for example, a single-phase relay only monitors the current of one phase, and the overload of the other two phases may not trigger the protection when there is a phase loss).

   - The "electronic type" and "bimetallic type" relays are not distinguished: The bimetallic type relies on the accumulation of temperature and reacts slowly to rapid overloads or short-term high currents; if the parameters of the electronic type are not set properly, it may also fail.

 II. Installation or wiring problemsCHINT surge protector price

1. Incorrect wiring or looseness

   - The wiring of the current transformer (if any) is incorrect, causing the current detected by the relay to not match the actual motor current.

   - The looseness of the wiring terminals increases the contact resistance, causing local heating. The relay fails to monitor the true current of the motor, or a phase loss is caused by poor contact (especially dangerous for a three-phase motor).

2. Not connected to the correct circuit

   - The relay is not connected in series in the main circuit of the motor, or the wiring of the control circuit is wrong, resulting in the inability to cut off the power supply during an overload.

 III. Environmental and heat dissipation problems

1. Excessive environmental temperature or poor heat dissipation

   - The relay is installed close to a heat source (such as the motor body or other heating equipment), causing its own temperature to rise and the action threshold to drift (for example, the bimetallic strip deforms in advance), which may cause misoperation or failure.

   - The motor's heat dissipation system fails (such as a damaged fan or blocked ventilation openings), causing the motor to overheat faster than the relay's response time.

2. Lack of temperature compensation for the relay

   - If the relay does not have a temperature compensation function, in a high-temperature environment, its operating current will decrease as the environmental temperature rises, and it may operate in advance; conversely, it may operate with a delay in a low-temperature environment.

 IV. Mismatch between the characteristics of the overload relay and the load

1. Mismatch of the load type

   - For loads with frequent start-stops and impact loads (such as compressors, conveyors), if a relay with functions such as "overload delay" or "anti-starting impact" is not selected, the relay may misoperate during starting, or it may fail due to a slow response when there is a real overload.

   - The protection requirements of DC motors and AC motors are different. If the relays are used interchangeably, effective monitoring may not be possible.

2. Problems with the reset mode

   - In the automatic reset mode, if the motor has not completely cooled down, and the relay automatically resets and restarts, it will lead to heat accumulation and aging of the insulation.

V. Faults or aging of the relay itself

1. Mechanical or electrical faults

   - The bimetallic strip deforms, and the contacts are oxidized or adhered, resulting in the inability to disconnect the circuit during an overload.

   - The internal components (such as sensors, chips) of the electronic relay fail, losing the monitoring function.

2. Performance drift after long-term use

   - The aging of the thermal element causes the action threshold to shift and the sensitivity to decrease, making it unable to respond to overloads in a timely manner.

 VI. Motor faults not caused by overload

1. Operation with a phase loss

   - When one phase of a three-phase motor is missing, the current in the other two phases surges (it can reach 1.7-2 times the rated current). If the relay has independent three-phase monitoring and insufficient sensitivity, it may trigger protection due to the overload of the two phases; however, if it is single-phase monitoring or the wiring is wrong, it may not be able to detect it, resulting in the rapid burnout of the motor.

2. Damage or short circuit of the winding insulation

   - Short circuits in the motor windings, inter-turn faults, or failure of the insulation to the ground generate abnormally large currents, which may exceed the response speed of the relay (for example, the instantaneous short-circuit current is much higher than the rated value, and the motor burns out before the relay operates).

3. Overload caused by mechanical faults

   - Sudden mechanical resistance such as a stuck bearing, a rubbing rotor, or a jammed load causes the current to surge. If the relay has a long response time (such as a bimetallic type taking several seconds to dozens of seconds), it may not be able to provide protection in time.

VII. Abnormalities in the power supply or voltage

1. Excessive voltage fluctuations

   - When the voltage is too low (such as below 85% of the rated voltage), the motor speed decreases and the torque is insufficient, and the current continues to rise ("stalling"). If the relay has a long action delay, the motor may overheat.

   - When the voltage is too high, the magnetic circuit is saturated and the excitation current surges, and the relay may not be able to respond in a timely manner.

2. Harmonics or power supply pollution

   - Harmonics are generated by variable frequency drives or non-linear loads, causing the current waveform to distort. Ordinary thermal overload relays may misjudge or be unable to accurately monitor the true effective value.

Suggestions for solutions:

1. Recheck the selection and setting

   - Confirm that the rated current of the relay is 1.05-1.1 times the rated current of the motor. For three-phase motors, select a three-phase monitoring type with phase loss protection function.

   - Adjust the delay parameters according to the load characteristics (for example, increase the delay for motors with a long starting time to avoid misoperation).

2. Check the installation and wiring

   - Ensure that the wiring is firm, the current detection circuit is correctly connected to the main circuit, and the control circuit can reliably cut off the power supply.

3. Improve the environment and maintenance

   - Keep the relay away from heat sources, regularly clean the motor's heat dissipation channels, and check the running status of the fan.

   - Regularly test the action characteristics of the relay (such as manual testing, simulating overload) and replace aging components.

4. Investigate non-overload faults of the motor

   - Detect the insulation resistance of the motor windings and the status of the bearings, and eliminate problems such as short circuits, phase losses, and mechanical jams.

   - Measure the power supply voltage and waveform to ensure stable power supply.

5. Upgrade the protection scheme

   - For high-demand scenarios, install an electronic multi-functional motor protector (with comprehensive protection functions such as current, voltage, phase loss, and blocking), or control it in conjunction with a PLC or frequency converter.

Summary:

The core reason for the failure of the thermal overload relay is the "mismatch between the protection mechanism and the actual fault". It is necessary to comprehensively analyze in combination with the motor type, load characteristics, installation environment, and fault phenomena, ensure the reasonable parameter setting and correct installation of the relay, conduct regular maintenance, and at the same time investigate the potential problems of the motor itself and the power supply system to effectively avoid burning out the motor.