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Reasons and solutions for excessive motor temperature

In general, at what temperature can the motor work normally? What is the maximum temperature the motor can withstand?

If the measured temperature of the motor cover exceeds the ambient temperature by more than 25 degrees, it indicates that the temperature rise of the motor has exceeded the normal range. Generally, the temperature rise of the motor should be below 20 degrees. Generally, the motor coil is wound by enameled wire, and when the temperature of the enameled wire is higher than about 150 degrees, the paint film will fall off due to the high temperature, causing the coil to short circuit. When the coil temperature is above 150 degrees, the temperature of the motor housing is around 100 degrees, so if the temperature of the housing is used as the basis, the maximum temperature the motor can withstand is 100 degrees.

 The temperature of the motor should be below 20 degrees Celsius, that is, the temperature of the motor end cover should be less than 20 degrees Celsius over the ambient temperature, but what is the reason for the motor heating over 20 degrees Celsius?

The direct cause of the motor heating is due to the large current. Generally, it may be caused by short-circuit or open-circuit of the coil, demagnetization of the magnetic steel, or low efficiency of the motor. The normal situation is that the motor runs for a long time with high current.

What causes the motor to heat up? What kind of process is this?

When the motor load is running, there is power loss in the motor, which will eventually become heat energy, which will increase the temperature of the motor and exceed the ambient temperature. The value at which the motor temperature is higher than the ambient temperature is called temperature rise. Once the temperature rises, the motor must dissipate heat to the surrounding; the higher the temperature, the faster the heat dissipation. When the heat emitted by the motor per unit time is equal to the heat dissipated, the temperature of the motor no longer increases, but maintains a stable temperature, that is, in a state of heat and heat balance.

Generally, what is the allowable temperature rise? The temperature rise of the motor has the greatest impact on which part of the motor? How is it defined?

Answer: When the motor is running with a load, start from playing its role as much as possible. The larger the load, that is, the greater the output power, the better (if the mechanical strength is not considered). But the greater the output power, the greater the power loss, and the higher the temperature. We know that the weakest thing in the motor is the insulating material, such as enameled wire. There is a limit to the temperature resistance of insulating materials. Within this limit, the physical, chemical, mechanical, and electrical properties of insulating materials are stable, and their working life is generally about 20 years.

Beyond this limit, the life of the insulating material will be shortened drastically, or even burned. This temperature limit is called the allowable temperature of the insulating material. The allowable temperature of the insulating material is the allowable temperature of the motor; the life of the insulating material is generally the life of the motor.

Excessive temperature is one of the common fault types of motors. So what causes the excessive temperature of the motor?

The following are common reasons and treatment methods for high motor temperature:

1. When the instant voltage of the motor exceeds 10% of the rated voltage, or the instant voltage of the motor is lower than 5% of the rated voltage, it will cause the motor to heat up and temperature rise under the rated load. In this case, check and adjust the voltage.

2. The unbalance of the three-phase power supply voltage of the motor will also cause the motor to heat up. This is because when the unbalanced degree of the three-phase power supply voltage exceeds 5%, it will cause the unbalance of the three-phase current. The countermeasure is to check and adjust the voltage.

3. The contact problem of the power switch of the motor and the open circuit of the one-phase fuse will cause the lack of phase operation, which will cause the temperature of the motor to rise. The solution is to repair or replace the damaged parts.

4. There is an error in the wiring of the motor winding, which causes the motor running under the rated load to produce overheating. The solution is to correct the wiring error of the winding wire.

5. The stator winding of the motor is short-circuited or grounded between turns or between phases. This situation will increase the current of the motor and cause the temperature to rise. The solution is to add insulation in the center or directly replace the winding.

6. Broken bars of the cage rotor of the motor or loose joints of the wound rotor coil will cause the current of the maintenance network to increase and increase the temperature. The countermeasures are to weld or replace the rotor.

7. When the bearing of the motor is severely worn, it will produce large friction and generate heat. The solution is to check whether the bearing is loose and whether the stator and rotor are poorly matched.

8. Excessive motor load is also the cause of overheating. Lightening the load or replacing the high-power motor can solve the overheating problem.

9. Too frequent starting of the motor, too high ambient temperature, poor ventilation, etc. will also cause the temperature of the motor to be too high, reducing the number of starts, lowering the ambient temperature, ensuring unobstructed air ducts, eliminating dust and oil, and keeping the fan running well. Can help solve similar overheating problems.

For a running motor, if the current does not exceed the rated current of the motor, it means that there is basically no problem in the circuit. If the original load is not changed, it is necessary to check whether the voltage is at the rated voltage. Generally, 380V is plus or minus 5% normal. See if the ambient temperature is too high. Whether the bearing is out of oil. Whether the cooling fan is damaged.

(1) The load is too large. Should reduce the load or replace a larger capacity motor.

(2) Two-phase operation. Check whether the fuse is blown, whether the contact of the switch is in good contact, and troubleshoot;

(3) The air duct of the motor is blocked. The air duct dust or grease should be removed;

(4) The ambient temperature rises. Cooling measures should be taken;

(5) Short circuit between turns or phases of stator windings. Use a megohmmeter or a multimeter to check the insulation resistance between the two-phase windings; use the current balance method to check the three-phase winding current, the phase with a large current is a short-circuit phase, and a short-circuit detector can also be used to check whether the winding turns are short-circuited;

(6) The stator winding is grounded. It can be checked with a multimeter or indicator light, and the resistance is zero for the grounding phase;

(7) The power supply voltage is too low or too high. Use the voltage file of a multimeter or a voltmeter to check the power supply voltage at the input end of the motor.

Article source: Wangcai Motor and Electronic Control, Today's Motor

Jul 05, 2021