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What should we do if the motor gets hot?

Date:2023-11-03   Author:XINDA MOTOR

As an indispensable and important power provider in people's production and life, motors will generate serious heat during use. However, many times we don't know how to solve it. What's more serious is that we don't know what the reason is. The motor heating caused by the motor should be grasped first during the use of the motor. Let's take a look at the common reasons why the motor heats up seriously.

1. The air gap between the stator and rotor of the motor is very small, which can easily cause the stator and rotor to collide with each other.

In medium and small motors, the air gap is generally 0.2mm ~ 1.5mm. When the air gap is large, the excitation current is required to be large, which affects the power factor of the motor; when the air gap is too small, friction or collision may occur in the rotor. Generally, due to serious out-of-tolerance bearings and wear and deformation of the inner hole of the end cover, the machine base, end cover, and rotor have different axes, causing the bore to sweep, which can easily cause the motor to heat up or even burn out. If the bearing is found to be worn, it should be replaced in time, and the end cover should be replaced or brush-plated. The simpler method is to set the end cover.  

2. Abnormal vibration or noise of the motor can easily cause the motor to heat up.

This situation is caused by vibration caused by the motor itself. Most of it is due to poor dynamic balance of the rotor, poor bearings, bent shafts, different axes of the end cover, machine base, and rotor, loose fasteners, or uneven motor installation foundations or improper installation. It may be caused by transmission from the mechanical end, which should be ruled out based on specific circumstances.

3. If the bearing is not working properly, it will inevitably cause the motor to heat up. Whether the bearing is working properly can be judged based on hearing and temperature experience.

You can use your hands or a thermometer to check the bearing end to determine whether its temperature is within the normal range; you can also use a listening rod (copper rod) to contact the bearing box. If you hear an impact sound, it means that one or several balls may have been crushed. A hissing sound means that the bearings have insufficient lubricating oil. The motor should be replaced with grease every 3,000 to 5,000 hours of operation. 

 4. The power supply voltage is too high, the excitation current increases, and the motor will overheat.

Excessive voltage can endanger the motor insulation, putting it at risk of breakdown. When the power supply voltage is too low, the electromagnetic torque will decrease. If the load torque does not decrease and the rotor rotation speed is too low, the increase in slip will cause the motor to be overloaded and generate heat. Long-term overloading will affect the life of the motor. When the three-phase voltage is asymmetrical, that is, when the voltage of one phase is too high or too low, the current in a certain phase will be too large, the motor will heat up, and the torque will decrease and it will make a "buzzing" sound. Over time, the winding will be damaged.

In short, whether the voltage is too high, too low or the voltage is asymmetrical, the current will increase, the motor will heat up and damage the motor. Therefore, according to national standards, the change in motor power supply voltage should not exceed ±5% of the rated value, and the motor output power can maintain the rated value. The motor power supply voltage is not allowed to exceed ±10% of the rated value, and the difference between the three-phase power supply voltages should not exceed ±5% of the rated value.

5. Winding short circuit, turn-to-turn short circuit, phase-to-phase short circuit and winding open circuit

After the insulation between two adjacent conductors in the winding is damaged, causing the two conductors to collide, it is called a winding short circuit. A winding short circuit occurring in the same winding is called an inter-turn short circuit. A winding short circuit that occurs between two phase windings is called a phase-to-phase short circuit. No matter which one it is, it will increase the current of one phase or two phases, causing local heating, aging the insulation and damaging the motor. Winding open circuit refers to a fault caused by the stator or rotor winding of the motor being broken or burned. Whether the winding is short-circuited or open-circuited, it may cause the motor to heat up or even burn out. Therefore, it must be shut down immediately after this happens. 

6. Material leaks into the inside of the motor, reducing the insulation of the motor, thereby reducing the allowable temperature rise of the motor. 

If solid materials or dust enter the motor from the junction box, they will reach the air gap between the motor stator and rotor, causing the motor to sweep, until the motor winding insulation is worn out, causing the motor to be damaged or scrapped. If liquid and gas media leak into the interior of the motor, it will directly cause the motor insulation to drop and trip. Generally, liquid and gas leaks have the following forms: 

 (1) Leakage from various containers and delivery pipelines, leakage from pump body seals, flushing equipment and floors, etc. 

   (2) After mechanical oil leaks, it enters the motor from the gap in the front bearing box. 

 (3) The oil seals of the reducer connected to the motor are worn, and the mechanical lubricating oil enters along the motor shaft. After accumulating inside the motor, it dissolves the motor insulating paint and gradually reduces the motor's insulation performance. 

7. Almost half of the motor burnouts are caused by the motor running out of phase.

Phase loss often causes the motor to fail to run or rotate slowly after starting, or the motor has no power to rotate and the current increases, causing a "buzzing" sound. If the load on the shaft does not change, the motor is in a severe overload state and the stator current will reach 2 times the rated value or even higher. The motor will heat up or even burn out in a short period of time. The main reasons causing phase loss operation are as follows: 


    (1) If one phase of the power line is cut off due to the failure of other equipment, the other three-phase equipment connected to the line will operate with a lack of phase.  

    (2) One phase of the circuit breaker or contactor is missing due to burnt bias voltage or poor contact.  

    (3) Phase loss in the motor’s incoming line due to aging, wear, etc.  

    (4) One phase winding of the motor is open, or one phase connector in the junction box is loose.  

8. Other non-mechanical electrical failure causes

The motor temperature rise caused by other non-mechanical and electrical faults may also lead to motor failure in severe cases. If the ambient temperature is high, the motor is missing a fan, the fan is incomplete, or the fan cover is missing. In this case, forced cooling must be performed to ensure ventilation or the fan blades must be replaced, otherwise the normal operation of the motor cannot be guaranteed.  

To sum up, in order to use the correct method to handle motor faults, you must be familiar with the characteristics and causes of common motor faults, grasp the key factors, and conduct regular inspections and maintenance. In this way, we can avoid detours, save time, troubleshoot as soon as possible, and keep the motor in normal operating condition. This ensures normal production in the workshop.