As an indispensable power provider in people's production and life, electric motors often experience severe overheating during use. However, many people don't know how to solve this problem, and even more seriously, they don't know the cause of the overheating. This should be the first thing to understand when using electric motors. Let's take a look at the common reasons why electric motors overheat.
1. The air gap between the stator and rotor of the motor is very small, which can easily lead to collisions between the stator and rotor.
In small and medium-sized motors, the air gap is typically 0.2mm to 1.5mm. A larger air gap requires a larger excitation current, thus affecting the motor's power factor; a smaller air gap may cause the rotor to rub or collide. Generally, severe bearing wear and deformation of the end cover's inner bore can cause misalignment between the frame, end cover, and rotor, leading to rotor rubbing and easily causing the motor to overheat or even burn out. If bearing wear is found, it should be replaced promptly. The end cover should be replaced or plated. A simpler solution is to insert a bushing into the end cover.
2. Abnormal vibration or noise of the motor can easily cause the motor to overheat.
This type of vibration is caused by the motor itself, mostly due to poor rotor dynamic balance, faulty bearings, bent shaft, misalignment of end cover, base, and rotor, loose fasteners, uneven motor mounting foundation, or improper installation. It may also be transmitted from the mechanical end. The specific cause should be identified and eliminated.
3. Malfunctioning bearings will inevitably cause the motor to overheat.
Whether a bearing is working properly can be judged by listening and temperature experience.
You can use your hand or a thermometer to check the bearing end to determine if its temperature is within the normal range; you can also use a listening rod (copper rod) to touch the bearing housing. If you hear an impact sound, it means that one or more balls may have been crushed. If you hear a hissing sound, it means that the bearing lubricating oil is insufficient. The motor should have its grease changed every 3,000 to 5,000 hours of operation.
4. If the power supply voltage is too high, the excitation current will increase, and the motor will overheat.
Excessive voltage can damage the motor insulation, posing a risk of breakdown. Low power supply voltage reduces electromagnetic torque; if the load torque doesn't decrease, the rotor speed will be too low, increasing slip and causing motor overload and overheating. Prolonged overload will shorten the motor's lifespan. Asymmetrical three-phase voltages (one phase being too high or too low) will result in excessive current in that phase, causing the motor to overheat and produce a humming sound due to reduced torque. Over time, this can damage the windings.
In summary, excessively high, low, or unbalanced voltage will all increase current, causing the motor to overheat and potentially damage it. Therefore, according to national standards, the variation in motor power supply voltage should not exceed ±5% of the rated value, and the motor output power should remain at 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, inter-turn short circuit, phase-to-phase short circuit, and winding open circuit.
When the insulation between two adjacent conductors in a winding fails, causing the two conductors to come into contact, it is called a winding short circuit. A winding short circuit occurring within the same winding is called an inter-turn short circuit. A winding short circuit occurring between two phase windings is called a phase-to-phase short circuit. Regardless of the type, it will increase the current in one or two phases, causing localized heating and leading to insulation aging and damage to the motor. A winding open circuit refers to a fault caused by the stator or rotor winding breaking or burning out. Both winding short circuits and open circuits can cause the motor to overheat or even burn out. Therefore, in such cases, the machine must be stopped immediately for repair.
6. Material leakage into the motor reduces its insulation, thereby lowering the allowable temperature rise of the motor.
If solid materials or dust enter the motor through the junction box, they will reach the air gap between the stator and rotor, causing the motor to rub against the rotor and eventually wear down the winding insulation, leading to motor damage or scrapping. If liquids or gases leak into the motor, it will directly cause a drop in insulation and trip the circuit breaker. Liquid and gas leaks generally manifest in the following ways:
Leaks in various containers and pipelines, pump seal leaks, and equipment and ground spills;
After the machine oil leaked, it entered the motor through the gap in the front bearing housing;
When the oil seals of components such as the speed reducer connected to the motor wear out, mechanical lubricating oil enters along the motor shaft. After accumulating inside the motor, it dissolves the motor's insulating varnish, gradually reducing the motor's insulation performance.
7. Nearly half of all motor burnouts are caused by single-phase operation.
Phase loss often causes a motor to fail to run, or to start slowly, or to rotate weakly with increased current and a "humming" sound. If the load on the shaft remains unchanged, the motor is in a state of severe overload, and the stator current will reach twice or even higher than the rated value. The motor will overheat and may even burn out within a short time. The main causes of single-phase operation are as follows:
If a phase of a power line is lost due to a fault in other equipment, the other three phases of equipment connected to that line will operate with a missing phase.
A phase loss occurs when one phase of a circuit breaker or contactor is burned out due to bias voltage or poor contact.
The motor connection line is missing a phase due to aging, wear, or other reasons.
One phase of the motor winding is open-circuited, or one phase connector in the junction box is loose.
8. Other non-mechanical and electrical fault causes
Other non-mechanical and electrical faults that cause motor temperature rise can also lead to motor failure in severe cases. These include high ambient temperatures, a missing or incomplete fan, or a missing fan cover. In such situations, forced cooling to ensure ventilation or replacement of the fan blades are necessary; otherwise, normal motor operation cannot be guaranteed.
In conclusion, to properly troubleshoot motor faults, it is essential to be familiar with the characteristics and causes of common motor malfunctions, identify key factors, and conduct regular inspections and maintenance. This will minimize wasted time and effort, allowing for faster fault resolution and ensuring the motor operates normally, thereby guaranteeing consistent production in the workshop.
