I. Causes of Motor Failure
1. Internal reasons
Internal causes refer to open circuits or burnout of internal coils, desoldering or corrosion of connection parts, oxidation of switches and contacts, etc.
2. External reasons
External causes refer to faults caused by external conditions of the motor, such as damage to the coil windings caused by abnormal grid voltage, long-term operation of the motor, or excessive load, as well as aging of the windings and short circuits between turns caused by excessive dust accumulation.
3. Human factors
Human-caused factors include severe vibrations and excessive jolting during transportation, as well as malfunctions caused by user-initiated disassembly or modification. Before repairing the machine, maintenance personnel should first determine the cause of the malfunction, and then inspect, analyze, and repair according to the different causes and symptoms. Generally, repairs should begin with external causes, as this method is simpler. Before repairs, it is also advisable to consult with the user and keep records during the repair process to facilitate analysis and judgment of the malfunction before addressing internal causes.
II. Motor Fault Repair Procedure
To quickly repair an electric motor, in addition to mastering its basic principles and correct repair methods, attention should also be paid to whether the repair steps are reasonable, ensuring that the repair work is carried out in an orderly manner. Whether it is a single-phase motor or a three-phase motor, the repair procedure is generally the same. During repair, the following steps can be followed.
1. Ask the user
When taking over a motor to be repaired, by questioning the user to understand the fault symptoms, and through analysis, the scope of the fault can be roughly determined. To find the fault point and sort out the clues, the questions asked can be roughly divided into the following categories.
(1) Operating status of the motor
For example, changes in speed and temperature, any abnormal noises or violent vibrations, and whether there is sparking, smoke, or a burning smell in the switches and motor windings.
(2) Electric motor usage
For example, whether the motor is operating indoors or outdoors, whether it is corroded by acids, alkalis, salts or other corrosive gases, and whether there is excessive dust or oil buildup; whether the motor's air duct is blocked, whether it is damp or has been rained on; whether the motor's operating mode complies with regulations, whether the nature of the load meets the motor's performance specifications, whether the power supply meets the requirements, and whether the three-phase power supply voltage is balanced (the difference between any two phase voltages should not exceed 5% of the three-phase average value), etc.
(3) Maintenance status of the electric motor
If the lubricating oil is not changed on time, the bearings will wear out; if the wire diameter is too thin or the number of coil turns is insufficient during motor repair, the motor will overheat; if the frame or end cover is cracked or poorly welded, it will cause port deformation, resulting in uneven air gap, and the motor will vibrate during operation or even cause the stator and rotor to rub against each other.
Electric motors are widely used in manufacturing and household appliances. During long-term use, they are bound to malfunction. Quickly identifying the cause of the malfunction and eliminating it is one of the important tasks of equipment maintenance.
Three-phase asynchronous motors are currently the most widely used type of motor, and they also have a variety of fault types. The following analysis will examine several typical faults:
1. The motor does not turn after being powered on, but there is no abnormal noise, smell or smoke.
This type of fault is basically caused by the main circuit control equipment (frequency converter, contactor, soft starter, etc.) not operating. You can check whether the main circuit control equipment receives control commands in a timely manner, such as whether the main contactor is engaged, whether the frequency converter is energized and in operating mode, etc.
If the main circuit control device does not operate, then the status of the control circuit components should be checked:
1. Check if the power supply to the control circuit is normal by measuring the voltage at the output terminal of the control switch. If it is abnormal, check if the fuse is blown and if the control switch is turned on.
2. Check if the control circuit is open. If it is a general relay control circuit, you can measure the voltage across the start button while it is powered on. When the control circuit is open, the measured voltage should be the control loop voltage; otherwise, the measured voltage value will be lower than the control loop voltage value.
You can check whether the protection device is activated, such as whether the thermal relay has not reset after activation, whether the normally closed contacts of the relevant relays are connected, whether there are any broken wires, poor contacts, or burnt screws in the circuit, whether the relay and contactor coils are open-circuited, and whether the intermediate relay contacts are burnt, etc.
If the main circuit control equipment is powered on, but the motor does not move, the main circuit loop should be suspected:
1. Two phases of the main circuit fuse have blown;
2. Two phases of the main circuit contactor's main contacts are burnt out;
3. Motor phase loss;
4. Main circuit disconnection.
These faults can all be diagnosed by measuring resistance when the power is off.
2. The motor does not turn after being powered on, but makes a "humming" sound.
Such faults are usually caused by single-phase faults or stall faults. You can check by turning the motor while the power is off. If the motor does not sink when turned, then you should consider whether it is a single-phase fault.
If the initial assessment is that it is a single-phase fault, then it is necessary to check whether it is a single-phase fault in the motor or the power supply. This can be done by measuring the motor windings. If the motor windings are normal, the power supply circuit should be checked in turn.
1. Inspect and measure the main fuse and the main power switch;
2. Inspect and measure the main contactor contacts;
3. Check all wiring points in the main circuit and measure the main circuit line.
If the motor is too heavy to rotate, you should distinguish whether it is due to the load or the motor itself. You should disconnect the motor from the load and rotate the motor under no-load conditions. If the motor is stuck, it may be due to bearing damage or rotor deformation.
III. Overcurrent protection trips after power-on.
Such faults are usually caused by overcurrent, or the overcurrent setting may be too low. The current value should be measured to make a judgment. If the current value is indeed high, then further investigation can be carried out.
1. The load is too heavy. This can be determined by rotating the motor rotor or running the motor under no-load conditions.
2. The bearing is faulty; this can be determined by rotating the motor rotor.
3. Motor fault. This type of fault is difficult to diagnose and often requires measuring the winding resistance or the insulation value between winding turns. However, it usually manifests as low speed or insufficient torque when under no-load conditions.
4. Poor wiring.
4. Difficulty starting the motor; under rated load, the motor speed is significantly lower than the rated speed.
To address the root cause of shaft current formation, the following preventative measures are typically taken on-site:
Current conduction: A grounding carbon brush is installed at the shaft extension end to ensure reliable grounding and contact with the shaft, guaranteeing zero potential on the shaft. This allows static charge on the motor shaft to be drawn to the ground at any time, thereby eliminating shaft current. This measure is commonly used in generators and occasionally in large AC motors.
Add insulating partition
To prevent shaft current caused by magnetic imbalance or other reasons, insulating baffles can be added to the bearing housing and bearing support at the non-shaft extension end to cut off the shaft current circuit.
To address shaft voltage caused by static charge, a grounded carbon brush can be added to the shaft on the load side of the motor. The carbon brush grounding must be reliable. This allows us to continuously draw static charge from the motor shaft to the ground, preventing the formation of shaft voltage and avoiding instantaneous shaft current caused by charge discharge.
Strengthen insulation protection
To avoid shaft current caused by damaged insulation of wires in other motor accessories, the insulation of wires or gaskets should be checked regularly and carefully, and the insulation should be strengthened to eliminate unnecessary shaft current hazards.
What are the causes of motor overheating?
Excessive temperature rise during motor operation not only shortens its lifespan but can also cause fires in severe cases. Overheating is often a symptom of motor malfunction and a major cause of motor damage.
Cause of the malfunction:
If the power supply voltage is too high or too low, or if the current is too high, the windings will overheat.
Improper use of hot disassembly method during winding repair can burn the iron core.
Broken bars of cage rotor
Motor with one phase missing, two phases running
Motor overload or frequent starting
Damaged motor bearings increase resistance
High ambient temperature can cause excessive dirt buildup on the motor surface, or blocked ventilation ducts.
stator winding fault
Stator and rotor core rubbing
Solutions
Motor overheating poses numerous hazards, including motor burnout, personal injury, production stoppages, and significant losses. Therefore, prevention and control measures are essential. Specific methods include the following:
First, the motor must have a good hygienic environment. The motor casing and heat dissipation slots must be clean and hygienic, and able to be in direct contact with the surrounding air to achieve good heat dissipation.
Prolonged overload operation of motors is prohibited. If short-term overload is necessary, appropriate cooling measures must be taken, such as installing a dedicated fan to cool the motor. It is also important to note that motor overload must not exceed 10% of the rated load.
If the cooling fan is installed properly, you must pay attention to the direction of rotation during operation to prevent reverse rotation.
Regularly clean the cooling air ducts to remove dust and scale, ensuring efficient heat exchange.
Regularly inspect and maintain the cooling equipment to ensure sufficient cooling medium and maintain effective cooling performance.
Improve airflow around the motor to promote heat dissipation; if necessary, add a fan to enhance airflow. Reduce the temperature around the motor.
What happens to a three-phase motor when one phase is missing?
Three-phase motor operation with one phase missing refers to the motor's stator windings being without one of the three phases of power.
