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Electric motors are commonly used power machines and play a key role in the safe operation of electromechanical systems. To prevent major explosions and fires in motors, fire prevention measures can be taken based on the causes of the fires. I. Main causes of motor fires: Various faults can cause excessive current to flow through the motor windings during operation, leading to overheating; this in turn burns the insulation material and results in a fire. There are many reasons that can cause a motor to overheat and burn out; the common ones include the following: 1. Excessively high or low three-phase voltage can both lead to motor overheating. When the voltage is too high, the current in the motor’s windings increases, causing the winding temperature to rise above acceptable levels; this leads to insulation damage and eventually a fire ； If the voltage is too low, both the motor’s speed and the impedance of the stator windings decrease, resulting in an increase in current; this can cause the insulating material to burn due to overheating, leading to a fire. 2. The three-phase voltages are not symmetric (unbalanced), usually due to grid issues or motor faults. If the three-phase voltage applied to the motor is asymmetric, various losses in the operating motor increase, resulting in additional heating of the motor. It is generally required that the difference between the three-phase voltages not exceed 5%; under such conditions, the motor can still operate at its rated power for an extended period of time. 3. Operation with a missing phase usually occurs when one phase of the motor’s three-phase power supply is broken, or when one phase of the windings is broken. If a phase loss occurs while the motor is running, it may still continue to operate, but its speed will decrease. The current in the other two phases will increase by about 1.7–1.8 times compared to normal operating conditions, which can easily lead to damage to the windings; therefore, the motor should not be allowed to run with a phase loss for an extended period of time. 4. Using a small motor to drive a large load represents mechanical overload operation, which can easily cause the motor to overheat due to prolonged excessive current, leading to the burning of the insulation material and even fires. 5. There is an error in the winding wiring. Generally, it is due to external wiring errors; or during maintenance, one or several coils in a certain pole phase group may be inserted in the wrong direction, or the pole phase groups may be connected incorrectly. All of these can cause the motor to vibrate, produce abnormal noises, have a low speed of rotation, severe imbalance in the three-phase currents, and overheating of the windings leading to burnout. 6. Rotor winding end fault. Issues such as local welding failure in the rotor of the motor, brush grades and sizes that do not match, insufficient or excessive brush pressure, poor contact between the brushes and the windings, or the presence of foreign objects after prolonged operation can all cause localized overheating in the relevant areas or spark formation between the slip rings and brushes. 7. Various short circuits occur in the stator or rotor windings. If there is an inter-phase short circuit in the motor windings, the insulation near the short-circuit point burns out, overheats due to the excessive current, which in turn causes the windings to catch fire. If the insulation of the windings in the stator is damaged and the conductors come into contact with each other, an inter-turn short circuit occurs. In the winding affected by this short circuit, a very large circulating current flows (2-10 times the normal current), which causes the winding to overheat significantly, leads to an imbalance in the three-phase currents, reduces the motor’s torque, and generates noise. 8. An excessively low insulation resistance can make the winding insulation of the motor prone to damage and breakdown during operation, leading to various short circuits and subsequent burning out. II. Main measures to prevent motor fires 1. To check whether the three-phase voltage is too high or too low, a multimeter set to the AC voltage function can be used to measure the bus voltage and the voltage at the motor terminals. If it is due to issues with the power grid, you can contact the power supply authority to request adjustments, or use the transformer’s control switch for adjustment ； If the voltage drop in such branches is too high, the cross-sectional area of the wires should be changed and the distance between the motor and the busbars should be reduced ； If the motor operates at around 340V for an extended period, a motor with a power rating 20% higher than that of the driving mechanical equipment can be used. However, replacing all the motors in large numbers is not economical or practical; it is better to install capacitors on the electrical network for compensation. Additionally, when the voltage is too low, combined devices such as AC contactors and three-phase thermal relays can be used to protect the motor ； When the voltage is too high, simply adjust the three-phase thermal relay to a higher value. 2. To determine whether the three-phase voltage is symmetrical, a multimeter set to the AC voltage function and a clamp meter can be used to measure the voltage and current values of the three-phase busbars. If a severe imbalance is detected, it can be attributed to an excessive number of single-phase high-power heaters and AC welders connected to the three-phase busbars. To rectify this abnormal situation, the installed capacity on the three-phase busbars can be readjusted and allocated more reasonably. Use the same method to check whether the voltage at the upper end of each motor and the load current are balanced. If severe imbalance is detected, first cut off the power to examine whether there are short circuits between phases or between turns in the stator windings, as well as whether the stator windings are grounded. Only after the fault location has been identified and repaired can the power be restored for a test run. To ensure the safe operation of the motor, combined devices such as automatic switches (circuit breakers), three-phase thermal relays, and AC contactors can be used in the three-phase feed lines. 3. To prevent fires caused by the motor operating with a missing phase, the following measures can be taken: (1) Install three low-power indicator lights at the output terminal of the three-phase thermal relay, which can help determine whether one phase of the power supply is disconnected or one phase of the stator winding is open-circuited during operation. If one phase of the power supply is disconnected, the indicator light for that phase should not shine or should dim. First, the power supply should be turned off to check for any broken wires in the three-phase feed lines, blown fuses, poor contact in the main contacts of the AC contactors or circuit breakers, as well as any loose connections. Only after the fault has been identified and repaired can the power be turned back on to test the system ； If all the indicator lights are on, a clamp meter can be used to check the three-phase currents in order to determine whether the motor’s stator windings are operating with a missing phase. Once it is confirmed that one phase is broken, the power supply should be turned off immediately, and the connection points of the motor windings should be inspected. The faulty area should be identified, then reconnected and welded securely. After covering it with insulation and applying insulating paint, the motor can be reassembled and tested. (2) When motors with a capacity of 1.7–20 kilowatts are connected in a Y configuration, a low-voltage relay of around 10–40 V can be connected between the neutral point of the Y connection and ground (zero). The normally closed contacts of this relay are then connected in series within the coil circuit of the AC contactor; thus, if one phase of the power supply or the stator windings is disconnected, the power supply to the motor will be automatically cut off. 4. When a small horse is found pulling a large cart, the following fire prevention measures and protection methods should be adopted: (1) For machinery with nameplates, a motor can be selected based on the power indicated on the nameplate ； If there is no nameplate indication, first try to reduce the mechanical load so that the motor’s load current does not exceed the rated current. (2) If it is not possible to reduce the mechanical load, the only option is to use a motor with a higher capacity to accommodate it, but the load current of such a motor should not exceed its rated current. (3) A combined device consisting of a three-phase thermal relay, an AC contactor, and an automatic switch (circuit breaker) can be installed in the motor feed line to provide overload protection for the motor. 5. To prevent errors in the wiring of the windings, the following points should be noted when repairing or modifying motors: (1) Do not connect the coils of some windings in the reverse order, do not reverse the start and end points of one of the three-phase windings, and do not use too few turns per winding. (2) If the nameplate indicates a motor with 380V/220V voltage and Y/△ connection, it must not be connected in △ configuration when the supply voltage is 380V ； When the supply voltage is 220V, it is generally connected in a Δ configuration, but sometimes it can also be connected in a Y configuration as needed ； If the nameplate indicates a 660V/380V motor, it should generally be connected in a △ configuration when used in a system with a line voltage of 380V. (3) When wiring the stator windings, be careful not to connect them in a Δ configuration by mistake as a Y configuration, as this could cause the motor to overheat and burn out. (4) For motors that use Y-△ starting, be sure not to get the numbers of the 6 lead wires mixed up or connected incorrectly when wiring them. (5) To check whether the wiring of the windings is correct, a circular piece of silicon steel can be used – a hole is drilled in its center and it is fitted around a copper bar to serve as the rotor. The silicon steel piece is placed at the center of the inner circular surface of the stator. When three-phase voltage equal to 30–50% of the rated voltage is applied to the stator windings, if there is any error in the wiring of the poles, phases, groups, or individual coils, the silicon steel piece will not rotate ； If the poles, phases, groups of windings, or one coil is connected correctly, the silicon steel sheets should all rotate. 6. For faults at the ends of the rotor windings, a test light or multimeter can be used to check for issues such as open circuits or solder loss in one of the windings. (1) If the contact between the brush and the slip ring is poor, the brush pressure can be adjusted and the contact area between the brush and the slip ring can be improved ； (2) If a broken wire or local welding failure is detected, it should be reconnected and welded firmly; after covering it with insulation material and applying insulating paint, it can be tested ； (3) If excessive sparking is observed between the brush and the slip ring, it may be due to a brush grade and size that do not match; replace it with an appropriate brush ； (4) If the brush pressure is insufficient or too high, it can be adjusted ； (5) If the brush gets stuck inside the brush holder, the brush can be ground down ； (6) If there is dirt or debris on the surface of the slip ring, it can be polished with sandpaper No. 0, and then cleaned with clean cotton yarn ； (7) If the slip ring is not round or has deep scratches, it can be polished with sandpaper No. 0 or turned over. 7. To prevent various short-circuit faults in the stator and rotor windings, the following protective measures can be employed: (1) Install appropriate DZ5 or DZ10 type automatic switches (circuit breakers) in the motor feeders as short-circuit protection. (2) Regularly use a clamp meter to check the motor’s load current; if it is found that the three-phase currents are severely unbalanced and exceed the rated current, it can be determined that there is a short circuit in the windings, and the machine should be stopped for inspection. In the case of an inter-phase short circuit in the windings, this may be caused by inadequate insulation between the turns or at the ends of the windings, improper fitting of the insulation sleeves on the winding leads or between the coil groups, moisture damage or aging of the winding insulation, mechanical damage to the windings, excessively high supply voltage, or overheating of the motor. A megohmmeter can be used to locate the fault point ； If there is an inter-turn short circuit in the windings, it may be caused by moisture in the windings or aging of the insulation, too high supply voltage, damaged coil terminals, damage to the insulation during winding, wear due to vibration, or improper installation of the connection sleeves between the coil groups. A short-circuit detector can be used to locate the point of short circuit in the windings ； If a pole phase group in the winding is short-circuited, 3–6V of direct current can be applied across the winding, and the short circuit location can be located using the pointer method ； Use a bridge to measure the resistance of each phase or of certain windings; the phase or winding with the lower resistance is the short-circuited phase or winding. To determine whether a winding is grounded, a megohmmeter or similar device can be used for testing. When a ground fault is detected, methods such as direct observation, the test light method, and the elimination method should be employed to locate the point of grounding. (3) Depending on the location of the short-circuit fault and its severity, the following actions can be taken: if the short-circuit occurs at the end of a winding and the damage is not severe, the insulation is generally strengthened ； If the end short-circuit damage is severe or the short circuit occurs within the slot, the winding should be replaced. 8. It can be determined whether the insulation resistance is too low using a megohmmeter. The insulation resistance of the motor is required to be greater than 0.5 megohms. The reasons for excessively low insulation resistance are generally moisture, dust accumulation, oil leakage, overload, poor heat dissipation, mechanical damage, and chemical corrosion that cause insulation to age or get damaged. To address these issues, measures such as drying, cleaning, stopping leaks, reducing the load, preventing damage or corrosion, and replacing faulty components are taken. If it remains low, use the testing method to locate the fault point and carry out repairs. III. Other points to note: 1. Protective motors that are suitable for the characteristics of their production environment and their fire-resistant properties should be used. 2. Prevent mechanical overload and failures, and pay attention to making timely adjustments as well as implementing fire prevention measures. 3. When installing the motor, its protective devices, and starter, they should be mounted on a sturdy, non-combustible material base or non-combustible building component ； It should also be kept at a certain distance from flammable materials, and no miscellaneous items should be piled up around it. 4. The causes of fire when the motor overheats but the current does not increase are generally high ambient temperatures (above 40 degrees Celsius), faults in the ventilation and cooling systems, lack of maintenance, and the absence of protective devices. Appropriate measures should be taken to address these issues and prevent fires. 5. To reduce the starting current, prevent damage to the power supply transformer due to sudden surges, avoid voltage drops that could affect the operation of other devices, and prevent overheating of the motor which could lead to burnout, it is required that a voltage-reducing starter be installed for heavy-load starts (power greater than 10 kilowatts) or no-load starts (power greater than 14 kilowatts). 6. To prevent excessive frequent starting, high loads, or large torque from prolonging the starting cycle and causing the motor to burn out due to overcurrent and overheating, slip-ring motors or dual-squirrel-cage motors should be used.