situations of three-phase motor over-current and solutions

I. Power problems

1.The power supply voltage is too high

When the power supply voltage is too high, the motor back-EMF, magnetic flux, and magnetic flux density all increase. Since the magnitude of the iron loss is proportional to the square of the magnetic flux density, the iron loss increases, causing the core to overheat. The increase of the magnetic flux causes the component of the exciting current to increase sharply, resulting in increased copper losses in the stator windings and overheating the windings. Therefore, when the power supply voltage exceeds the rated voltage of the motor, the motor will overheat.

2.The power supply voltage is too low

When the power supply voltage is too low, if the electromagnetic torque of the motor remains unchanged, the magnetic flux will decrease, and the rotor current will increase accordingly. The load power component in the stator current will increase accordingly, causing the copper loss of the winding to increase, resulting in stator and rotor The winding is overheated.

3.Asymmetric power supply voltage

When one phase of the power line is disconnected, one phase of the fuse is blown, or one phase of the brake knife starting equipment is burned, which causes one phase to fail, it will cause the three-phase motor to go single-phase, causing the running two-phase winding to overheat by high current and burn out. Therefore, fuses are generally not suitable for protection of three-phase motors.

4. Unbalanced three-phase power

When the three-phase power supply is unbalanced, the three-phase current of the motor will be unbalanced, causing the winding to overheat.

It can be seen from the above that when the motor is overheating, the cause of the power supply should be considered first (soft start, inverter, servo drive can also be considered as power supply). After confirming that there are no problems with the power supply, consider other factors.

Second, the load problem

1.Motor overload operation

When the equipment is not matched and the load power of the motor is greater than the rated power of the motor, the motor will be overloaded for a long time (that is, a small horse-drawn cart), which will cause the motor to overheat. When repairing an overheated motor, you should first find out whether the load power matches the motor power to prevent blind and unintended disassembly.

2.The mechanical load of the drag is not working properly

Although the equipment is matched, the mechanical load being dragged is not working properly, the load is large and small during operation, and the motor is overloaded and generates heat.

3.The dragging machinery is faulty

When the machine being towed is faulty, inflexible or stuck, it will overload the motor and cause the motor winding to overheat. Therefore, when the motor is overheated, the load factor cannot be ignored.

Third, the motor itself

1.The motor winding is open

When one phase of the motor winding is disconnected or one of the parallel branches is disconnected, the three-phase current will be unbalanced and the motor will overheat.

2.Motor winding short circuit

When the motor winding has a short-circuit fault, the short-circuit current is much larger than the normal working current, which increases the copper loss in the winding, causing the winding to overheat and even burn out.

3, the motor star angle connection method is wrong

When the delta-connected motor is connected in a star shape by mistake, the motor is still running at full load. The current flowing through the stator windings must exceed the rated current, and even cause the motor to stop. Overheating and burning. When a star-connected motor is wrongly connected into a triangle, or a number of coils are connected in parallel to form a branch, a motor is connected in parallel to two branches in parallel, which will overheat the winding and the core and burn the winding in severe cases.

4.The connection of the motor coil is wrong

When a coil, coil group or one-phase winding is connected reversely, the three-phase current will be severely unbalanced and the winding will overheat.

5.Mechanical failure of the motor

When the motor shaft is bent, poorly assembled, and the bearing is faulty, it will increase the motor current, increase copper loss and mechanical friction loss, and cause the motor to overheat.

4. Ventilation and heat dissipation problems

1. The ambient temperature is too high, making the inlet air temperature high.

2. The air inlet is blocked by debris, which makes the air intake not smooth, resulting in a small air intake.

3. Excessive dust inside the motor affects heat dissipation.

3. The fan is damaged or installed backwards, causing no wind or small air volume.

4. The windshield is not installed or the windshield is not installed in the motor end cover, which causes the motor to have a certain air path.

Fifth, the repair motor problem The starting current of the repaired motor reaches more than 66%. At the same time, the frequent operation of the motor will also cause the high current and the motor to overheat.

6. Series resistance problem The winding motor does not match the series resistor, etc. At the same time, the frequent operation of the motor will also cause high current and overheating of the motor.

Seven, motor vibration problems 1, rotor imbalance-leveling balance

2. Unbalanced pulley or shaft extension bending-check and correct

3.The motor is not aligned with the load axis-check the axis of the adjustment unit

4, the motor is not properly installed-check the installation and foot screws

5. Suddenly heavy load-reduce the load