7 situations and solutions for excessive current in three-phase motors
In terms of power supply, the causes of motor overheating include the following:
1. The power supply voltage is too high
When the power supply voltage is too high, the motor's back electromotive force, magnetic flux and magnetic flux density all increase. Since the size 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 in magnetic flux will cause the excitation current component to increase sharply, causing the stator winding copper loss to increase and causing the winding to overheat. Therefore, when the power supply voltage exceeds the rated voltage of the motor, it will cause the motor to 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, the rotor current will increase accordingly, and the load power component of the stator current will increase, causing the copper loss of the winding to increase, resulting in damage to the stator and rotor. Winding overheated.
3. The power supply voltage is asymmetrical
When one phase of the power cord is disconnected, one phase of the fuse is blown, or the corner of the knife starting device is burned and one phase is blocked, it will cause the three-phase motor to run in single phase, causing the running two-phase winding to overheat through large current and even burn out. Therefore, fuses are generally not suitable for protection of three-phase motors .
4. Three-phase power supply is unbalanced
When the three-phase power supply is unbalanced, the three-phase current of the motor will be unbalanced, causing the windings to overheat.
It can be seen from the above that when the motor overheats, the power source should be considered first ( soft start, frequency converter, and servo drive can also be regarded as power sources ). After confirming that there is no problem with the power supply, consider other factors.
load problem
The reasons for motor overheating in terms of load are as follows:
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 run overloaded for a long time (that is, a small horse pulls a large 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 aimless disassembly.
2. The dragged mechanical load is not working properly.
Although the equipment is complete, the mechanical load being driven does not work properly. The load fluctuates during operation, and the motor is overloaded and generates heat.
3. The dragging machine is faulty
When the towed machinery is faulty, inflexible or stuck, it will overload the motor and cause the motor windings to overheat. Therefore, when inspecting and repairing motor overheating, load factors cannot be ignored.
Problems with the motor itself
1. Motor winding circuit breakage
When one phase winding in the motor winding is open circuit, or one branch circuit in the parallel branch circuit is open circuit, it will cause the three-phase current to be unbalanced and cause the motor to overheat.
2. Motor winding short circuit
When a short-circuit fault occurs in the motor winding, the short-circuit current is much larger than the normal operating current, which increases the copper loss of the winding, causing the winding to overheat or even burn out.
3. Wrong star-angle connection of the motor
When a delta-connected motor is mistakenly connected to a star, the motor is still running with full load, and the current flowing through the stator winding will exceed the rated current, which may even cause the motor to stop by itself. If the stalling time is longer and the power supply is not cut off, the windings will not only be seriously damaged. If overheated, it will burn. When a star-connected motor is mistakenly connected into a triangle, or a motor with several coil groups in one branch is mistakenly connected into two branches in parallel, the winding and core will overheat, and in severe cases, the winding will be burned.
4. Wrong connection of motor coil
When a coil, coil group or one-phase winding is connected reversely, it will cause a serious imbalance in the three-phase current and cause the winding to overheat.
5. Mechanical failure of the motor
When the motor shaft is bent, poorly assembled, or the bearings are faulty, the motor current will increase, copper loss and mechanical friction loss will increase, causing the motor to overheat.
Ventilation and heat dissipation issues
1. The ambient temperature is too high, causing the inlet air temperature to be high.
2. The air inlet is blocked by debris, making the air inlet unsmooth and resulting in a small air inlet volume.
3. There is too much dust inside the motor, which affects heat dissipation.
4. The fan is damaged or installed backwards, resulting in no air or low air volume.
5. The windshield is not installed or the windshield is not installed in the motor end cover, causing the motor to have no certain air path.
Repair motor problems
The starting current of the repaired motor reaches more than 66%. At the same time, frequent operation of the motor will also cause high current and cause the motor to overheat.
Series resistance problem
The mismatch between the wound motor and the series resistor, etc., and the frequent operation of the motor will also cause high current and cause the motor to overheat.
Motor vibration problem
Excessive motor vibration may also cause high motor current. The reasons and solutions are:
1. The rotor is unbalanced - leveling and balancing
2. The pulley is unbalanced or the shaft extension is bent —check and correct
3. The motor and load axis are not aligned - check and adjust the axis of the unit
4. The motor is improperly installed - check the installation condition and foot screws
5. The load is suddenly too heavy - reduce the load
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