Theoretical Integration with Practice - Analysis of the Causes of Motor Heating

The power supply voltage is too high. When the power supply voltage is too high, the back electromotive force, magnetic flux, and magnetic flux density of the motor all increase accordingly. Due to the proportionality between the magnitude of iron loss and the square of magnetic flux density, the iron loss increases, leading to overheating of the iron core. And the increase in magnetic flux leads to a sharp increase in the excitation current component, causing an increase in copper loss in the stator winding and overheating of the winding. Therefore, when the power supply voltage exceeds the rated voltage of the motor, it will cause the motor to overheat.

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

Asymmetric power supply voltage. When one phase of the power cord breaks, it will cause single-phase operation of the three-phase motor, causing the two-phase windings to overheat due to high current, resulting in motor burnout. The commonly known motor is damaged due to phase loss. The three-phase power supply is unbalanced. When the three-phase power supply is unbalanced, it will cause the three-phase current of the motor to be unbalanced, and zero sequence and negative sequence currents that do not do work will flow through the motor winding, causing the winding to overheat.

When the motor overheats, the power supply should be considered first when using the elimination method.

1. Load problem causing motor overheating

2. Overloaded operation of the electric motor. 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 operate under overload for a long time (i.e. a small horse pulling a large car), which will cause the motor to overheat.

When repairing an overheated motor, it is necessary to first determine whether the load power matches the motor power to prevent blind and aimless disassembly.

4. The mechanical load being dragged is not working properly. Although the equipment is matched, the mechanical load it drags is not working properly, with varying loads during operation. For example, when the motor is paired with a thresher, if the feeding amount is too large, the motor will generate heat due to overload.

5. Mechanical failure caused by dragging. When the dragged machinery malfunctions, rotates inflexibly or gets stuck, it will overload the motor and cause the motor winding to overheat.

Overheating caused by the motor itself

1. The motor winding is open circuited. When one phase of the motor winding is broken or one branch of the parallel branch is broken, it will cause three-phase current imbalance and cause the motor to overheat.

When a short circuit fault occurs in the motor winding, the short-circuit current is much higher than the normal operating current, which increases the copper loss of the winding, causing it to overheat or even burn out.

3. Incorrect connection of the electric motor. When a delta connected motor is misconnected into a star shape, if the motor still runs at full load, the current flowing through the stator winding will exceed the rated current, and even cause the motor to stop on its own. If the stop time is slightly longer and the power is not cut off, the winding will not only overheat severely, but also burn out the motor.

When a star connected motor is misconnected into a triangle, or when several coil groups are connected in series as one branch of the motor are misconnected into two parallel branches, it will cause the winding and iron core to overheat, and in severe cases, it will burn out the winding.

Wiring of electric motor process. When a coil, a group of coils, or a phase winding group is connected in reverse, it will cause serious imbalance of three-phase current and cause the winding to overheat.

Mechanical failure of the electric motor. When the motor shaft is bent, poorly assembled, or the bearings are faulty, it will increase the motor current, copper and mechanical losses, and cause the motor to overheat.

Poor ventilation and heat dissipation cause the motor to overheat

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

2. There are debris blocking the air inlet, causing poor air flow and resulting in low air intake.

3. Excessive dust inside the motor affects heat dissipation.

4. The fan is damaged or installed backwards, resulting in no wind or low air volume.

5. Failure to install a wind cover or wind deflector inside the motor end cover results in the motor not having a suitable air path.