Why does the motor burn

Update:10-12-2020
Summary:

Why does the motor burn out? As one of the most commonl […]

Why does the motor burn out? As one of the most commonly used moving equipment, what is the cause of the burning phenomenon?
Starting with load can cause damage to the motor;
The actual use head is too much lower than the pump nameplate head;
Some special pumps work too long without water;
Many conditions can cause burning
First of all, first find out whether the motor burned the bearing (mechanical failure) or burned the coil (electrical failure)? If it is the failure of the burning coil, it is mainly caused by overcurrent, and sometimes the voltage is too high or too low. The coil is heated and short-circuited, so first check whether the voltage during operation is too much different from the rated voltage.
There may be several reasons for the current short circuit
1. Overload operation of equipment. The motor runs at the rated current or over-rated current for a long time. It should be noted that the starting current of the motor is 3-5 times of the rated current, so try to avoid running the equipment with load or full load (mainly depending on the motor’s Matching margin between rated current and normal operating current).
2. The motor works in a humid working environment. The ground insulation and phase-to-phase insulation of the coil should be checked before the motor is started. The insulation requirements of different voltage levels are also different. You can refer to the relevant national standards for checking. Pay attention to the waterproof and moisture-proof of the motor during the operation of the motor.
3. The mechanical failure of the pump causes the motor to overload, the current is too large and the coil is burnt.
4. There is a problem with the heat dissipation of the motor. Generally, motor coils are air-cooled, and submersible pumps are water-cooled. Large motors are mostly cooled by air-to-air heat exchangers and air-to-water heat exchangers. If the cooling water (air) is broken, the coil cannot dissipate heat, and the coil may be burnt.
The actual use head is too much lower than the pump nameplate head:
The head of a centrifugal pump is used to overcome height and resistance. When a high-lift pump is working at a high-head point, its flow is the flow of the design point. If it is working at a low-head, the output resistance of the pump is reduced. The flow of the centrifugal pump will increase, the motor will be overloaded, and the motor will be burnt if it exceeds a certain level.
For example, a feed water pump has a head of 50 meters and a flow rate of 50 cubic meters per hour. When it feeds water to a height of 50 meters, its flow rate is 50 cubic meters per hour. When it feeds water to a height of 40 meters, Its height and resistance reduce its flow rate may reach 80-90 cubic meters per hour or more, then the motor will heat up or burn.
A summary of the reasons for motor burn-in can be divided into: load, power supply, motor insulation, and lack of equivalence.
1 lack of phase
Reason: Generally, it is caused by the lack of phase of the power supply (one phase is not supplied or the supply voltage is insufficient) or the contact point of the contactor in the line is not closed, the wire connection point is disconnected, loose or the contact position is oxidized.
1. The motor is delta connection: only one phase winding will be burned, and the insulation damage of one phase winding to the ground can be measured with a megohmmeter (shaking meter).
2. The motor is star-shaped (Y) connection: two-phase windings will burn out. You can use a megohmmeter (shaking meter) to measure the insulation damage of the two-phase windings to the ground.
Features: One or two phases (level 4) in the winding are all blackened, the coil is damaged symmetrically, and there are rules for phase loss. In short: if the motor is burned out due to lack of phase, then there will be windings that have not been burned. If the motor is burned out due to overload, all three-phase windings will be damaged to the ground.
2 overload
Reason: Generally, the motor runs for a long time with overcurrent, overheated operation, frequent start or braking, and wiring errors (delta connection to star connection). The cause of overheating is: over-load operation, causing the motor to heat up. Or the motor starts frequently, causing the motor to overheat.
Features: all windings turn black, and the end straps turn color and become brittle or even break.
In this kind of burning, both ends of the stator and rotor inside the motor will burn black, and the black spots are relatively uniform (generally, the motor has a fixed operating power, called rated power, in watts (W). In this case, if the actual power of the motor exceeds the rated power of the motor, this phenomenon is called motor overload).
3 turns
Reason: Enameled wire breakage caused by the motor manufacturing process.
Feature: The windings are partially burnt, usually when the motor cavity is clean, there is only one explosion point.
4 phases
Cause: The interphase paper is not put in place, or the interphase paper (casing) is damaged.
Feature: Burn out between two adjacent phases of the motor.
5 ground strike
Reason: The distance between the coil and the end cover base is not enough.
Feature: There are burnt black marks between the coil and end cap or end cap.

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