Motor burnout is mainly caused by abnormalities in the power supply, the motor itself, the load, ventilation and heat dissipation. When there is a problem with the burning motor, first of all, it is necessary to judge whether the motor is burnt out, and the following steps are performed:
First: The motor will be very hot. After opening the junction box, there will be a very pungent smell that is unbearable, but it does not prove that the motor is burnt and there is a possibility of high temperature burning.
Second: Use a multimeter to measure the three-phase resistance to see if the resistance of the three-phase resistor is balanced. If the three-phase resistance is unbalanced, the motor is abnormal. For a motor with a large capacity, measure the DC resistance, which can be measured using a bridge.
Third: Use a megger to measure the resistance of one phase to the ground or the base. If the resistance is zero or the resistance is less than 0.5 megohm, the motor insulation is faulty or burned.
Generally, after the above three points, it is basically judged whether the motor is burned. The most direct causes of motors that are usually burned during user use are: overload, single phase, phase loss, and daytime.
(1) overload
If the motor is overloaded, the winding will be overheated and burned, and the wire package will be completely black.
(2) Single-phase angular connection The single-phase winding of the motor is burned due to lack of phase.
(3) Phase loss The two-phase winding of the motor connected by Y is burned due to lack of phase.
(4) Inter-turn short circuit caused by short circuit of motor phase winding
1 Power supply reasons and treatment
(1) When the pulse du/dt and di/dt (slope) of the inverter output are too large, the PWM wave spike voltage rise time is too short, causing the voltage drop of about 80% of this voltage to drop in the first group of the phase. On the windings. The low-voltage motor windings are inevitably tied together at the beginning and end of the same winding. That is to say, if it is a 380V inverter, there will be a voltage of more than 1000V added to the insulating paint of the enameled wire (the paint is difficult to reach) and there will be corona discharge. Problem performance: The performance of the motor burnout is a short circuit between turns.
Processing: In this case, the drive resistance and the output reactor must be increased to reduce the du/dt, di/dt slope, and the power line must not be too long.
(2) The output pulse spike of the inverter (the absolute value is too large) causes the insulation to break down. Problem performance: The performance of motor burnout is phase-to-phase short circuit and short circuit to ground. Handling: hardware measures for peak absorption or filtering must be taken
(3) The output harmonic content of the inverter is large, and the harmonic reflection of the injected motor causes the voltage of the motor terminal to rise, which increases the insulation pressure of the motor, and the cumulative effect of the motor loss heat causes the motor insulation to accelerate and age and burn. Problem performance: short circuit between turns, short circuit between phases, short circuit to ground. Processing: The carrier frequency must be increased to reduce the current distortion rate, and the power line must not be too long.
(4) The inverter output voltage is too low. When the mechanical load is constant, the low voltage will cause the motor stator and rotor current to increase, and the winding copper loss will increase, causing the stator and rotor windings to overheat. The motor will age due to overheating and even the windings will burn out. According to national standards: When the deviation between the power supply voltage and the rated voltage does not exceed ± 5%, the motor output power is maintained at the rated value, and the motor power supply voltage is not allowed to exceed ±10% of the rated value.
(5) When the output voltage of the inverter is too high, the magnetic flux density B of the stator core of the motor increases with the voltage, and the iron loss increases, causing the core to overheat. At the same time, the excitation current rises sharply, causing the stator current to increase and the copper loss to increase, causing the motor winding to overheat.
(6) Asymmetric three-phase power supply of the inverter or disconnection of a phase power supply will cause single-phase operation of the motor, which will cause the stator and rotor winding currents to increase and the motor to overheat. According to the national standard: Generally, the difference between the average voltage of any one of the three-phase voltage and the three-phase voltage does not exceed 5% of the average value of the three-phase voltage, both [(Un- Up)/Up]*100%<±5% Where Up is the three-phase average voltage, Up=(Ua+Ub+Uc)/3, and Un is the voltage of any phase.
(7) Inverter parameter setting problem:
a Torque coefficient setting is too large, the magnetic is easy to saturate at low speed but still less than In does not jump off the motor
b The ratio of the V/F curve selection is too high, so that the voltage increases and the core heat increases.
c Carrier frequency setting is too high to generate overvoltage breakdown winding
d carrier frequency setting is too low to generate a large number of harmonics, so that the stator windings are heated
e electronic thermistor setting is too high to properly protect the motor
f The inverter output voltage setting is too high or too low
- The cause and treatment of the motor itself
(1) Motor selection error: power is too small, insulation level is not enough (frequency converter requires F grade or above), etc.
(2) The three-phase current imbalance caused by the inter-turn winding of the phase winding, or a short-circuit fault between the windings, causes the phase current to increase, causing the winding to heat up, causing the motor to overheat.
(3) The stator winding wiring is wrong, such as the angular connection is staggered into a star connection
(4) High ambient temperature The motor surface has a lot of dirt, or the air duct is blocked. The motor fan is faulty and the ventilation is poor. Handling: Keep the motor surface clean and the motor fan is normal.
(5) Frequent startup. The starting current of the asynchronous motor is 5 times to 7 times of the normal working current. If the motor starts frequently, the temperature of the motor will rise.
(6) Long startup time. Load start with load often results in an overload condition with long start-up time and high motor temperature.
(7) The cage rotor is broken, the coil is disconnected, there is foreign matter in the motor, the rotor core is rubbed, and the mechanical jam is blocked. The motor is overloaded due to damage to the motor bearing, the rotor is stuck, or the load dragged by the motor is stuck.
(8) Due to long-term overload or overheating operation, the winding insulation aging accelerates, and the weakest point of insulation causes carbonization to cause inter-turn short circuit, phase-to-phase short circuit or short circuit to ground, causing partial burning of the winding. Handling: Replace large motor and reduce motor temperature
(9) The insulation of the motor winding is affected by mechanical vibration (such as high current impact at start-up, vibration of the driven equipment, unbalanced rotor of the motor, etc.), causing undesirable phenomena such as loose turns and insulating cracks in the winding, and the damage effect is continuously accumulated. The thermal expansion and contraction causes the winding to be rubbed, thereby accelerating the insulation aging, eventually leading to the destruction of the first carbonized insulation until the winding is burned. Handling: The motor base may be loose. Tighten the screws, adjust the center line and motor rotation, and the stator gap. When the load changes too much, the special motor must be replaced.
- Load reasons and treatment
(1) The load power is too large, so that the motor is overloaded for a long time. The overload causes the stator current to increase and the winding copper loss to increase. Performance: The motor winding temperature rise exceeds the allowable value. Treatment: Replace the large motor
(2) The mechanical load of the motor is high or low or stuck. The load dragged by the motor is not normal, causing the motor to not operate normally or even stall. Performance: The motor is overheated. Processing: Correct the load
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