Burning two new motors in a row ! The output current and voltage of the inverter are balanced, and the input voltage is balanced. The current is unstable and fluctuates between 20% and 50%. The inverter burned out due to a short circuit between turns at 35Hz. The grid voltage is very stable, 720V. The distance from the motor to the inverter is 120m. Reactors are added to the input and output ends of the frequency converter. The motor current is very small and the ambient temperature is about -10 degrees, which can rule out the cause of motor overheating. What may cause motor failure in this case?
Analysis reason: The current of the load motor is not large, which is caused by the inverter. The inverter is too far away from the motor. The harmonics of the inverter output current break down the insulation between the parts, causing a short circuit. You can use an oscilloscope to measure the current on the motor side. Voltage quality.
There is a certain distributed capacitance between the cable length and the earth. This capacitance will affect the propagation of electricity, causing a certain phase shift between the current and the voltage, which will cause the (peak) applied to the motor. The oscilloscope can See, ordinary multimeter can not see) high voltage, high voltage breakdown motor windings.
Solution: If the withstand voltage is really not enough to burn the motor, you can use an inverter that has passed international certification (CE, UL) + a suitable reactor to avoid this problem.
-Does the frequency converter have any impact on the motor-
Ordinary asynchronous motors are designed according to constant frequency and constant voltage , and cannot fully adapt to the requirements of variable frequency speed regulation. The following is the impact of the frequency converter on the motor:
1. Motor efficiency and temperature rise issues
Regardless of the type of frequency converter, varying degrees of harmonic voltages and currents are generated during operation, causing the motor to operate under non-sinusoidal voltages and currents. According to information, at present
Taking the commonly used sine wave PWM type inverter as an example, its low-order harmonics are basically zero, and the remaining high-order harmonic components that are about twice the carrier frequency are: 2u+1 (u is the modulation ratio).
High-order harmonics will cause an increase in stator copper loss, rotor copper (aluminum) loss, iron loss and additional losses, the most significant of which is rotor copper (aluminum) loss. Because the asynchronous motor rotates at a synchronous speed close to the fundamental frequency, high-order harmonic voltages cut the rotor bar with a large slip, resulting in large rotor losses. In addition, additional copper losses due to skin effect need to be considered. These losses will cause the motor to generate additional heat, reduce efficiency, and reduce output power . If an ordinary three-phase asynchronous motor is operated under a non-sinusoidal power supply output by a frequency converter , its temperature rise will generally increase by 10%-20%.
2. Motor insulation strength problem
At present, many small and medium-sized inverters adopt PWM control method. Its carrier frequency has a rise rate of about several thousand to more than ten kilohertz, which is equivalent to applying a steep impact voltage to the motor, causing the motor’s inter-turn insulation to withstand a relatively harsh 4 to 6 times voltage superimposed on the motor’s operating voltage. , will pose a threat to the motor’s ground insulation. In addition to the repeated impact of high voltage on the ground insulation, the motor stator winding has to withstand high voltage, and the rectangular chopper impulse generated by the PWM inverter accelerates aging.
3. Harmonic electromagnetic noise and vibration
When ordinary asynchronous motors are powered by frequency converters, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complicated. The time harmonics contained in the variable frequency power supply interfere with the inherent space harmonics of the electromagnetic part of the motor, forming various electromagnetic excitation forces. When the frequency of the electromagnetic force wave is consistent with or close to the natural vibration frequency of the motor body, resonance will occur, thereby increasing noise. Since the motor has a wide operating frequency range and a large rotational speed variation range, it is difficult for the frequencies of various electromagnetic force waves to avoid the natural vibration frequencies of each component of the motor.
4. The motor ’s adaptability to frequent starting and braking
Since the frequency converter is used for power supply, the motor can be started at very low frequency and voltage without inrush current, and can be braked quickly using various braking methods provided by the frequency converter, in order to achieve frequent starting and braking. Conditions are created, so the mechanical system and electromagnetic system of the motor are under the action of cyclic alternating forces, which brings fatigue and accelerated aging problems to the mechanical structure and insulation structure.
-The frequency converter will produce high odd harmonics-
Mainly, the 5th and 7th orders have a greater impact on the inverter and motor. Usually, reactors, absorption capacitors, etc. are added during design to reduce the impact of harmonics. A filter can also be added to the output end of the frequency converter.
How to calculate the harmonic power of a frequency converter-powered motor?
method one:
The Fourier transform obtains the amplitude and phase of each harmonic of voltage and current. The active power of each harmonic is calculated according to P=√3UIcosφ. The active power of all harmonics is added to obtain the harmonic power.
Method Two:
The total active power is measured, and the fundamental wave amplitude and phase of the voltage and current are obtained by Fourier transform. The fundamental wave active power is calculated according to P=√3UIcosφ. The total active power minus the fundamental wave active power is the harmonic power.
The harmonic power measurement accuracy is low. Generally, the higher the harmonic frequency, the lower the accuracy. The second method is recommended.
-Causes and improvement methods of the impact of frequency converter harmonics on motors-
1. The reason for motor damage is that the frequency converter generates a high peak voltage on the stator winding of the motor. The amplitude of the peak voltage exceeds the insulation strength of the winding, causing damage to the winding. The amplitude of the peak voltage will reach more than 3 times the rated operating voltage of the inverter. For example, for an inverter with a rated voltage of 380V, the amplitude of the peak voltage exceeds 1200V. This peak voltage impacts the stator winding of the motor thousands of times per second, which will soon cause damage to the stator winding.
2. The reason for motor damage is that the frequency converter will also generate bearing current in the bearings of the motor. Bearing current flowing in the bearing for a long time will cause the bearing to burn. The smaller the power of the motor, the easier it is for the stator winding to be damaged; the greater the power, the more likely it is that the stator winding will be damaged. The motor bearings are more susceptible to damage.
Weak protection : Because all motor protectors currently on the market achieve the purpose of protecting the motor by collecting values of current or voltage changes; however, due to various reasons, the motor bearings are damaged and the rotor is eccentric, which in turn causes the motor to sweep. The problem of burned motor: These protectors cannot protect the motor, because only when the motor is swept and the windings are burned out and short-circuited, this type of protector will act, but it is too late; so far, there is no An intelligent product for protecting motor bearings; many users can only artificially protect some large motors by manually monitoring or regularly inspecting and testing the temperature changes at the bearings. This method has two drawbacks:
1. It increases the workload of personnel and increases the personnel expenses of the enterprise. At the same time, it is impossible to take care of all motors.
2. There is a time limit for manual inspection. It is impossible to leave people alone within 24 hours. If the bearing is damaged during the non-inspection period, the rotor will be eccentric, the motor will be swept, and the motor will be burned. Accidents such as ordinary motors cannot be avoided. When driven by a frequency converter, the service life is greatly shortened. In severe cases, the stator winding may be damaged within a few months. The resulting shutdown caused huge losses to the company.
