Attention to the shortcomings of the inverter - hope Senlan

The wiring interval between the motor and the inverter is too long, causing the new motor to catch fire

Burn two new motors in a row! The output current and voltage of the inverter are balanced, the input voltage is balanced, the current is unstable, the fluctuation is 20%-50%, the inverter is burned out by a short circuit between 35Hz turns, the grid voltage is very stable, 720V, the interval between the motor and the inverter is 120m, and the input and output terminals of the inverter are equipped with reactors. The motor current is very small, and the ambient temperature is about -10 degrees, which can rule out the cause of motor overheating. What is the cause of this situation?

Attention to the shortcomings of the inverter - hope Senlan

Attention to the shortcomings of the inverter - hope Senlan

Analysis reason: the current of the load motor is not large, it is caused by the inverter, the inverter is too far away from the motor, the harmonic of the output current of the inverter breaks down the insulation between the smashing rooms, resulting in a short circuit, and the quality of the current and voltage on the motor side can be measured with an oscilloscope.

The cable is long, and there is a certain distribution capacitance between the ground and the earth, which will affect the propagation of electricity, so that a certain phase shift occurs between the current and the voltage, which will make the high voltage and high voltage breakdown of the motor winding (the spike, the oscilloscope can be seen, but the general multimeter cannot be seen).

Solution: If the withstand voltage is not enough to burn the motor, you can use the inverter + suitable reactor that has passed the world certification (CE, UL) to avoid such a problem.

The inverter has no effect on the motor

Generally, asynchronous motors are planned according to constant frequency and constant voltage, and it is impossible to completely adapt to the requirements of frequency conversion speed regulation. The following is the influence of the inverter on the motor:

1. The power and temperature rise of the motor

Regardless of the method of the inverter, different degrees of harmonic voltage and current occur during operation, so that the motor runs under non-sinusoidal voltage and current. According to the material, to the present

For example, the commonly used sine wave PWM inverter has zero lower harmonics at all, and the weight of the remaining higher harmonics that is about twice the carrier frequency is: 2u+1 (u is the modulation ratio).

Higher harmonics will cause the addition of stator copper loss, rotor copper (aluminum) loss, iron loss and additional loss, and Z is the most significant rotor copper (aluminum) loss. Because the asynchronous motor rotates at a synchronous speed close to the fundamental frequency, a large rotor loss will occur when the higher harmonic voltage cuts the rotor guide with a large slip. In addition to this, additional copper loss due to the skin effect needs to be taken into account. These losses will make the rated heat of the motor, the power decreased, and the output power decreased, such as the general three-phase asynchronous motor running under the condition of non-sinusoidal power supply output of the inverter, its temperature rise generally needs to increase by 10%-20%.

2. The dielectric strength of the motor

At present, many small and medium-sized inverters use PWM control methods. His carrier frequency is about a few thousand to more than ten thousand hertz rise rate, which is equivalent to applying a steep impulse voltage to the motor, so that the inter-turn insulation of the motor accepts a more severe 4-6 times the voltage superimposed on the motor operating voltage, which will pose a threat to the ground insulation of the motor

3. Harmonic electromagnetic noise and vibration

Generally, when the asynchronous motor is powered by an inverter, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complicated. The harmonics contained in the variable frequency power supply at each time and the inherent space harmonics of the electromagnetic part of the motor interfere with each other to form various electromagnetic excitation forces. When the frequency of the electromagnetic force wave is the same as the natural oscillation frequency of the motor body or close to each other, the resonance phenomenon will occur, thereby increasing the noise. Because the working frequency of the motor is wide and the speed change scale is large, it is difficult for the frequency of various electromagnetic force waves to avoid the natural vibration frequency of each component of the motor.

4. The adaptability of the motor to frequent start and brake

Because after the inverter is selected for power supply, the motor can be started without impulse current at very low frequency and voltage, and various braking methods provided by the inverter can be used for rapid braking, which creates conditions for the realization of frequent starting and braking, so the mechanical system and electromagnetic system of the motor are under the effect of cyclic alternating force, which brings fatigue and accelerated aging to the mechanical structure and insulation structure.

High odd harmonics occur in frequency converters

Mainly 5th and 7th times have a greater impact on the inverter and motor, usually in the planning of the reduction of harmonic influence, the addition of reactors, absorption capacitors, etc. It is also possible to add a filter at the output of the inverter.

How to calculate the harmonic power of the inverter-powered motor?

Option 1:

The Fourier transform obtains the amplitude and phase of each harmonic of voltage and current, calculates the active power of each harmonic according to P=√3UIcosφ, and adds the active power of all harmonics to obtain the harmonic power.

Method 2:

The total active power is measured, the fundamental amplitude and phase of the voltage and current are obtained by the Fourier transform, and the fundamental active power is calculated according to P=√3UIcosφ, and the total active power minus the fundamental active power is the harmonic power.

The accuracy of harmonic power measurement is low, and the higher the harmonic frequency, the lower the accuracy, so the second method is recommended.

Attention to the shortcomings of the inverter - hope Senlan

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