Why is it not recommended for ordinary motors to link frequency converters with frequency conversion speed regulation

Update:01 Jul 2021

Ordinary motors can indeed be used with frequency converters for speed regulation, but general frequency conversion motor manufacturers do not initiate customers to use the motor in this way, because such use of the motor can easily cause the motor to burn!

One. The influence of frequency converter on the efficiency and temperature rise of ordinary motors

Regardless of which form of frequency converter and frequency conversion cabinet, different levels of high-order harmonic voltage and current occur during operation. High-order harmonics will cause motor stator copper loss, rotor copper loss (aluminum loss), iron loss and The most significant increase in additional loss is the loss of the rotor. These loss cities make the motor particularly feverish, low efficiency, and reduced output power. If an ordinary motor is operated under variable frequency conditions, its temperature rise will generally increase by 10%-20%.

2. Cooling problem at low speed

First of all, ordinary motors are not designed for variable frequency speed regulation at the beginning, so the impedance of ordinary motors is not ideal. When the power supply frequency is low, the loss caused by the higher harmonics in the power supply is large. Secondly, the ordinary asynchronous motors are in the low frequency state. When running down, it belongs to constant torque operation. Although the voltage, frequency, and power at this time are all lower than the rated operating value, but the current of the motor will not change much, so the fever of the motor will not decrease, and the The cooling air volume is proportional to the third power of the speed. As the speed decreases, the cooling environment of the motor becomes worse, and the temperature rise of the motor will increase rapidly.

3. The shaft voltage and shaft current of the motor

The bearing voltage refers to the capacitive coupling voltage present in the radial gap of the bearing. The bearing voltage is initially caused by the exchange voltage between the average potential of the stator windings and the grounded stator core. When the motor is powered by a sinusoidal voltage, the bearing voltage is actually zero. Rectangular wave power supply, the neutral point potential is not zero, so common mode voltage will occur. If the motor is not insulated on the bearing housing, shaft cycle current, shaft ground current and common mode current will occur accordingly. Great damage to bearings and loads. The insulation treatment of ordinary motors is not suitable for the use of frequency converters for speed regulation.

Four. Insulation problems of the motor

As the motor recycles the power supply from the inverter, a reliable amplitude voltage rectangular pulse of different width and frequency (tens to thousands of kilohertz) occurs, and the voltage rise time is between 50ns-400ns. In such a short rise time, the inverter passes through the cable When power is supplied to the motor, repeated voltage mutations (surge voltage) occur on the motor terminals, which will inevitably reduce the insulation life. For ordinary motors, electromagnetic wires, insulation, and lead wire cables are not subject to special punishment. In the inverter power supply environment, there are major hidden dangers.

5. Harmonic electromagnetic noise and vibration issues

When an ordinary asynchronous motor is running at a variable frequency, the vibration and noise caused by electromagnetic, machinery, ventilation and other factors will become more and more huge. The time harmonics contained in the variable frequency power supply and the inherent spatial harmonics of the electromagnetic sector of the motor interfere with each other, forming various electromagnetic exciting forces. When the frequency of the electromagnetic force wave is consistent with or close to the natural frequency of the motor, it will occur Resonance phenomenon increases noise. If the working frequency range of the motor is wide and the speed changes greatly, it is difficult for the frequency of various electromagnetic force waves to avoid the natural vibration of the motor's components.

Therefore, if you need to use frequency conversion speed control, you should choose frequency conversion speed control motor as usual. The frequency conversion speed control motor is a series of three-phase asynchronous motors developed based on the use of frequency conversion speed control. It has higher insulation grade and overload resistance. Accurately control the speed of the frequency conversion motor.

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